第四章贵州大学java实验及练习

例题：4—18 Jsva中多态示例
import java.util.*;
//定义Shape类
class Shape {
void draw() {}
void erase() {}
}
//定义Circle类
class Circle extends Shape {
void draw() {
System.out.println("Calling Circle.draw()"); }
void erase() {
System.out.println("Calling Circle.erase()"); }
}
//定义Square类
class Square extends Shape {
void draw() {
System.out.println("Calling Square.draw()"); }
void erase() {
System.out.println("Calling Square.erase()"); }
}
//定义Triangle类
class Triangle extends Shape {
void draw() {
System.out.println("Calling Triangle.draw()"); }
void erase() {
System.out.println("Calling Triangle.erase()"); }
}
//包含main()的测试类
public class Shapes{
static void drawOneShape(Shape s){
s.draw();
}
static void drawShapes(Shape[] ss){
for(int i = 0; i < ss.length; i++){
ss[i].draw();
}
}
public static void main(String[] args) {
Random rand = new Random();
Shape[] s = new Shape[9];
for(int i = 0; i < s.length; i++){
switch(rand.nextInt(3)) {
case 0: s[i] = new Circle();break;
case 1: s[i] = new Square();break;
case 2: s[i] = new Triangle();break;
}
}
drawShapes(s);
}
}
习题4—11给出下列程序的运行结果：
class Meal{
Meal(){System.out.println("Meal()");}
}
class Bread{
Bread(){System.out.println("Bread()");}
}
class Cheese{
Cheese(){System.out.println("Cheese()");}
}
class Lettuce{
Lettuce(){System.out.println("Lettuce()");}
}
class Lunch extends Meal{
Lunch(){System.out.println("Lunch()");}
}
class PortableLunch extends Meal{
PortableLunch(){System.out.println(" PortableLunch()");} }
public class Sandwich extends PortableLunch{
private Bread b = new Bread ();
private Cheese c = new Cheese();
private Lettuce l= new Lettuce();
public Sandwich (){
System.out.println("Sandwich()");
}
public static void main(String args[]) {
new Sandwich();
}
}。