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 super-class reference to access the protected member.
• Sub-class reference can be used to access a protected member.
• The keyword super can be used to access the protected member from within a subclass.

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();
                d.mope(); //OK subclass reference.
	}
	
	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.

	}
}

=== protected static methods and constructors ===

* Protected static methods can be invoked by all classes in the same package AND by child classes which are in other packages.
* Since it is static, the rules regarding which reference (child/super) to invoke it do not apply.
* It can be invoked using the Class name.
* For classes in same package, invoking the protected constructor is not a problem.
* FOr class outside the package, the protected constructor can only be invoked by child-classes using a call to super().

<syntaxhighlight lang="java5">

package prexams.exam4.util;

public class Utils {
	
	protected Utils() {
	}
	
	public Utils(String s) {
	}
	
	protected static void do3() {
		System.out.println("do3");
	}

}

package prexams.exam4;

import prexams.exam4.util.Utils;

class NonChild {
	
	public void go() {
		Utils u = new Utils(); //Non-child class cant access protected constructor
	}
	
	public void bar() {
		Utils.do3(); // Non-child class - Protected static method is inaccessible.
	}
}

public class UPS extends Utils {
	
	UPS() {
		super(); //OK since UPS is a subclass.
	}
	
	public static void main(String[] args) {
		Utils.do3(); //static is accessible
		UPS ups = new UPS();
		ups.do3(); //meaningless but OK
		Utils u = new Utils("u");
		u.do3(); //meaningless but OK
	}
	
	public void foo() {
		Utils u = new Utils(); //NOT OK - UPS Constructor can only called through super().
	}

}

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! If arguments dont match, then the methods will overload.
• Example:

class Masher {
	
	public void mash(Object o) {
		System.out.println("Masher.mash");
	}
	
}

class Jaws extends Masher {
	
	public void mash(String o) { //Overloads not Overrides the superclass method
		System.out.println("Jaws.mash");
	}
}

public class Override {

	public static void main(String[] args) {
		Masher m = new Jaws();
		m.mash("Hello"); //Will print Masher.mash
	}
	
}

• The return type must be the same or a subtype (Covariant return) of the superclass method.
• If the return types are not the same and arguments are the same - then this will be a name clash and the code wont compile.
• Example:

interface Moveable {
	
	Number moveIt();
	
}

class Bike implements Moveable {
	
	public Integer moveIt() {
		return new Integer(42);
	}
	
}

• Access level CANT be more restrictive but CAN be less restrictive.

• Overriding method can throw ANY runtime exception regardless of what superclass method declares.
• Overriding method cannot throw checked exceptions that are NEWER or BROADER than superclass method.
• Overriding method can throw narrower or fewer checked exceptions.
• It also need NOT redeclare any exceptions (checked or unchecked) from the superclass.

class Handler {
	
	public void handle() throws IOException {
		System.out.println("Handler.handle()");
	}
	
	public void handleIt() throws NumberFormatException {
		System.out.println("Handler.handleIt()");
	}
	
	public void handleMe() {
		System.out.println("Handler.handleMe()");
	}
}

class FileHandler extends Handler {
	
	public void handle() { //OK need not redeclare checked exception
		System.out.println("FileHandler.handle()");
	}
	
	public void handleIt() { //OK need not redeclare unchecked exception
		System.out.println("FileHandler.handleIt()");
	}
	
	public void handleMe() throws IllegalArgumentException { //OK can declare new unchecked exception
		System.out.println("Handler.handleMe()");
	}
}

class Handler {
	
	public void handle() throws IOException {
		System.out.println("Handler.handle()");
	}
	
	public void handleIt() throws InterruptedException {
		System.out.println("Handler.handleIt()");
	}
	
	public void handleMe() {
		System.out.println("Handler.handleMe()");
	}
}

class FileHandler extends Handler {
	
	public void handle() throws FileNotFoundException { //OK, narrower checked exception can be declared to be thrown
		System.out.println("FileHandler.handle()");
	}
	
	public void handleIt() throws Exception { //NOT OK, broader checked Exception being declared
		System.out.println("FileHandler.handleIt()");
	}
	
	public void handleMe() throws UnknownHostException  { //NOT OK, newer checked exception being declared
		System.out.println("Handler.handleMe()");
	}
}

Static Members

• When it comes to statics, subclasses can redefine static memebers (variables and methods). They are not overrides.
• Which member will be used depends on the reference type ! not the actual object type. See below:

class Vehicle {
	
	public static int I = 10;
	
	static String stop() {
		return "Vehicle Stop";
	}
	
	static String move() {
		return "Vehicle Move";
	}
	
}

class Ship extends Vehicle {
	
	public static int J = 20;
	
	static String move() {
		return "Ship Move";
	}
	
}

public class Statics {

	public static void main(String[] args) {
		
		Vehicle v = new Vehicle();
		
		System.out.println(v.I);
		System.out.println(v.move()); //10
		System.out.println(v.stop()); //Vehicle Stop
		
		Vehicle sv = new Ship();
		System.out.println(sv.I); //10
		System.out.println(sv.move()); //Vehicle Move
	
		Ship s = new Ship();   
		System.out.println(s.I);  //10
		System.out.println(s.J);  //20
		System.out.println(s.stop()); //Vehicle Stop
		System.out.println(s.move()); //Ship Move

	}
}

• When static methods are redefined in the sub-class they dont override the superclasses's, they "hide" it.
• However a static method CANNOT hide an instance method and an instance method CANNOT override a static method.

class Veg {

	void eat() {
		
	}
	
	static void grow() {
		
	}
}

class Potato extends Veg {

	static void eat() { //COMPILER ERROR - overriding method is static
		
	}
	
	void grow() { //COMPILER ERROR - overriden method is static
		
	}
}

Overloading

• Overloaded methods differ only in argument list, no other restrictions.
• Methods can be overloaded with argument types that are in the same hierarchy.
• The decision to invoke a particular overloaded method is made at COMPILE time, not at RUN time like polymorphic methods.
• See below: compiler will invoke overloaded method depending on the reference type.

class Gadget {
}

class Phone extends Gadget {
}

public class Overlord {

	public static void main(String[] args) {
		new Overlord().go();
	}
	
	void go() {
		Gadget g = new Gadget();
		Phone p = new Phone();
		Gadget gp = new Phone();
		
		foo(g); //Prints: Gadget
		foo(p); //Prints: Phone
		foo(gp);//Prints: Gadget
		
	}
	
	void foo(Gadget g) {
		System.out.println("Gadget");
	}
	
	void foo(Phone p) {
		System.out.println("Phone");
	}

}

Casting

• References can be cast if the types belong in the same hierarchy.
• This means that an Object reference can be cast to anything.... because everything is an Object.
• However, the compiler can figure out if two references are of completely different types and they cannot be cast

                Object obj = new String();
		String str = (String) obj;          //OK, an object can be a String.

                Object obj2 = new Integer();
                String str2 = (String) obj2; //Compiler says OK - but this will fail at runtime with a ClassCastException
		
		Exception oie = new InterruptedException();
		OutOfMemoryError ome = (OutOfMemoryError) oie;     //An Exception can never be an Error

• The cast need not be of the same type as that of the reference variable, it can be of a sub-type as well:

interface Computable {
}

class Electronics {
}

class Computer extends Electronics implements Computable {
}

class Laptop extends Computer {
}

public class Casting {

	public static void main(String[] args) {
		
		Electronics e = new Laptop();
		
		Electronics e2 = (Computer) e;
		
		Computer comp = (Laptop) e;
		
	}
}