Java Basics III: Methods and Encapsulation in Java

3.  Methods and Encapsulation in Java

3.1 Scope of Variables

The scope defines lifespan of a variable. There are four different scopes in Java:

  • Local variables
  • Method parameters
  • Instance variables
  • Class variables

3.1.1 Local Variables

Variables defined within a method. These are used mainly to store immediate results of a calculation. They have the shortest life span. In the following code, a local variable avg is defined within the method getAverage():

class Student {

     private double marks1, marks2, marks3;    //instance variables

     private double maxMarks = 100;     //instance variable

     public double getAverage() {

          double avg= 0;   //local variable avg

          avg= ((marks1 + marks2 + marks3) / (maxMarks*3)) * 100;
          //avg inaccessible outside getAverage

          return avg;

The scope of a variable can be reduced to a for block or an if statement within a method:

public void localVariableInLoop() {

     for (int ctr= 0; ctr < 5; ++ctr) {  //ctr defined within the loop



     System.out.println(ctr);//ctr inaccessible outside the loop, code would not compile

public double getAverage() {

     if (maxMarks > 0) {

          double avg = 0;  //avg local to block

          avg = (marks1 + marks2 + marks3)/(maxMarks*3) * 100;

     return avg;


     else {

          avg = 0;   //avg cannot be accessed from here, code would not compile

          return avg;



3.1.2 Method Parameters

Variables that accept values within a method those are accessible only in the method that defines them.

class Phone {

     private boolean tested;

     public void setTested(boolean val) {

          tested = val;    //accessible only in setTested method


     public boolean isTested() {

          val= false;   //this line won’t compile

          return tested;



The scope of a method parameter may be as long as that of a local variable, or longer, but it can never be shorter.

3.1.3 Instance Variables

An instance variable is available for the life of an object. Declared inside the class, outside of the methods and accessible to all nonstatic methods within the class.

class Phone {
     private boolean tested;   //instance variable
     public void setTested(boolean val) { 
          tested= val;  //tested is accessible here
     public boolean isTested() { 
           return tested;   //tested is accessible here

The scope of instance variables is longer than that of local variables or method parameters.

3.1.4 Class Variables

A class variable is defined by using the keyword static. It belongs to a class and not to individual objects of the class, therefore it is shared across all objects.

class Phone {
     static boolean softKeyboard= true; //class variable

class TestPhone {
     public static void main(String[] args) {
          Phone.softKeyboard= false; //accessing variable by using class name
          Phone p1 = new Phone();
          Phone p2 = new Phone();
          System.out.println(p1.softKeyboard);//accessing variable by object name
          p1.softKeyboard = true;

A class variable can be changed using the name of the class or an object.

3.1.5. Overlapping Variable Scopes

Different local variables can have different scopes. The scope of local variables may be shorter, equal or as long as the scope of method parameters. Local variables can have a shorter scope if declared in a sub-block in a method.
In order to prevent conflicts with variable names, some rules are necessary:
• A static variable cannot be defined with the same name of an instance variable within the same class.
• Local variables and method parameters cannot use the same name.

3.2.  Object’s Life Cycle

An object’s life cycle lasts from its creation until it goes out of its scope or is no longer referenced by a variable. When an object is accessible, it can be referenced by a variable and can be used by calling its methods and accessing its variables.

3.2.1.    An Object is born

An object is created when the keyword new is used.

class Person {}

class ObjectLifeCycle {

     Person person;    //declaring only a reference

class ObjectLifeCycle2 {

     Person person = new Person();//declaring and initializing a person type variable.


When an unreferenced object is created, it only executes the relevant constructors of the class, but it cannot be accessed using any reference variable.

class ObjectLifeCycle2 {

     Person person = new Person();  // unreferenced object

     ObjectLifeCycle2() {

          new Person();



3.2.2.    Object is Accessible

Once an object is created, it remains accessible until it goes out of scope or its reference variable is explicitly set to null. Also if a reference variable gets reassigned to another object, the previous object becomes inaccessible.

class ObjectLife1 {

     public static void main(String args[]) {

          Exam myExam = new Exam();     //Object created

          myExam.setName("OCA Java Programmer 1");

          myExam = null;      //reference set to null

          myExam = new Exam();       //another object creation




After the reference variable gets assigned to null in the previous example, the first object is considered garbage by Java.

3.2.3.    Object is Inaccessible

An object can become inaccessible if it can no longer be referenced by any variable, if it goes out of scope, if an object’s reference variable is assigned an explicit null value or if it is reinitialized.

When an object can no longer be accessed it is marked as eligible to be garbage collected. A user cannot control or determine the execution of a garbage collector as it is controlled by the Java Virtual Machine. It can be never be determined when a particular object will be garbage collected.

3.3.  Create methods with arguments and return values

A method is a group of statements identified with a name. Methods are used to define the behavior of an object. A method can perform different functions:

3.3.1.    Return type of a method

A method may or may not return a value, a void method does not return a value, a method can return a primitive value or an object of any class. The return type can be any of the eight primitive types defined in Java, the name of any class or an interface.

A void method result cannot be assigned to a variable, in that case the code would not compile. The assigned variable must also be compatible with the returned value.

3.3.2.    Method Parameters

Method parameters are the variables that appear in the definition of a method and specify the type and number of values that a method can accept. No limit exists on the number of parameters that can be put within a method, but it is not a good practice to use more than five or six. A parameter that can accept variable arguments can be defined with ellipsis (…). The ellipsis indicates that the method parameter may be passed as an array or multiple comma-separated values. It can only be done once per method and must be the last variable in the parameter list:

Public int daysOff(int… days){

     Int daysOff = 0;

for(int i = 0; i < days.length; i++ )

          daysOff += days[i];

     return daysOff;



3.3.3.    Return Statement

This statement is used to exit from a method, with or without a value. If a method returns a value, the return statement must be followed by a return value. Methods that do not return a value (void) are not required to define a return statement. The return statement must be the last statement to execute in a method, the compiler will fail to compile if there’s code after it:


void setNumber(double val){


     val = 3;    //return must be the last statement to execute



void setNumber2(double val){

     if(val < 0)

          return;   //return is the last statement to be executed




In this scenario, the return statement is not the last statement, but it will be the last statement to execute when the parameter val is less than zero.

3.4.  Create an overloaded method

Overloaded methods are methods with the same name, defined In the same class but with different argument lists.

