import java.util.ArrayList;
这里写代码片
class Machine {
@Override
public String toString() {
return "I am a Machine";
}
public String start() {
return "the machine starting";
}
}
class Camera extends Machine {
@Override
public String toString() {
return “I am a Camera”;
}
public String start() {
return "the camera starting";
}
}
public class App {
public static void main(String[] args) {
ArrayList<Machine> list1 = new ArrayList<>();
list1.add(new Machine());
list1.add(new Machine());
ArrayList<Camera> list2 = new ArrayList<>();
list2.add(new Camera());
list2.add(new Camera());
showList(list2);
showList2(list2);
showList3(list1);
}
public static void showList(ArrayList<?> list) {
for (Object value : list) {
System.out.println(value);
}
}
public static void showList2(ArrayList<? extends Machine> list) {
for (Machine value : list) {
System.out.println(value);
}
}
public static void showList3(ArrayList<? super Camera> list) {
for (Object value : list) {
System.out.println(value);
}
}
}
子类的ArrayList并不是父类的子ArrayList,因此,方法中的参数是父类时,并不能利用Polymophism来传递子ArrayList ,此时,需要用到Wildcard ; showList2中的extends 表示可以传递父类或者父类的子类,此时类似于Polymophism;showList3中的super表示可以传递子类及其父类,但是此时object并不能表示为Camera,
类似于downcasting中,强制转换过来的父类并不能使用子类的methods.