首页 > itarticle > liuchen’s blog【笔记】ArrayList

liuchen’s blog【笔记】ArrayList

admin 11月 12, 2020 0

前言

读源码读出来的一些东东

源码和分析

#####构造函数
private static final Object[] EMPTY_ELEMENTDATA = {};
private transient Object[] elementData;

public ArrayList(int initialCapacity) {
    super();
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    this.elementData = new Object[initialCapacity];
}

public ArrayList() {
    super();
    this.elementData = EMPTY_ELEMENTDATA;
} *   刚创建出来ArrayList如果没指定大小，那么数组元素是个空数组。 *   可以通过调用带int参数的构造函数来创建一个数组为指定大小的ArrayList

#####添加元素
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return true (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
    if (elementData == EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

/**
 * The maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    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);
}

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
} *   如果elementData是空数组，elementData的大小会被设置成DEFAULT_CAPACITY，也就是10 *   ensureExplicitCapacity调用了modCount++方法，是为了保证线程安全 *   ArrayList添加元素时，会进行容量检查，如果发现size不够，会添加 当前size/2 和 添加元素数量 的最大值。