For example the System.out.println() accepts different types of parameters:

int intVal = 10;

boolean boolVal = false;

String name = "eJava";

System.out.println(intVal);   //prints an integer

System.out.println(boolVal);  //prints a boolean

System.out.println(name); // prints a string


3.4.1.    Argument list

The argument lists of an overloaded method can differ in terms of any of these options:

  • Change in the number of parameters that are accepted
  • Change in the types of parameters that are accepted
  • Change in the positions of the parameters that are accepted (based on parameter type, not variable names)


double calcAverage(int marks1, double marks2) {

     return (marks1 + marks2)/2.0;


double calcAverage(int marks1, int marks2, int marks3) {

     return (marks1 + marks2 + marks3)/3.0;



In the previous example the methods differ in the number of parameters.


class MyClass {

      double calcAverage(double marks1, int marks2) {

            return (marks1 + marks2)/2.0;


     double calcAverage(int marks1, double marks2) {

            return (marks1 + marks2)/2.0;


      public static void main(String args[]) {

            MyClass myClass = new MyClass();

            myClass.calcAverage(2, 3);    // compiler can’t determine method to use



Because int can be passed to a variable type of double, the values 2 and 3 can be passed to both methods, in this scenario the compilation fails.

3.4.2.    Return type

Methods can’t be defined as overloaded if they only differ in their return types.

3.4.3.    Access modifier

Methods can’t be defined as overloaded if they only differ in their access modifiers.

3.5. Constructors of a class

Constructors are methods that create and return an object of the class in which they are defined. They have the same name of the class where they are defined and they don’t specify a return type, not even void.

3.5.1.    User defined constructors

If the author of the class defines a constructor it is known as user defined constructor. It can be used to assign default values to the variables of the class. If a return type is specified, Java will treat it as another method and not as a constructor.

class Employee {

     void Employee() { //return type specified, not a constructor

           System.out.println("not a Constructor ");



class Office {

     public static void main(String args[]) {

           Employee emp = new Employee();

          emp.Employee();// calling the Employee method with void return type

} Initializer block

It is a block defined in a class that is not a part of any method. It gets executed for every object that is created for a class.

class example {


           System.out.println("Initializer Block");




3.5.2.    Default constructor

In the absence of a user defined constructor, Java inserts a default constructor; it doesn’t accept method arguments and assigns default values to all the instance variables.

3.5.3.    Overloaded constructors

Overloaded constructors can be defined in the same way as overloaded methods. Overloaded constructors can be called within other constructors by using the keyword this.


class Employee {

     String name;

     int age;

     Employee() {   //no arg constructor

         this(null, 0); //invoking the other constructor


     Employee(String newName, int newAge) {   //constructor with 2 arguments

           name = newName;

           age = newAge;



Constructors cannot be called from any other method that is not a constructor of the class.


3.6.  Accessing object fields


3.6.1.    What is an object field?

It is another name for an instance variable defined in a class.

3.6.2.    Read and write object fields

To access an object field of a class you can use the set method or use the variable name. = "Selvan";//as long as access modifier of the variable name is not private



3.6.3.    Calling methods on objects

Java uses the dot notation to execute a method on a reference variable. When calling a method, the exact number of parameters must be passed on it, literal values and variables are also acceptable when invoking a method.

Employee e1 = new Employee();

String anotherVal = "Harry";



Values returned from a method are also accepted.



3.7. Apply encapsulation principles to a class


3.7.1.    Need for encapsulation

Sometimes when a class is defined, some parts of it must be hidden to other classes to prevent unexpected behavior or security problems.

3.7.2.    Apply encapsulation

Encapsulation is the concept of defining variables and methods together in a class. The private members of a class are used to hide the internal information to other classes.

class example {

     private double num1;



3.8.  Passing objects and primitives to methods


3.8.1.    Passing primitives to methods

It is okay to define a method parameter with the same name as an instance variable or object field. Within a method, a method parameter takes precedence over an object field.

class Employee {

     int age;

     void modifyVal(int age) {

           age= age+ 1;




In the previous example, in order to reference the instance variable age, the keyword this must be used before.

3.8.2.    Passing object references to methods

There are two main cases:

  • When a method reassigns the object reference passed to it to another variable
  • When a method modifies the state of the object reference passed to it reassign the object references passed to them


In the following example, since the object references are passed to other variables, the state of the objects remain intact.

class Person {

     private String name;

     Person(String newName) {

          name = newName;


     public String getName() {

          return name;


     public void setName(String val) {

          name = val;



class Test {

     public static void swap(Person p1, Person p2) {

          Person temp = p1;

          p1 = p2;

          p2 = temp;


     public static void main(String args[]) {

          Person person1 = new Person("John");

          Person person2 = new Person("Paul");


          + ":" + person2.getName());      //prints: John : Paul

          swap(person1, person2);


          + ":" + person2.getName());   //prints: John : Paul


} Methods modify the state of the object references passed to them

In the following example, the method modifies the value of both objects person1 and p1:

class Test {

     public static void resetValueOfMemberVariable(Person p1) {



     public static void main(String args[]) {

          Person person1 = new Person("John");

          System.out.println(person1.getName());   //prints John

          resetValueOfMemberVariable(person1);  //changes the name

          System.out.println(person1.getName());  //Prints Rodrigue



This entry is the third part of of a series on Java Basics, for further reading:
Java Basics I
Java Basics II


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Java Basics II

2.     Working With Java data types

2.1. Primitive Variables

Simplest data type in a programming language
. In Java there are eight primitive data types:

  • char
  • boolean
  • byte
  • short
  • int
  • long
  • float
  • double

2.1.1.    Boolean

Boolean variables can only store one of two values: true or false.

boolean answer =  false;

In this case, the variable answer is assigned a fixed value with a literal (false). A literal is a fixed value that doesn’t need calculations to be assigned to any variable. true and false are the boolean literals.

2.1.2.    Numeric

There are two subcategories for numeric variables: integers and decimals.           Integers

When the value can be counted in whole numbers, it is an integer. They can be stored in different data types:

  • byte
  • int
  • short
  • long

The difference between those types is the range of values they can support:

  • byte –128 to 127, inclusive
  • short –32,768 to 32,767, inclusive
  • int –2,147,483,648 to 2,147,483,647, inclusive
  • long –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807, inclusive

For designation of an integer literal value as a long value, the suffix L or l must be added to the literal:

long bigNumber = 584677658897L;

Integer variables can be defined as binary, decimal, octal or hexadecimal numbers. In order to specify the number system, a prefix must be used:

  • decimal use no prefix: int value = 267;
  • octal use the prefix 0: int octVal = 0413;
  • hexadecimal use the prefix 0x or 0X: int hexVal = 0x10B;
  • binary use the prefix 0b or 0B : int binVal = 0b100001011;

Java version 7 allows the use of underscores to help group individual digits or letters of literal values:

long baseDecimal = 100_267_760;

long octVal = 04_13;

long hexVal = 0x10_BA_75;

long binVal = 0b1_0000_10_11;

Underscores cannot be placed at the start or the end of a literal value, cannot be placed right after prefixes for binary and hexadecimal values (0b, 0B, 0x and 0X) but it can be placed right after the prefix for octal values (0). Underscores cannot be placed prior to a long suffix (l or L), finally, they cannot be placed in positions where a string of digits is expected

int i = Integer.parseInt(“3_14”);  //invalid use of underscore           Float and Double

When expecting decimal numbers in Java float and double primitive data types can be used. Float has smaller range than double and requires less space. Capture To initialize a decimal literal value as a float value, the suffix F or f must be used because the default type of a decimal literal in java is double.

float average = 17.925F;

After Java version 7, underscores can be used with floating-point literal values following these rules:

  • You can’t place an underscore prior to a D, d, F, or f suffix (these suffixes are used to mark a floating-point literal as double or float).
  • You can’t place an underscore adjacent to a decimal point.

