謹慎的覆蓋clone

Stack.java

源碼： 

import java.util.Arrays;

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


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


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


    public Object pop() {
        if (size == 0)
            throw new EmptyStackException();
        Object result = elements[--size];
        elements[size] = null; // Eliminate obsolete reference
        return result;
    }


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


    @Override public Stack clone() {
        try {
            Stack result = (Stack) super.clone();
            result.elements = elements.clone();
            return result;
        } catch (CloneNotSupportedException e) {
            throw new AssertionError();
        }
    }


    // Ensure space for at least one more element.
    private void ensureCapacity() {
        if (elements.length == size)
            elements = Arrays.copyOf(elements, 2 * size + 1);
    }


    // To see that clone works, call with several command line arguments
    public static void main(String[] args) {
        Stack stack = new Stack();
        for (String arg : args)
            stack.push(arg);
        Stack copy = stack.clone();
        while (!stack.isEmpty())
            System.out.print(stack.pop() + " ");
        System.out.println();
        while (!copy.isEmpty())
            System.out.print(copy.pop() + " ");
    }
}

總結： 

clone方法就是另一個構造器，要確保它不會傷害到原始的對象，

並確保正確地創建被克隆對象中的約束條件（invariant）

 @Override public Stack clone() {
        try {
            Stack result = (Stack) super.clone();
            result.elements = elements.clone();
            return result;
        } catch (CloneNotSupportedException e) {
            throw new AssertionError();
        }
    }

有的時候需要單獨的拷貝