DCO10103 Object-Oriented Programming and Design

is-a relationship

create a class hierarchy (super-sub,parent-child,ancester-descendant) with Object at the top
```
public class SubClass extends SuperClass implements SuperInterface  {
	...
}
```
"A class inherits from its direct superclass and direct superinterfaces all the non-private methods (whether abstract or not) of the superclass and superinterfaces that are accessible to code in the class and are neither overridden nor hidden by a declaration in the class." JLS
Hiding and Overriding
- a field is said to be hidden, and consequently not inherited, if the child defines a field of its own with the same identifier
- it is permissible for an instance variable to hide a static variable
- a method is said to be overridden, and consequently not inherited, if the child defines a method of its own with the same signature and return type
- a compile-time error occurs if an instance method overrides a static method
class that can not be extended anymore e.g. java.lang.Math
```
public final class CannotBeSuper  {
	...
}
```

method that can not be verridden

public final void  cannotBeOverridden()  { 
	... 
}

Example: Overloading, Overriding, and Hiding

//file: RealPointTester.java
class Point {
	int x = 0, y = 0;
	void move(int dx, int dy) { x += dx; y += dy; }
	int getX() { return x; }
	int getY() { return y; }
	int color;
}
class RealPoint extends Point {
	float x = 0.0f, y = 0.0f;                                    // Hiding super.x, and super.y 
	void move(int dx, int dy) { move((float)dx, (float)dy); }    // Overriding super.move() 
	void move(float dx, float dy) { x += dx; y += dy; }          // Overloading move() 
	int getX() { return (int)Math.floor(x); }                    // Overriding super.getX() 
	int getY() { return (int)Math.floor(y); }                    // Overriding super.getY() 
}

public class RealPointTester {
	public static void main(String[] args) {
		RealPoint rp = new RealPoint();
		Point p = rp;
		rp.move(1.718298f, 4.14159f);
		p.move(1, -1);
		show(p.x, p.y);                 // (0,0)
		show(rp.x, rp.y);               // (2.7182798,3.14159) 
		show(p.getX(), p.getY());       // (2,3)
		show(rp.getX(), rp.getY());     // (2,3)
	}
	static void show(int x, int y) {        // Overloading show()
		System.out.println("(" + x + ", " + y + ")");
	}
	static void show(float x, float y) {
		System.out.println("(" + x + ", " + y + ")");
	}
}

hidden fields and methods of Superclass can be accessed using the keyword super
Example: Overriding, Hiding, and accessing parent's field and method

Parent

public class Parent  {

	public static int  x = 37;

	public static void  cling()  {

		stdOut.println("Called parent's cling method.");
	}

	public void  print()  {
	
		stdOut.println("Called parent's print method.");
	}
}

Child

public class Child extends Parent  {

	public double  x;  // hiding

	public Child()  {

		x = 2.5;
	}

	public static void  cling()  {  // hiding

		stdOut.println("Called child's cling method.");
	}

	public void   print()  {  // overriding
	
		stdOut.println("Called child's print method.");
	}

	public void  printLots()  {

		stdOut.println("From within child...");
		stdOut.println("Calling:  print()");
		print();

		stdOut.println("Calling:  super.print()");
		super.print();

		stdOut.println("Calling:  cling()");
		cling();

		stdOut.println("Calling:  super.cling()");
		super.cling();

		stdOut.println("Calling:  Parent.cling()");
		Parent.cling();

		stdOut.println("x:  " + x);
		stdOut.println("super.x:  " + super.x);
		stdOut.println("Parent.x:  " + Parent.x);
	}
}

ChildTester.java

import java.io.*;

class Parent  {
	private static PrintWriter  stdOut = new PrintWriter(System.out, true);

	public static int  x = 37;

	public static void  cling()  {

		stdOut.println("Called parent's cling method.");
	}

	public void  print()  {
	
		stdOut.println("Called parent's print method.");
	}
}

class Child extends Parent  {
	private static PrintWriter  stdOut = new PrintWriter(System.out, true);

	public double  x;  // hiding

	public Child()  {

		x = 2.5;
	}

	public static void  cling()  {  // hiding

		stdOut.println("Called child's cling method.");
	}

	public void   print()  {  // overriding
	
		stdOut.println("Called child's print method.");
	}

	public void  printLots()  {

		stdOut.println("From within child...");
		stdOut.println("Calling:  print()");
		print();

		stdOut.println("Calling:  super.print()");
		super.print();

		stdOut.println("Calling:  cling()");
		cling();

		stdOut.println("Calling:  super.cling()");
		super.cling();

		stdOut.println("Calling:  Parent.cling()");
		Parent.cling();

		stdOut.println("x:  " + x);
		stdOut.println("super.x:  " + super.x);
		stdOut.println("Parent.x:  " + Parent.x);
	}
}

public class ChildTester {

    public static void main(String[] arg) throws IOException {

      Child  son = new  Child();
      son.printLots();
    }
}

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