2.1.3.    Character

A char can store a single character from all the existing scripts and languages. To assign a value to a char variable, single quotes must be used instead of double quotes, otherwise the code will fail to compile.

char v1 = ‘v’;

Java stores char data as a positive integer, therefore it is acceptable to assign an integer to a char.

char a1 = 97;   // a assigned to a1           Casting

Casting is used to forcefully convert data to another data type and must be used between compatible data types, when casting a bigger value to a smaller data type the compiler chops the extra bits that does not fit into the smaller variable. Negative integer numbers can be casted into a char as follows:

char a2 = (char) -97;  //successfully compiled

2.2.  Identifiers

Identifiers are names of packages, classes, interfaces, methods, and variables.

2.2.1.    Valid and Invalid Identifiers

Rules to define valid and invalid identifiers: Identifiers         Java reserved words and keywords:

Words that cannot be used as an identifier name: reserved_words

2.3.  Object Reference Variables

An Object Reference is a memory address that points to a memory area where an object’s data is located.

Class1 refVal = new Class1(); //refVal is the name of the object reference variable

In the previous line, a reference variable refVal of type Class1 is created and an object is assigned to it. The literal value of reference variables is null, it can also be assigned explicitly:

Class1 refVal = null;

2.3.1.    Object reference variables and primitive variables

The main difference between these variables is that primitive variables store the values and reference variables store the memory address of the object they refer to.

2.4.  Operators

Some of the most important Java operators: Java_Operators

2.4.1.    Assignment Operators

The most frequently used operator is the = and it is used to initialize variables with values and to reassign new values to them. The +=, -=, *=, and /=operators are short forms of addition, subtraction, multiplication and division with assignment and work as follows: a -= b is equal to a = a – b a += b is equal to a = a + b a *= b is equal to a = a * b a /= b is equal to a = a / b Multiple values can be assigned in the same line of code as follows:

int a = 7, b = 10, c = 8;

a = b = c; // here a = 8 because the assignment starts from right to left.

2.4.2.    Arithmetic Operators

Arithmetic operators in Java: Arithmetic_Operators           ++ And – (Unary increment and decrement operators)

Used to increment or decrement the value of a single variable by 1. Can be used in prefix or postfix notation:

int a = 10;

++a;   // a = 11

int b = 10;

b++;  //b = 11

When a unary operator is used in a expression, its placement with respect to its operand decides whether its value will increment or decrement before the evaluation of the expression or after the evaluation of the expression.

int a = 20;

int b = 10;

int c = a - ++b;

System.out.println(c);    // c = 9

System.out.println(b);   // b = 11


int a = 20;

int b = 10;

int c = a - b++;

System.out.println(c);  // c = 10

System.out.println(b);  // b = 11

int a = 10;

a = a++ + a + a-- - a-- + ++a;

System.out.println(a); //a = 32(a = 10 + 11 + 11 - 10 + 10)evaluates from left to right

2.4.3.    Relational Operators

Relational operators are used to check a condition. The result of the relational operator is always a Boolean value. Usually subdivided in two categories:

  • Comparing greater (>, >=) and lesser values (<, <=)
  • Comparing values for equality (==) and nonequality (!=)

The operators (>, >=) and (<, <=) work with all types of numbers, char and floating point variables that can be added and subtracted.

int a = 10;

int b = 15;

System.out.println(a >=  b);  //prints false

The operators (==) and (!=) can be used to compare all types of primitives: char, byte, short, int, long, float, double, and boolean. != returns true if the values compared are not equal and false otherwise. For the same set of values if == returns true, != will return false. These operators cannot be used to compare incomparable types, for example comparing an int with a boolean will fail to compile.

2.4.4.    Logical Operators

These Operators are used to evaluate one or more expressions and should return a boolean value.

  • And (&&) operator in Java returns true when both conditions are true.
  • Or (||) operator in Java returns true when at least one condition is true.
  • Not (!) operator reverses the outcome of a Boolean value.
int x = 1;

int y = 5;

System.out.println(x == y); //prints false

System.out.println(x != y); //prints true

boolean b1 = false;

System.out.println(b1 == true);  //prints false

System.out.println(b1 != true);  //prints true

System.out.println(b1 == false); //prints true

System.out.println(b1 != false); //prints false

2.4.5.    Operator Precedence

The next list illustrates the precedence of operators in Java, the operators on top have the highest precedence and operators within the same group are evaluated from left to right. Operator Precedence

This entry is the second part of  of a series on Java Basics, for further reading:
Java Basics I
Java Basics III

Java Basics

1.1   The structure of a Java class and source code file

1.1.1       Structure of a Java class

A class can define multiple components for example:

  • Package statement
  • Import statement
  • Comments
  • Class declarations and definitions
  • Variables
  • Methods
  • Constructors       Package Statement

All Java classes are part of a package; if a class is not defined in a named package it becomes part of a default package, which doesn´t have a name. If a class includes a package statement, it must be the first statement in the class definition (it cannot appear within a class declaration or after the class declaration) and if present, it must appear exactly once in a class:

package certification;
//should be the first statement in a class

class Course{
} Import Statement

Classes and interfaces from the same package can use each other without prefixing the package name. But to use a class or interface from another package, the import statement can be used:

package University;
import certification.ExamQuestion;
// import must be placed after the package

class AnnualExam{
     examQuestion eq;

The import statement follows the package but precedes the class declaration. Comments

The comments in Java code can be placed at multiple places in a class. To place multiline comments, start with /* and end with */ . End-of-line comments start with // and are placed at the end of a line of code. Comments can be placed before the package statement.

* @author JRamirez // first name initial + last name End-of-line within a multiline
* @version 0.0.2
* Class to store the details of a monument
package uni; // package uni End-of-line comment

class Monument {
     int startYear;
} Class Declaration

Components of a class declaration:

  • Access modifiers
  • Nonaccess modifiers
  • Class name
  • Name of the base class, if the class is extending another class
  • All implemented interfaces, if the class is implementing any interfaces
  • Class body (class fields, methods, constructors), included within a pair of curly

braces, {}


public final class Runner extends Person implements Athlete {} Compulsory and optional elements of a class:


  • Keyword class
  • Name of the class
  • Class body, marked by the opening and closing curly braces, {}


  • Keyword class Access modifier, such as public
  • Nonaccess modifier, such as final
  • Keyword extends together with the name of the base class
  • Keyword implements together with the name of the interfaces being implemented Class Definition

The use of a class is to specify the behavior and properties of an object using methods and variables. It is a design from which an object can be created.

