Declarations and Access Control - this and super

this (JLS §15.8.3)

• this is an Object-Oriented Programming (OOP) operator
• it is used to refer to the current instance of an object
• it can be used in the body of a class constructor to refer to the object being created
• it can be used in the body of an instance method or initializer to refer to the object whose method is being executed
• it cannot be used in a static method or initializer
• most commonly appears in constructors
• can be used to explicitly call another constructor
• when used in a constructor it must be the first statment in the constructors body

class Super {

    int x;
    int y;

    Super(){

        System.out.println("Super object being created.");

	}
	
    Super( int x, int y ) {

        this();             // call no-arg constructor

        this.x = x;

        this.y = y;    

    }

}	
	

super

• super is an Object-Oriented Programming (OOP) operator
• used to call a constructor declared in a classes superclass
• commonly used in constructors and to access hidden fields or invoke overridden methods in the superclass
• if used in a constructor, must be the first statment in constructor body

 class Subclass extends Super {
    int w;

    Subclass(){
        this(0,0,0);        // call 3-param constructor
    }

    Subclass( int x, int y ) {
        this(x,y,0);        // call 3-param constructor
    }

    Subclass( int x, int y, int w ) {    

        super(x,y);         // call superclass constructor
        this.w = w;
    }
}
 

Subclass( int x, int y, int w) {

        this();

        super(x,y);         // compile-error

    }

Example Code

TestThisAndSuper.java