Effective Java 第六条：消除过期的对象引用

public class Stack {
  private Object[] elements;
  private int size = 0;
  private static final int DEFAULT_INITIAL_CAPACITY = 16;

  public Stack() {
    elements = new Object[DEFAULT_INITIAL_CAPACITY];
  }

  public void push(Object e) {
    ensureCapacity();
    elements[size++] = e;
  }

  public Object pop() {
    if (size == 0)
      throw new EmptyStackException();
    return elements[--size];
  }

  private void ensureCapacity() {
    if (elements.length == size) {
      elements = Arrays.copyOf(elements, 2 * size + 1);
    }
  }
}

public Object pop() {
  if (size == 0)
    throw new EmptyStackException();
  Object result = elements[--size];
  elements[size] = null; //消除过期引用
  return result;
}