1.1.2       Structure and components of a Java source code file

A  Java  source code  file  is  used  to  define  classes  and  interfaces.  All your Java code should be defined in Java source code files (text files whose names end with .java)   Definition of interfaces in a Java source code file

An interface is a grouping of related methods and constants, but the methods in an

interface cannot define any implementation.

interface Controls {
     void changeChannel(int channelNumber);
     void increaseVolume();
     void decreaseVolume();
} Definition of single and multiple classes in a single java source code file

A single class or interface can be defined in a single Java source code or multiple classes and interfaces can be within the same source code. The classes and interfaces can be defined in any order of occurrence in a Java source code file. If a public class or interface is defined, its name should match the name of the Java source code file.  Application of package and import statements in Java source code files

When an import or package statement is used within a java source code file, it applies to all classes and interfaces defined in that code.

1.2 Executable Java applications

1.2.1    Executable Java classes versus nonexecutable Java clases

An executable Java class is a class which, when handed over to the Java Virtual Machine, starts its execution at a particular point in the class in the main method. A nonexecutable class doesn’t have it. The programmer designates an executable class from all of the files of an application.

1.2.2  Main method

The first requirement in creating an executable Java application is to create a class with  a  method  whose  signature  (name  and  method  arguments)  match  the main method.

public class HelloExam {
         public static void main(String args[]) {
         System.out.println("Hello exam");

This main method should comply with the following rules:

  • The method must be marked as a public method.
  • The method must be marked as a static method.
  • The name of the method must be main.
  • The return type of this method must be void.
  • The method must accept a method argument of a String array or a variable argument of type String

The keywords public and static can be interchanged:

public static void main(String[] args)

static public void main(String[] args)

1.3       Java Packages

1.3.1       The need for packages

Packages are used to group classes and interfaces, they also provide protection and namespace management. Subpackages can also be created within the packages.

1.3.2       Defining classes in a package using the package statement

//The first statement in a class or interface must be the package statement:

package certification;

class ExamQuestion {



Rules about packages:

  • Per Java naming conventions, package names should all be in lowercase.
  • The package and subpackage names are separated using a dot (.).
  • Package names follow the rules defined for valid identifiers in Java.
  • For packaged classes and interfaces, the package statement is the first statement in a Java source file (a .java file). The exception is that comments can appear before or after a package statement.
  • There can be a maximum of one package statement per Java source code file (.java file).
  • All the classes and interfaces defined in a Java source code file will be defined in the same package. There is no way to package classes and interfaces defined within the same Java source code file in different packages.       Directory Structure and Package Hierarchy

The hierarchy of the packaged classes and interfaces should match the hierarchy of the directories in which those classes are defined.

1.3.3       Using simple names with import statements

For using classes and interfaces in other classes of your code, there are two options, using the fully qualified name or using the import statement with a simple name of the class or package.


package office;

 class Cubicle {

     home.LivingRoom livingRoom;


package office;

import home.LivingRoom;

class Cubicle {

     LivingRoom livingRoom;


1.3.4       Using packaged classes without using the import statement

By using its fully qualified name, a class or interface can be used without an import statement.

class AnnualExam {

     certification.ExamQuestion eq;


This is often used when there are multiple classes and interfaces with the same name, because the import statement cannot be used in that case.

class AnnualExam {

     java.util.Date date1;

     java.sql.Date date2;


1.3.5       Importing a single or all members of a package

By using an asterisk, all the public classes, members and interfaces of a package can be imported at once.

import certification.*;

 class AnnualExam {

     ExamQuestion eq;

     MultipleChoice mc;


Importing a class in Java doesn’t add to the size of the file.

1.3.6       Importing Recursively

By using an asterisk, classes from a subpackage are not imported, only classes from the main package.

1.3.7       Default Package Import

If no explicit package is defined for classes or interfaces, they are imported in the default package automatically in all the classes and interfaces in the same directory.

class Person {

     // code


class Office {

     Person p;


A class from a default package can’t be used in any named packaged class.

1.3.8 Static Imports

To import an individual static member of a class or all its static members, the import static statement must be used.

package certification;

public class ExamQuestion {

     static public int marks;

     public static void print() {



package university;

import static certification.ExamQuestion.marks;

class AnnualExam {

     AnnualExam() {

           marks = 20;



//Importing all of the static members:

package university;

import static certification.ExamQuestion.*;

class AnnualExam {

     AnnualExam() {

          marks = 20;




1.4       Java Access Modifiers

1.4.1       Access Modifiers

Access modifiers control the accessibility of a class or interface and its members, by other classes and interfaces.

They can be applied to classes, interfaces, and their members (instance and class variables and methods). Local variables and method parameters can’t be defined using access modifiers.

Java defines four access modifiers:

  • public(least restrictive)
  • protected
  • default
  • private(most restrictive)

1.4.3       Public Access Modifier

Classes and interfaces defined using the public access  modifier  are  accessible  across  all  packages,  from  derived  to  unrelated classes.

1.4.4       Protected Access Modifier

Classes and interfaces defined using the protected access modifier are accessible to classes and interfaces in the same package and all derived classes even in separate packages. They cannot be accessed by unrelated classes in other packages.

1.4.5       Default Access (package access)

Classes and interfaces defined without any explicit access modifier are defined with package accessibility (default accessibility). They can only be accessed by classes and interfaces defined in the same package.

1.4.6       Private Access Modifier

The members of a class defined using the private access modifier are accessible only to themselves. Private members are not accessible outside the class they are defined.

1.5       Nonaccess Modifiers

1.5.1       Abstract Modifier

When added to the definition of a class, interface, or method, the abstract modifier changes  its  default  behavior.       Abstract Class

When  the abstract keyword  is  prefixed  to  the  definition  of  a  concrete  class,  it changes it to an abstract class. An abstract class can’t be instantiated. An abstract class can be defined without any abstract methods but a concrete class cannot define an abstract method.       Abstract Interface

An interface is an abstract entity by default. The Java compiler automatically adds the keyword abstract to the definition of an interface.       Abstract Method

An abstract method doesn’t have  a  body.  Usually, an abstract method is implemented by a derived class.       Abstract Variables

No type of variable can be defined as abstract.

1.5.2       Final Modifier

The keyword final changes the default behavior of a class, variable, or method.       Final Class

A class defined final cannot be extended by other classes.       Final Interface

No interface can be marked as final.       Final Variable

A final variable can only be assigned a value once.       Final Method

A final method defined in a base class cannot be overridden in a derived class.

