ArrayList扩容源码解读

最新推荐文章于 2025-02-05 21:12:01 发布

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最新推荐文章于 2025-02-05 21:12:01 发布

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分类专栏： JavaSE 文章标签： java 源码

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1. ArrayList的构造方法，源码如下：

 
    
  
/**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
​
    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

 
/**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

2. ArrayList类add方法的详解

 
    
  
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

3. 调用的是ensureCapacityInternal方法，我们就看一看这个方法的源码

 
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {//如果是第一次添加元素，则执行下面方法
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
​
        ensureExplicitCapacity(minCapacity);
    }

4. ensureExplicitCapacity方法的解析

 
    
  
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;    //记录修改的次数
​
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);//如果minCapacity比原数组的容量大，则进行扩容操作
    }

5. grow方法如下所示：

 
 private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);//数组扩容1.5倍
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);//扩容核心方法，将elementData复制到新的数组中并扩容
    }

6. 按照index的位置添加元素

 
    public void add(int index, E element) {
        rangeCheckForAdd(index);
​
        ensureCapacityInternal(size + 1);  // Increments modCount!!//先扩容
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);//复制数组后半部分
        elementData[index] = element;//插入元素
        size++; //增加计数器
    }