//Chapter 9 inheritance stuff crammed into here
//part 2

public class Inheritance2 {
    public static void main( String[] args ) {
	X x1 = new X();
	Y y1 = new Y();

	System.out.println( "x1.i= " + x1.i );
	System.out.println( "y1.i= " + y1.i );  //i inherited from X
	//System.out.println( "x1.j= " + x1.j );  //syntax error: X has no j
	System.out.println( "y1.j= " + y1.j );  

	x1.m1();
	y1.m1();   //m1() inherited from X
	y1.m2();   //added to Y
    }
}


//super class is more general.
class X {
    int i=3;
    void m1() {
	System.out.println( "hello from X m1()" );
    }
}


//subclass is more specific, specialized.  Extends the superclass.
//subclass inherits members of superclass (and all ancestors back to Object).
//subclass is customized with additional members (fields and methods) and
//by overriding (redefining) superclass methods.
//subclass is everything the superclass is, plus more.

class Y extends X {
    int j=4;   //field added to subclass Y. not in superclass X
    void m2() {
	System.out.println( "hello from Y m2()" );
    }

}

//inheritance (IS-A): form of software reuse: new subclass gets all the data and
//methods of superclass without duplicating code.
// share common code (data and methods) among the instances of related classes.

//composition (HAS-A): form of software reuse too.

//can derive a subclass without access to superclass's source code.
//thus can tweak or embellish existing wheel functionality without having to
//reinvent the entire wheel.