1.5.3       Static Modifier

Can be applied to the definitions of variables, methods,  classes,  and  interfaces.       Static Variables

They are common to all instances of a class and are not unique. They are shared by all the objects of the class. Static variables may be accessed even when no instances of a class have been created.       Static Methods

Static methods aren’t associated with objects and can’t use any of the instance variables of a class. They can be used to use or manipulate static variables of a class.

Nonprivate static variables and methods can be inherited by derived classes and can be redefined within the derived class.       What can a static method access?

Non-static variables and methods can access static variables and methods. Static methods and variables cannot access the instance methods of a class.




This entry is the first part of of a series on Java Basics, for further reading:
Java Basics II
Java Basics III

Java 8 Streams and JPA

Since the definition of the JPA Standard predates the release of Java 8 it is not surprising that the JPA API is only based on collections. There is no way to obtain a Java 8 Stream out of a query object.

For those out there using Hibernate as their JPA provider there is an interesting way to create a stream for your JPQL queries.

Hibernate Scrollable Results

Hibernate supports many more features than those offered by the JPA Standard only. Hibernate’s Query class allows us to obtain an iterator-like object for a JQPL query. This iterator-like class is named ScrollableResults.

You can see a great example of this feature in this blog post about Reading Large Result Sets with Hibernate and MySQL. I have taken the liberty of copying one of their examples:

Query query = session.createQuery(query);
ScrollableResults results = query.scroll(ScrollMode.FORWARD_ONLY);
while ( {
   Object row = results.get();
   // process the entity here

Creating an Iterator Wrapper around Scrollable Results

Let’s take this a bit further now by implementing a Java Iterator wrapper around the Hibernate’s scrollable results object as shown below:

class ScrollableResultIterator<T> implements Iterator<T> {
   private final ScrollableResults results;
   private final Class<T> type;

   ScrollableResultIterator(ScrollableResults results, Class<T> type) {
      this.results = results;
      this.type = type;

   public boolean hasNext() {

   public T next() {
      return type.cast(results.get(0));

Gaining Access to Hibernate Session through JPA Entity Manager

As you can see the examples above use a Hibernate session object to obtain an instance of a Hibernate query object. Supposing we are using JPA, we typically have access to an EntityManager, but not to any particular implementation objects of the underlaying provider.

To gain access to the Hibernate’s Session object we can use a special method in the entity manger.

Session session = entityManager.unwrap(Session.class);

Be warned that at this point we are escaping from the safety of vendor agnostic code. In other words, if we ever wanted to use another provider like OpenJPA or EclipseLink, we would be forced to find a different alternative to implement our code here since those other providers won’t offer anything like Hibernate’s Session and ScrollableResults.

From Iterators to Spliterators to Streams

Our next step consists in obtaining a Java 8 stream out of this iterator. Lukas Eder recently wrote an interesting article about the difficulties to obtain a stream out of an iterable object which will be pretty handy here.

To achieve our goal we now need to use two utility classes in Java 8 named StreamSupport and Spliterators.

For instance, we could take an iterator, like the one we defined above and transform it into a Spliterator by doing:

private Spliterator<T> toSplitIterator(ScrollableResults scroll, Class<T> type){
   return Spliterators.spliteratorUnknownSize(
      new ScrollableResultIterator<>(scroll, type),
         Spliterator.DISTINCT | Spliterator.NONNULL | 
         Spliterator.CONCURRENT | Spliterator.IMMUTABLE

This Splititerator is an intermediate product that we can now use to create a Java 8 stream out of it:, false);

At this point we have built enough to create our first stream out of a JPQL query. We can do it as follows:

public Stream<T> getResultStream(
      String sql, 
      Integer fetchSize, 
      Map<String,Object> parameters) 
   Query query = session.createQuery(sql);
   if (fetchSize != null) {
   for (Map.Entry<String, Object> parameter : parameters.entrySet()) {
      query.setParameter(parameter.getKey(), parameter.getValue());
   ScrollableResults scroll = query.scroll(ScrollMode.FORWARD_ONLY);
   return, type), false)

A JPA Stream API

We can do a little bit better by defining the basics of the type of query object we are currently missing in JPA Standard API:

public interface StreamQuery<T> {
   Stream<T> getResultStream();
   StreamQuery<T> setParameter(String name, Object value);
   StreamQuery<T> setFetchSize(int fetchSize);

And putting together all we have learned so far we could create this implementation of our new StreamQuery interface:

public class HibernateQueryStream<T> implements StreamQuery<T> {

   private final Session session;
   private final String sql;
   private final Class<T> type;
   private final Map<String, Object> parameters = new HashMap<>();
   private Integer fetchSize;

   public HibernateQueryStream(
      EntityManager entityManager, 
      String sql, 
      Class<T> type) 
     this.session = entityManager.unwrap(Session.class);
      this.sql = sql;
      this.type = type;

   public StreamQuery<T> setParameter(String name, Object value) {
      parameters.put(name, value);
      return this;
   public StreamQuery<T> setFetchSize(int fetchSize) {
      this.fetchSize = fetchSize;
      return this;

   public Stream<T> getResultStream() {
      Query query = session.createQuery(sql);
      if (fetchSize != null) {
      for (Map.Entry<String, Object> parameter : parameters.entrySet()) {
         query.setParameter(parameter.getKey(), parameter.getValue());
      ScrollableResults scroll = query.scroll(ScrollMode.FORWARD_ONLY);
      return, type), false)
   private Spliterator<T> toSplitIterator(ScrollableResults scroll, Class<T> type){
      return Spliterators.spliteratorUnknownSize(
         new ScrollableResultIterator<>(scroll, type),
            Spliterator.DISTINCT | Spliterator.NONNULL | 
            Spliterator.CONCURRENT | Spliterator.IMMUTABLE

   private static class ScrollableResultIterator<T> implements Iterator<T> {

      private final ScrollableResults results;
      private final Class<T> type;
      ScrollableResultIterator(ScrollableResults results, Class<T> type) {
         this.results = results;
         this.type = type;
      public boolean hasNext() {
      public T next() {
         return type.cast(results.get(0));

The Repository Layer

On top of this component we can now build our repository layer.

public class StreamRepository {

   private EntityManager entityManager;

   public Stream<Order> getOrderHistory(String email) {
      String jpql = "SELECT o FROM Order o WHERE";
      StreamQuery<Order> query = new HibernateQueryStream<>(
      return query.getResultStream();

And from here the rest is a piece of cake:

   .filter(order -> > 100)
   .reduce(0, (x,y) -> x + y);

It is important to highlight that the Hibernate session must be alive by the time we actually consume the stream, because it is at that point that the entities will be mapped. So, make sure that wherever you consume the stream the Hibernate session or you entity manager context is still alive.

