ArrayList
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:
add() 的时候需要判断当前数组 size+1 是否等于此时定义的数组大小;
若小于直接添加即可; 否则, 需要先扩容再进行添加.
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
- /**
- * 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); // 进行扩容校验
- elementData[size++] = e; // 将值添加到数组后面, 并将 size+1
- return true;
- }
- /**
- * 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
- private void ensureCapacityInternal(int minCapacity) {
- ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); // elementData 数组
- }
- /**
- * Default initial capacity.
- */
- private static final int DEFAULT_CAPACITY = 10;
- /**
- * 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 = {};
- // 返回最大的 index
- private static int calculateCapacity(Object[] elementData, int minCapacity) {
- if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { // 与空数组实例对比
- return Math.max(DEFAULT_CAPACITY, minCapacity);
- }
- return minCapacity;
- }
- private void ensureExplicitCapacity(int minCapacity) {
- modCount++;
- // overflow-conscious code
- if (minCapacity - elementData.length> 0)
- grow(minCapacity);
- }
扩容调用方法, 实际也就是数组复制的过程
- /**
- * 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);
- }
- add(int index, E element)
源码分析
- /**
- * 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); // 校验 index 是否超过当前定义的数组大小范围, 超过则抛出 IndexOutOfBoundsException
- ensureCapacityInternal(size + 1); // Increments modCount!!
- System.arraycopy(elementData, index, elementData, index + 1,
- size - index); // 复制, 向后移动
- elementData[index] = element;
- size++;
- }
- /**
- * A version of rangeCheck used by add and addAll.
- */
- private void rangeCheckForAdd(int index) {
- if (index> size || index < 0)
- throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
- }
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
封装了一个动态再分配的对象数组
使用索引进行随机访问效率高
缺陷
在数组中增删一个元素, 所有元素都要往后往前移动, 效率低下
知识脑图
From Java Core Knowledge Tree
ArrayList
来源: http://www.jianshu.com/p/e18b358357b0