Java、利用继承实现MyStack

package pack1;
import java.util.ArrayList;
public class MyStack<E> extends ArrayList<E> {
    public MyStack() {
    }
    public MyStack(int initialCapacity) {
        super(initialCapacity);
    }
    public boolean push(E element) {
        if (element == null) return false;
        super.add(element);
        return true;
    }
    public E pop() {
        if (super.size() == 0) return null;
        return super.remove(super.size() - 1);
    }
    public E peek() {
        if (super.size() == 0) return null;
        return super.get(super.size() - 1);
    }
    public int getSize() {
        return super.size();
    }
    public boolean isEmpty() {
        return super.size() == 0;
    }
    @Override
    public String toString() {
        return super.toString();
    }
}

package pack1;
public class TestMyStack {
    public static void main(String[] args) {
        MyStack<String> stack = new MyStack<>();
        stack.push("One");
        stack.push("Two");
        stack.push("Three");
        stack.push("Four");
        stack.push("Five");
        while (!stack.isEmpty())
            System.out.println(stack.pop());
    }
}