Further Reading

Functional Programming with Java 8 Functions

Edwin Dalorzo I have finally had the opportunity to work in a couple of projects being entirely developed with Java 8 and during the past months I’ve experienced, first hand, many of the new functional programming features in the language. So I decided to write a series of articles about some of these things I’ve being learning and how some of the well known functional programming constructs can be translated into Java 8.


Ok, let’s start with something simple. The following is a lambda expression (i.e. an anonymous function) that takes an argument x and increments it by one. In other words a function that receives, apparently an integer, and returns a new incremented integer:

x -> x + 1

And what is the type of this function in Java?

Well, the answer is that it depends. In Java the same lambda expression could be bound to variables of different types. For instance, the following two are valid declarations in Java:

Function<Integer,Integer> add1 = x -> x + 1;
Function<String,String> concat1 = x -> x + 1;

The first one increments an integer x by one, whereas the second one concatenates the integer 1 to any string x.

And how can we invoke these functions?

Well, now that they are bound to a reference we can treat them pretty much like we treat any object:

Integer two = add1.apply(1); //yields 2
String answer = concat1.apply("0 + 1 = "); //yields "0 + 1 = 1"

So, as you can see every function has a method apply that we use to invoke it and pass it an argument.

And what if I already have a method that does that, can I use it as a function?

Yes, since an alternative way to create functions is by using methods we had already defined and that are compatible with our function definition.

Suppose that we have the following class definition with methods as defined below:

public class Utils {
   public static Integer add1(Integer x) { return x + 1; }
   public static String concat1(String x) { return x + 1; }

As you can see the methods in this class are compatible with our original function definitions, so we could use them as “method references” to create the same functions we did before with lambda expressions.

Function<Integer,Integer> add1 = Utils::add1;
Function<String,String> concat1 = Utils::concat1;

These two are just the same thing as the ones we did before.

High Order Programming

The cool thing about functions is that they encapsulate behavior, and now we can take a piece of code, put it into a function and pass it around to other functions or methods for them to use it. This type of functions that operate on (or produce new) functions are typically called high order functions and the programming style based on exploiting this powerful feature is called, unsurprisingly, high order programming.

About Functions that Create Functions

Let’s see a couple of examples of how we can do this using function objects. Let’s consider the following example:

Function<Integer, Function<Integer,Integer>> makeAdder = x -> y -> x + y;

Above we have a function called makeAdder that takes an integer x and creates a new function that takes an integer y and when invoked adds x to y. We can tell this is a high order function because it produces a new function.

Now, we use this to create our original add1 function:

Function<Integer,Integer> add1 = makeAdder.apply(1);
Function<Integer,Integer> add2 = makeAdder.apply(2);
Function<Integer,Integer> add3 = makeAdder.apply(3);

With our new high order function, however, we could also create add2, add3, ..., addn, right?

Can’t we define this in a simpler way as we did before with the Utils class methods?

Yes, we can. Consider the following addition to the Utils class:

public class Utils {
    public static Function<Intger, Integer> adder(Integer x) {
       return y -> x + y;

This signature is a bit simpler to read than that in our lambda expression, but as you can see it is pretty much the same thing. The function continues to receive the same number and type of arguments and continues to return the same type of result.

We can now use this simpler function factory to create our makeAdder and add1 functions again:

Function<Integer, Function<Integer,Integer>> makeAdder = Utils::adder;
Function<Integer,Integer> add1 = makeAdder.apply(1);

And there you have it, this is exactly the same thing as before.

About Functions that Receive Functions as Arguments

Let’s suppose we had the following two functions defined:

Function<Integer,Integer> add1 = x -> x + 1;
Function<Integer,Integer> mul3 = x -> x * 3;

Now, naively, we could invoke this two functions together to increment and multiply a number by 3, right?. Like this:

Integer x = 10;
Integer res = mul3.apply(add1.apply(x)); //yields 33

But what if we created a function that did both things instead?

Consider the following pseudocode:

(f,g) -> x -> g( f(x) )

This would be a function that takes two other unary functions and creates yet another function that applies the original two in certain order. This is a classical example of what is called function composition.

In some languages there is even an operator to compose two functions in this way:

h = f o g

Where o would be a binary operator that would compose functions f and g pretty much as we did in pseudocode above and produce a new function h.

How can we do function composition in Java?

I can think of two ways to do this in Java, one more difficult than the other. Let’s start with the more difficult first, because that will let us appreciate the value of the simpler solution later on.

Function Composition Strategy 1

First, we must start by realizing that the function in pseudocode above is a binary function (i.e. a function that receives two arguments). But all our examples so far have dealt only with unary functions.

It would seem this is not important, but in Java it is, since functions of different arities have different target functional interfaces. In Java, a function that receives two arguments is called BinaryOperator.

For instance, using a BinaryOperator we could implement a sum operator:

BinaryOperator<Integer> sum = (a,b) -> a + b;
Integer res = sum.apply(1,2); // yields 3

Well, just as easily we could implement the compose operator, right? Only that in this case, instead of two simple integers, we receive two unary functions:

BinaryOperator<Function<Integer,Integer>> compose;
compose = (f,g) -> x -> g.apply(f.apply(x));

Now we can easily use this to fabricate a compound function that adds 1 and multiplies by 3, one after the other.

Function<Integer,Integer> h = compose.apply(add1,mul3);
Integer res = h.apply(10); //yields 33	

And now we can beautifully, and in really simple way, combine two unary integer functions.

Function Composition Strategy 2

Now, function composition is something so common that it would have been a mistake if the Java Expert Group would have not considered it in their API design, and so, to make our lives simpler, all Function objects have a method called compose that allows us to very easily compose two functions together.

The following code produces the exact same result as above:

Function<Integer,Integer> h = mul3.compose(add1);
Integer res = h.apply(10);	

Partial Function Application or Currying

In most functional programming languages it is possible to create partially applied functions. That is, if a function is receiving multiple arguments, we can partially invoke the function providing just a few arguments and receive a partially applied function out of it. This is typically called currying.

Although you have not noticed it, we have already covered that in this article, but now we are going to make it much more evident 🙂

So, consider the following pseudo code

sum = x -> y -> x + y

Then we say that sum is a function that accepts one parameter x and fabricates another anonymous function that in turn accepts one parameter y that, when invoked, sums x and y.

In many functional programming languages a construct like this can be invoked as if this was just one simple function by doing:

sum 10 5 //yields 15

But the truth is that this is just syntactic sugar to do:

sum(10)(5) //yields 15

Since sum is a function that returns a function.

The beauty of this idiom is that now we could partially apply sum:

plus10 = sum 10

And now plus10 is a partially applied function of sum. Can you see now where we had already talked about this?

plus10(5) //yields 15

Can we do this with Java?

