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);}}