Java ArrayList学习

import java.util.ArrayList;

class Apple {
private static long counter;
private final long id = counter++;
public void printId() { System.out.println(id);}
}

class Orange {
}

public class Test {
public static void main(String[] args) {
ArrayList<Apple> apples = new ArrayList<Apple>();
//      ArrayList<Apple> apples = new ArrayList(); 这样写也好像没有运行错误，但是有警告
for(int i = 0; i < 3; i++) {
apples.add(new Apple());
}

//      apples.add(new Orange());

for(int i = 0; i < apples.size(); i++) {
((Apple) apples.get(i)).printId();
}

for(Apple a : apples) {
a.printId();
}
}
}