The truth is that we have already done it above, we just probably did not notice. Unfortunately, in Java we do not have the syntactic sugar that some other language have, and therefore this is a bit more verbose:

Function<Integer,Function<Integer,Integer>> sum = x -> y -> x + y;

Well, you can see sum is declared in a “currified” way. And now we can partially apply it:

Function<Integer, Integer> plus10 = sum.apply(10);
Integer res = plus10.apply(5); //yields 15

Unary, Binary, Ternary and n-ary Functions

So, as mentioned above, Java uses different functional interfaces for different function arities. And so Function<T,R> is a functional interface for any unary function where the domain and the codomain of the function may be of different types.

For instance, we could define a function that receives a string value and parses it as an integer:

Function<String,Integer> atoi = s -> Integer.valueOf(s);

But most of our examples above are for integer functions whose argument and return value are of this same type. For those cases we could alternatively use the UnaryOperator<T> instead. This is just a Function<T,T>.

Thus, some our declarations above could be slightly simplified with this functional interface:

UnaryOperator<Integer> add1 = n -> n + 1;
UnaryOperator<String> concat1 = s -> s + 1;
Function<Integer, UnaryOperator<Integer>> sum = x -> y -> x + y;
UnaryOperator<Integer> sum10 = sum.apply(10);

I have already written another article that explains Why Java 8 has Interface Pollution like this in case you are interested in an explanation.

Value Types and Primitive Type Functions?

Evidently using a type like Integer incurs into the costs of boxing and unboxing when our functions have to deal with a primitive type like int.

As you know, Java does not support value types as type arguments in generic declarations, so to deal with this problem we can use alternative functional interfaces like ToIntFunction, IntFunction or IntUnaryOperator.

Or we can define our own primitive function.

interface IntFx {
	public int apply(int value);

Then we can do:

IntFx add1 = n -> n + 1;
IntFunction<IntFx> sum = x -> y -> x + y;
IntFx sum10 = sum.apply(10);
sum10.apply(4); //yields 14

Similar functional interfaces can be found in the Java API for types double and long as well. This topic is also covered in the alternative article mentioned above about interface pollution.

And that’s it for the time being. I hope that in a future article I can take this a bit further and show some practical applications derived from my experience in actual projects.

Further Reading

JSApps 101: AngularJS In A Nutshell


Moving on with the topic, in the previous part of this series, we discussed about that interesting new tendency to let the browser do all the user interface heavy-lifting on the client-side, since it is such a good guy. Of course, this involves a LOT of JavaScript going on, since it is the only language the browser understands besides HTML.

We all know how complex and crazy things can get when you involve JavaScript in the picture. The language is usually seen as over-complicated, it used to be hard to debug, learning curve is steep as hell and it can get confusing quickly; also, there used to be a lack of tools and IDEs with strong support for its development just sums up for bad reputation.

However, fear not! After this article, you will have a solid base on how you can turn the fiendish JavaScript you have known for years from a simple DOM traversal helper to a whole reliable application development framework with the aid of AngularJS; man, now THAT’S a catchy name.

What In The World Is AngularJS?

AngularJS is a JavaScript library that allows you to implement the Model-view-controller pattern (in some sort of way) at a client-side level (dude, the browser). Why is this? Because you have two options in life:

  • Option A    You implement a +1000 lines JavaScript source file with a bunch of code that not only is in charge of handling manipulation of the HTML structure and creation of UI components, but also handles all of the data validation and display logic of it, all of this while your fellow teammates start hating the guts out of you.
  • Option B    You be a good developer, who cares about separation of concerns, and split all those tasks in several components where each is in charge of a single thing; those written in separate files for your code maintainability’s sake, while making a lot of friends in the process.

In the good old days, option A was “kind” of affordable (heavy quotations), since JavaScript was used for simple things like adding and removing elements from the HTML structure, changing colors and such. But when we talk about implementing a client application only using JavaScript, option A starts struggling. Mainly because we are not only moving HTML elements around, animating stuff and other user interface tasks; but also performing data validation, error handling and server communications in the client-side.

Of course, option B is the way to go. Since there is a ton of things going on in the browser, we need a way to create components that handle each task separately. This is known as the Separation of Concerns principle, or SOC; which states that your code has to be split in several parts, handling a single task each, orchestrated in order to achieve a common good. And this is where AngularJS shines. It allows you to:

  • Organize your client-side code in a Model-view-controller fashion, so you have separate components where each handles a different task in the logic stack: from user input to server data posting (Don’t worry, we’ll get on this later on.)
  • Live template processing and data-binding; more specifically: munch a template, bind it to specific data coming from the server or anywhere else and then produce a valid HTML piece that can be displayed in the browser.
  • Creation of re-usable user interface components.
  • Implementation of advanced patterns, like dependency injection; which is tied to…
  • Ability to implement unit tests for the JavaScript code.

Client-side Model-View-Whatever

We all know Model-View-Controller (MVC from now on) was conceived for a good reason: Roles and responsibilities matters. It was designed as a practical way to separate all of the front-end logic into three interconnected parts, so code having to do with how data is displayed is clearly separated from the code that validates, stores and retrieves that data. Also, it was thought with unit testing in mind. MVC Flow Now, MVC is commonly used server-side; think of ASP.NET MVC or Spring MVC, where you have a class representing the “controller”, which fetches data into “models” and then binds them to a template producing a “view” in the form of an HTML document, which is returned to the browser for display on the user screen. Remember, each time we talk about MVC, we are generally referring to the Presentation Layer of an application.

Now, why go crazy and MVC-fy my JavaScript code? Well, because what’s trendy right now are dynamic web interfaces where pretty much everything is asynchronous, little post-backs are done to the server unless really necessary and you have complex UI components like pop-ups, multi-level grids and others living on the screen (think of Facebook, for example). The only way to do this is delegate the browser (through JavaScript) with the user-interface composition, interaction between all of the components and lastly fetching and posting data back and forth from the server. You really, really NEED a way to achieve law and order so the client-side code does not become an uncontrollable mess.

OK, enough talk already. Let’s dive into how to implement a simple application using AngularJS.

Hello, Angular

Consider the classic “To-do List” example. We want a screen that displays a list of items to be completed; also we need to show a mini-form that allows us to enter and add new items to the list. Here is how it should look: To-do List UI So, in order to get this done, we need to complete some steps:

  • Create the HTML document which will be representing our view.
  • Include the AngularJS library in the HTML document so we can start creating controllers that will handle behavior of the user interface.
  • Create and include a JavaScript file where we are going to create our application object (wait for it) and define some controllers.
  • Add scope (almost there…) variables and functions that will be bound to the view through special AngularJS HTML attributes.

