ArrayList源码笔记

ArrayList-JDK1.8

0. 参考资料

transient关键字

ArrayList详解，看这篇就够了

1. 成员变量

private int size 实际元素个数

transient Object[] elementData

• transient 关键字，标识后，序列化时不会序列化该属性
  • 实现原理：java的serialization把对象状态存储到硬盘，需要的时候读取出来使用。当我们不希望某字段，如密码，卡号等在网络上传输，transient把字段的生命周期仅存于调用者的内存中，而不会被持久化到磁盘。
  • 被transient修饰的变量也能被序列化。序列化有实现Serializable接口，实现Exteranlizable接口两种方式。第二种方式需要重写writeExternal和readExternal方法，可以决定哪些属性需要序列化（包括被transient修饰的），且效率更高。但是对大量对象、重复对象则效率低。
  • 静态变量不能被序列化。静态变量存在于全局区，而序列化是写到磁盘上面的，当我们获取该变量，JVM会去全局区查找，而不是磁盘。（参考原理，静态变量的生命周期也在内存中。）

用于构造函数的

/**
 * Shared empty array instance used for empty instances.
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
 * 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 = {};

2. 构造函数

/**
 * Constructs an empty list with the specified initial capacity.
 *
 * @param  initialCapacity  the initial capacity of the list
 * @throws IllegalArgumentException if the specified initial capacity
 *         is negative
 */
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}

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

构造函数size为空的情况下，只构建了一个空的数组。所谓的默认初始扩容为10，其实是在第一次add时扩容的。

/**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

将集合c转为数组，赋值非elementData。

如果数组为空，赋EMPTY_ELEMENTDATA；如果不为空，判断是否为Object类型，如果不是要转化为Object。

3. add方法

/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return <tt>true</tt> (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) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

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

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

添加之前会进行判断是否需要扩容，然后再添加数据

/**
 * 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);
}

扩容的方法如上。oldCapacity>>1是做了一个位移的操作，相当于除以二，也就是扩容了原来的1.5倍。扩容后，如果容量比最小需要的容量还小的话，就将需要的容量作为新容量扩容（扩容后太小的情况）；如果容量比数组能扩容的最大容量还要大，那么使用hugeCapacity方法，判断是否内存溢出抛异常（扩容后太大的情况）。如果没有异常，有两种情况，需要容量比数组最大容量大的，返回Integer能存的最大值；否则返回数组最大值。

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

add（index，element）的情况

这种情况下，除了add操作，还会将目标位置的元素后移，给新元素腾出一个位置，然后再赋值。

如果index为最后一位，相当于没有后移。

/**
 * Inserts the specified element at the specified position in this
 * list. Shifts the element currently at that position (if any) and
 * any subsequent elements to the right (adds one to their indices).
 *
 * @param index index at which the specified element is to be inserted
 * @param element element to be inserted
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public void add(int index, E element) {
    rangeCheckForAdd(index);

    ensureCapacityInternal(size + 1);  // Increments modCount!!
    System.arraycopy(elementData, index, elementData, index + 1,
                     size - index);
    elementData[index] = element;
    size++;
}

4. remove方法

/**
 * Removes the element at the specified position in this list.
 * Shifts any subsequent elements to the left (subtracts one from their
 * indices).
 *
 * @param index the index of the element to be removed
 * @return the element that was removed from the list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E remove(int index) {
    rangeCheck(index);

    modCount++;
    E oldValue = elementData(index);

    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

首先会检查index是否合法，如果合法，使用arraycopy直接覆盖掉要remove的那个元素。这个时候可能是前移了一位，所以对最后一个元素，即size所指向的元素，是重复的，使用

elementData[--size] = null 将冗余的元素回收。（为什么是--size不是size--，因为最后一位下标应该是size-1）

5. get 方法

/**
 * Returns the element at the specified position in this list.
 *
 * @param  index index of the element to return
 * @return the element at the specified position in this list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

直接get即可