如果堆栈为空，则重写stack.peek()以返回null

import java.util.EmptyStackException;

public class CustomStack<T> {
    private Object[] elements;
    private int size;
    private static final int DEFAULT_CAPACITY = 10;

    public CustomStack() {
        this(DEFAULT_CAPACITY);
    }

    public CustomStack(int capacity) {
        elements = new Object[capacity];
        size = 0;
    }

    public void push(T item) {
        ensureCapacity();
        elements[size++] = item;
    }

    public T pop() {
        if (isEmpty()) {
            throw new EmptyStackException();
        }
        @SuppressWarnings("unchecked")
        T item = (T) elements[--size];
        elements[size] = null; // 帮助垃圾回收
        return item;
    }

    public T peek() {
        if (isEmpty()) {
            return null;
        }
        @SuppressWarnings("unchecked")
        T item = (T) elements[size - 1];
        return item;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    private void ensureCapacity() {
        if (elements.length == size) {
            Object[] newElements = new Object[elements.length * 2];
            System.arraycopy(elements, 0, newElements, 0, size);
            elements = newElements;
        }
    }

    public static void main(String[] args) {
        CustomStack<Integer> stack = new CustomStack<>();
        System.out.println(stack.peek()); // 输出 null
        stack.push(1);
        stack.push(2);
        System.out.println(stack.peek()); // 输出 2
        stack.pop();
        System.out.println(stack.peek()); // 输出 1
        stack.pop();
        System.out.println(stack.peek()); // 输出 null
    }
}