Basic View Markup

Let’s start this in a natural way: let’s create the user interface first. We will create a file called todolist.html and we will add the following HTML to it:
Now, nothing fancy right there; we just included the AngularJS library at line 8 and another file at line 9, which we will create in a moment. However, just notice the structure of the document: We have an UL element which will represent our list of to-do items and then a couple input controls so we can add new items to the list. It does not have any information on it for now, but eventually it will… And it will be awesome.

Let’s leave this code as it is for now, let’s move to the app.js file which has all the fun going on.

Adding Logic: The Controller

Just by looking at the previous HTML document, you will realize that there are two things we need regarding to view state data:

  • An array to store the to-do items to be displayed in the UL element at line 13.
  • A function to add new items when I click the Add button at line 15.

Simple, right? Well, let’s create a new file called app.js and add the following code; I’ll explain it later:


First things first: let’s take a look at line 1. Before we start building controllers that handle UI logic, we need to create what is known as an AngularJS application module; this is done by calling the angular.module() function and passing it the module name, which returns the created module. This object will later be used as a container where you will store all of the controllers belonging to this particular JS App.

The variable angular is available after we included the AngularJS library in our HTML document at line 8.

After we create the application object, we can start creating controllers; being done in line 3 through the controller() function, which is now callable from the application module we created in line 1. This function takes two arguments: the name of the controller and a callback function that will be responsible of initializing the controller and which will be called each time a controller is instantiated by AngularJS. AngularJS also passes a special variable to that function called $scope; everything that is added to the $scope variable during controller initialization, will be accessible to the view for data binding; better said: it represents what the view can see.

The rest of the lines are quite self-explanatory:

  • Lines 7 to 11 adds a pre-initialized array of to-do items to the scope, that will be used as storage for new items. It represents the items that will be displayed in the empty UL element we have right now.
  • Line 13 declares a variable that will hold the description of new items to be added after the Add button is clicked.
  • Lines 17 to 21 define a function that will add new items to the to-do items array; this should be called each time the Add button is clicked.

Binding The View

Now that we have AngularJS, the application module and the to-do list controller, we are ready to start binding HTML elements in the view to the scope. But before that, we need to change our <html> tag a little and add some special AngularJS attributes:


Notice the ng-app attribute we added to the <html> tag; this is your first contact with one of the several directives that are part of the AngularJS data binding framework.

Directives are simply HTML element markers that are to be processed by the data binding engine. What does ng-app do? Well, it tells AngularJS which application module it should use for this HTML document; you might notice we specified ToDoListApp, which is the one we created in out app.js file at line 1.

After we associate our HTML document with the ToDoListApp, second step is to specify a controller for our view; this is done through the ng-controller directive. The ng-controller directive assigns a controller to a section of the HTML document; in other words, this is how we tell Angular where a view starts and when it ends.

Anyways, modify the <body> tag so it looks like this:


Same with the ng-app directive, the previous code tells AngularJS that everything inside the <body> tag will be governed by the ToDoListController, which we created in our app.js file at line 3.

Now we are ready to start binding elements to members added to the $scope variable during the ToDoListController controller initialization.

The ng-repeat Directive

Let’s start with the empty UL list element. In this case we want to create a new child LI element per item that is contained in the to-do items array. Let’s use the ng-repeat directive for this matter; add the following code inside the UL tags:


OK, hold on tight. The ng-repeat directive will repeat an element per item in an array used as data source; the quoted value represents the iterator expression, which defines the array to be iterated and an alias used to refer the current item being iterated.

In this case, we specified that it should repeat the LI element for all of the items in the items array defined in the $scope variable from line 7 through 11 in the app.js file.

Lastly, inside the LI tags we define an Angular template, which is an expression enclosed in double curly braces that will be compiled dynamically during run-time. In this case, the template is extracting a member named desc from each item and is going to display its value inside the LI tags being repeated.

Go ahead, save your files and open todolist.html in your preferred browser; you will see how the list gets filled. Awesome stuff, huh?

The ng-model Directive

Next in our list is the ng-model directive, which is used to bind the value of an input element, a text box, text area, option list, etc.; to a variable of the $scope. But before I give you more talk, change the input element at line 15:


Binding the value of an element means that each time the value of the input element changes, the variable specified in the ng-model directive will change with that value.

AngularJS enables two-way binding by default, meaning that if the value of the variable the input element is bound to changes, the change will be reflected in the input element on screen. This is the true magic of AngularJS: You can change the value of elements displayed on screen just by modifying values of the $scope variable; no need of selectors, no need to access the DOM from JavaScript, no need of extra code.

In this case, the input element has been bound to the newItemDescription variable of the $scope, defined at line 13 of the app.js file. Each time the value of the input element changes, the variable at the scope will be updated and viceversa.

The ng-click Directive

What if I want to do something when I click that? For that, AngularJS provides a bunch of event handler directives. These directives can be used to invoke functions defined on the $scope each time the user performs an action over an element.

The only one we are going to use for this example is the ng-click, which handles the user click on a button or input element; the expression it takes is basically the code to be executed on each click. In our case, we will modify the button element at line 15 and add the following directive:


If you look closely, we are telling AngularJS to call the addItem function defined in the $scope. If you see the code of that function from line 17 to 21, you will see that it adds a new item to the to-do list array, based on the value of the newItemDescription variable.

If you save your files and open todolist.html, you will see how the list is automatically updated each time you enter a new description in the text box and click on Add.

Your HTML document is dynamic and alive. How cool is that?

What Kind Of Sorcery Is This!?

OK, I must tell you that all of this does not happen through magic. AngularJS is what’s called an unobtrusive library; which means that everything happens automatically as soon as the library finishes loading.

At the very beginning, when the HTML document finished loading, AngularJS crawls through HTML document structure looking for ng-app directives which tells it that something has to be done there. After all of the directives have been found, the marked elements are processed separately: elements are bound to controllers and templates are compiled.

The $scope variable lifetime is automatically handled by AngularJS, each time something happens on-screen, it is notified and performs anything that has to be done to ensure the controller and view are synced; from updating values in the $scope bound to a particular element to refreshing an element or template. This is done through the implementation of some sort of the observable pattern that allows AngularJS to react to changes on elements and the $scope itself.


Woah! That was a bunch of stuff to digest. Hopefully, you will make sense out of it with some practice. Of course those three directives are not everything that there is on AngularJS; for a complete list on all of the possible directives you can use out-of-the-box, take a look here.

Have in mind that you can also extend the library with custom directives, but that is an advanced topic we might cover in a future article.

AngularJS is not everything there is about JS Apps, this was a very basic introduction and I want you to have knowledge you can use in real life, so we will be learning how to build more complex JS Apps in my next article. Topics to be covered will be:

  • Creating custom Angular directives.
  • Complex user interfaces (windows, redirection and data validation.)
  • RequireJS for on-demand asynchronous JavaScript files loading.

Hope that sounds exciting enough for you. Stay tuned! 😀

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Further Reading