ArrayList 是通过一个数组来实现的,因此它是在连续的存储位置存放对象的引用,只不过它比 Array 更智能,能够根据集合长度进行自动扩容。
假设让我们来实现一个简单的能够自动扩容的数组,我们最容易想到的点就是:
实际上,ArrayList的内部实现原理也是这样子,我们可以来研究分析一下ArrayList的源码
add(E e)
源码分析 1 /**
2 * Appends the specified element to the end of this list.
3 *
4 * @param e element to be appended to this list
5 * @return <tt>true</tt> (as specified by {@link Collection#add})
6 */
7 public boolean add(E e) {
8 ensureCapacityInternal(size + 1); // 进行扩容校验
9 elementData[size++] = e; // 将值添加到数组后面,并将 size+1
10 return true;
11 }
12
13
14
15 /**
16 * The array buffer into which the elements of the ArrayList are stored.
17 * The capacity of the ArrayList is the length of this array buffer. Any
18 * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
19 * will be expanded to DEFAULT_CAPACITY when the first element is added.
20 */
21 transient Object[] elementData; // non-private to simplify nested class access
22
23 private void ensureCapacityInternal(int minCapacity) {
24 ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); // elementData 数组
25 }
26
27
28
29 /**
30 * Default initial capacity.
31 */
32 private static final int DEFAULT_CAPACITY = 10;
33
34 /**
35 * Shared empty array instance used for default sized empty instances. We
36 * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
37 * first element is added.
38 */
39 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
40
41 // 返回最大的 index
42 private static int calculateCapacity(Object[] elementData, int minCapacity) {
43 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { // 与空数组实例对比
44 return Math.max(DEFAULT_CAPACITY, minCapacity);
45 }
46 return minCapacity;
47 }
48
49
50
51 private void ensureExplicitCapacity(int minCapacity) {
52 modCount++;
53
54 // overflow-conscious code
55 if (minCapacity - elementData.length > 0)
56 grow(minCapacity);
57 }
扩容调用方法,实际也就是数组复制的过程
1 /**
2 * Increases the capacity to ensure that it can hold at least the
3 * number of elements specified by the minimum capacity argument.
4 *
5 * @param minCapacity the desired minimum capacity
6 */
7 private void grow(int minCapacity) {
8 // overflow-conscious code
9 int oldCapacity = elementData.length;
10 int newCapacity = oldCapacity + (oldCapacity >> 1);
11 if (newCapacity - minCapacity < 0)
12 newCapacity = minCapacity;
13 if (newCapacity - MAX_ARRAY_SIZE > 0)
14 newCapacity = hugeCapacity(minCapacity);
15 // minCapacity is usually close to size, so this is a win:
16 elementData = Arrays.copyOf(elementData, newCapacity);
17 }
add(int index, E element)
源码分析 1 /**
2 * Inserts the specified element at the specified position in this
3 * list. Shifts the element currently at that position (if any) and
4 * any subsequent elements to the right (adds one to their indices).
5 *
6 * @param index index at which the specified element is to be inserted
7 * @param element element to be inserted
8 * @throws IndexOutOfBoundsException {@inheritDoc}
9 */
10 public void add(int index, E element) {
11 rangeCheckForAdd(index); // 校验index是否超过当前定义的数组大小范围,超过则抛出 IndexOutOfBoundsException
12
13 ensureCapacityInternal(size + 1); // Increments modCount!!
14 System.arraycopy(elementData, index, elementData, index + 1,
15 size - index); // 复制,向后移动
16 elementData[index] = element;
17 size++;
18 }
19
20
21 /**
22 * A version of rangeCheck used by add and addAll.
23 */
24 private void rangeCheckForAdd(int index) {
25 if (index > size || index < 0)
26 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
27 }
从上面的源码分析可知,扩容和随机插入元素的消耗比较大,因此在实际开发中,应尽量指定ArrayList大小,减少在随机插入操作。
优点
缺陷
在 github 上建了一个 repository ,Java Core Knowledge Tree,各位看官若是喜欢请给个star,以示鼓励,谢谢。 https://github.com/suifeng412/JCKTree
(以上是自己的一些见解,若有不足或者错误的地方请各位指出)