Java Modifiers

Modifiers

By now, you are quite familiar with the public keyword that appears in almost all of our examples:

public class MyClass

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 

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

Modifier Description
final Attributes and methods cannot be overridden/modified
static Attributes and methods belong 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 MyClass {
  final int x = 10;
  final double PI = 3.14;

  public static void main(String[] args) {
    MyClass myObj = new MyClass();
    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

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

Example

An example to demonstrate the differences between static and public methods:

public class MyClass {
  // 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

    MyClass myObj = new MyClass(); // Create an object of MyClass
    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: Person.java 
// abstract class
abstract class Person {
  public String fname = "John";
  public int age = 24;
  public abstract void study(); // abstract method 
}

// Subclass (inherit from Person)
class Student extends Person {
  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: Person.java

// Code from filename: MyClass.java
class MyClass {
  public static void main(String[] args) {
    // create an object of the Student class (which inherits attributes and methods from Person)
    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
  }
}