初始化(Map,List)容器类的容量会有一定的性能提升

 容器类底层的数据结构大多使用数组，我们知道数组是不能动态修改它的size，假如非要修改的话，只要初始化另外一个更大容量的数据，并将原来的数据复制过去。对容器类的插入操作，首先就得check是否容量满足，如不满足先扩展

 public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

 private void ensureCapacityInternal(int minCapacity) {

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

 private void grow(int minCapacity) {

        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);//扩展为原来容量的1.5倍
        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);//复制操作