Java-Java编程思想第四版 第九章 练习

练习

abstract class A{
    abstract void print();
    public A(){print();}
}
public class Ja9_1_3 extends A{
    int i=5;
    void print(){
        System.out.println(i);
    }
    public static void main(String[] args){
        Ja9_1_3 j=new Ja9_1_3();
        j.print();
    }
}
/*
 *output:
 0
 5
 * */

练习4：

import static net.mindview.util.Print.*;
abstract class Dad {}

class Son extends Dad {
	protected void print() { System.out.println("Son"); }
}

abstract class SecondDad {
	abstract protected void print();
}

class SecondSon extends SecondDad {
	protected void print() { System.out.println("SecondSon"); }
}

public class Ja9_1_4 {
	public static void testPrint(Dad d) {
		((Son)d).print();//it's necessary
	}
	public static void secondTestPrint(SecondDad sd) {
		sd.print();//don't need ((SecondSon)sd)
	}
	public static void main(String[] args) {
		Son s = new Son();
		Ja9_1_4.testPrint(s);
		SecondSon ss = new SecondSon();
		Ja9_1_4.secondTestPrint(ss);			
	}
}