ArrayList源码分析

1、初始化对象

  1.    /**
  2.      * Default initial capacity.
  3.      */
  4.     private static final int DEFAULT_CAPACITY = 10;
  6.     /**
  7.      * Shared empty array instance used for empty instances.
  8.      */
  9.     private static final Object[] EMPTY_ELEMENTDATA = {};
  11.     /**
  12.      * Shared empty array instance used for default sized empty instances. We
  13.      * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
  14.      * first element is added.
  15.      */
  16.     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
  18.     public ArrayList() {
  19.         this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
  20.     }

默认构造一个长度为0的数组。

2、向集合中添加元素

  1.    public boolean add(E e) {
  2.         ensureCapacityInternal(size + 1);  // Increments modCount!!
  3.         elementData[size++] = e;
  4.         return true;
  5.    }
  6.    private void ensureCapacityInternal(int minCapacity) {
  7.         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
  8.             minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
  9.         }
  11.         ensureExplicitCapacity(minCapacity);
  12.     }
  14.     private void ensureExplicitCapacity(int minCapacity) {
  15.         modCount++;
  17.         // overflow-conscious code
  18.         if (minCapacity - elementData.length > 0)
  19.             grow(minCapacity);
  20.     }
  22.     private void grow(int minCapacity) {
  23.         // overflow-conscious code
  24.         int oldCapacity = elementData.length;
  25.         // 新数组长度 = 旧数组长度 + 旧数组长度/2
  26.         int newCapacity = oldCapacity + (oldCapacity >> 1);
  27.         if (newCapacity - minCapacity < 0)
  28.             newCapacity = minCapacity;
  29.         if (newCapacity - MAX_ARRAY_SIZE > 0)
  30.             newCapacity = hugeCapacity(minCapacity);
  31.         // minCapacity is usually close to size, so this is a win:
  32.         elementData = Arrays.copyOf(elementData, newCapacity);
  33.     }

当像集合中添加元素时，系统会判断是否需要扩容（如果没有没有设置集合的大小，添加元素时系统会默认创建10个大小空间的数组），当此时已经超出数组的容量，就会进行扩容，扩容规则为：将旧数组大小加上旧数组大小右移一位（旧数组大小除以2），创建一个长度为扩容后的数组，并将旧数组里面的数据复制到新数组中，然后向数组尾部插入添加的那个元素。