Object Orientation

Constructors

• NO return type.
• Can have all the access modifiers.
• Can't be marked as abstract or final or static.

Class Modifiers

Regular Classes:

• Access Modifiers: public and default only
• Non access: strictfp, abstract and final only
• Adding any other modifier will result in a compiler error.

• A final class cannot be overriden (e.g. String)
• An abstract class cannot be instantiated (e.g. DateFormat)

Class Members

• Members can use all the access modifiers: public, private, default(package), protected
• Private members are not inherited.
• This means private members can be redeclared in subclasses.
• Instance level variables cannot be synchronized, abstract, strictfp, native.

Protected

• Protected is wider than package. Protected = package + subclasses.
• For classes and subclasses in the same package protected works in exactly the same way as package(default).
• For subclasses in other packages, protected members (both fields and methods) are only accessible through inheritance in the subclass.
• The subclass in a different package cannot use a reference to super to access the protected member.

package pkg1;

public class Foo {
  protected String str = "Hello";
}

package pkg2;

public class Bar extends Foo {
  public void go() {
     Foo f = new Foo();
     String s = f.str; //Wont work ! Compiler error
     System.out.println(str); //Will work - through inheritance
  }
}

package pkg2;

public class Fubar {
   public void goo() {
        Bar b = new Bar();
        System.out.println(b.str); //wont work - str only accessible through inheritance
   }
}

• One more example

package pkgA;

public class Animal {
	
	int count = 10;
	protected int secret = 42;
	
	void eat() {
		System.out.println("Animal Eat");
	}
	
	protected void mope() {
		System.out.println("Animal Moping");
	}
	
}

//Same package - different class

package pkgA;

public class Zoo {

	public static void main(String[] args) {
		Zoo z = new Zoo();
		z.go();
	}
	
	public void go() {
		Animal a = new Animal();
                //Package and protected are accessible
		a.eat();
		a.mope();
		System.out.printf("Package : %d %n", a.count);
		System.out.printf("Protected : %d %n", a.secret);
	}

}

//Same package - subclass

package pkgA;

public class Cat extends Animal {
	
	public static void main(String[] args) {
		Cat c = new Cat();
		c.catnap();
	}
	
	public void catnap() {
		System.out.println("Animal count :" + count); //package variable through inheritance
		System.out.println("Secret : " + secret); //protected variable through inheritance
                eat(); //package method through inheritance
		mope(); //protected method through inheritance

                Animal a = new Animal();
                //Access through super-class reference or super in all cases will work.
                a.count = 20;      
                super.secret = 18;
                super.eat();  
                a.mope(); 
	}
}


//Subclass in a different package

package pkgB;

import pkgA.Animal;

public class Dog extends Animal {

	public static void main(String[] args) {
		Dog d = new Dog();
		d.dawg();
	}
	
	public void dawg() {
                //None of the package level members will be accessible.

		System.out.println(secret); //This will work
                System.out.println(super.secret) // OR this will too
                Animal a = new Animal();
                System.out.println(a.secret); //This wont. Compile error.
                
                System.out.println(mope()); //OK
                System.out.println(super.mope()); //OK
                System.out.println(a.mope()); //NO. Compiler error.

	}
}

Abstract Methods

As they go against the logic of overriding :

• abstract methods cannot be static.
• abstract methods cannot be private.

Overriding

• Polymorphic method invocations apply ONLY to instance methods.
• The argument list for the overriding method must EXACTLY match that of the overriden method. Covariant arguments wont work!