Java Modifiers

Example

 public class Main

The public keyword is an access modifier, meaning that it is used to set the access level for classes, attributes, methods and constructors.

We divide modifiers into two groups:

Access Modifiers - controls the access level
Non-Access Modifiers - do not control access level, but provides other functionality

Access Modifiers

For classes, you can use either public or default:

Modifier	Description
public	The class is accessible by any other class
default	The class is only accessible by classes in the same package. This is used when you don't specify a modifier.

For attributes, methods and constructors, you can use the one of the following:

Modifier	Description
public	The code is accessible for all classes
private	The code is only accessible within the declared class
default	The code is only accessible in the same package. This is used when you don't specify a modifier.
protected	The code is accessible in the same package and subclasses.

Non-Access Modifiers

For classes, you can use either final or abstract:

Modifier	Description
final	The class cannot be inherited by other classes
abstract	The class cannot be used to create objects (To access an abstract class, it must be inherited from another class

For attributes and methods, you can use the one of the following:

Modifier	Description
final	Attributes and methods cannot be overridden/modified
static	Attributes and methods belongs to the class, rather than an object
abstract	Can only be used in an abstract class, and can only be used on methods. The method does not have a body, for example abstract void run();. The body is provided by the subclass (inherited from).
transient	Attributes and methods are skipped when serializing the object containing them
synchronized	Methods can only be accessed by one thread at a time
volatile	The value of an attribute is not cached thread-locally, and is always read from the "main memory"

Final

If you don't want the ability to override existing attribute values, declare attributes as final:

Example

    
 public class Main {
  final int x = 10;
  final double PI = 3.14;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 50; // will generate an error: cannot assign a value to a final variable
    myObj.PI = 25; // will generate an error: cannot assign a value to a final variable
    System.out.println(myObj.x);
  }
}

Static

A static method means that it can be accessed without creating an object of the class, unlike public:

Example

    
public class Main {
  // Static method
  static void myStaticMethod() {
    System.out.println("Static methods can be called without creating objects");
  }

  // Public method
  public void myPublicMethod() {
    System.out.println("Public methods must be called by creating objects");
  }

  // Main method
  public static void main(String[ ] args) {
    myStaticMethod(); // Call the static method
    // myPublicMethod(); This would output an error

    Main myObj = new Main(); // Create an object of Main
    myObj.myPublicMethod(); // Call the public method
  }
}

Abstract

An abstract method belongs to an abstract class, and it does not have a body. The body is provided by the subclass:

Example

    
 // Code from filename: Main.java
// abstract class
abstract class Main {
  public String fname = "John";
  public int age = 24;
  public abstract void study(); // abstract method
}

// Subclass (inherit from Main)
class Student extends Main {
  public int graduationYear = 2018;
  public void study() { // the body of the abstract method is provided here
    System.out.println("Studying all day long");
  }
}
// End code from filename: Main.java

// Code from filename: Second.java
class Second {
  public static void main(String[] args) {
    // create an object of the Student class (which inherits attributes and methods from Main)
    Student myObj = new Student();

    System.out.println("Name: " + myObj.fname);
    System.out.println("Age: " + myObj.age);
    System.out.println("Graduation Year: " + myObj.graduationYear);
    myObj.study(); // call abstract method
  }
}