基于ArrayList初始化长度的作用及影响

List<String> list = new ArrayList<>();

    
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    
    private static final int DEFAULT_CAPACITY = 10;

    
    transient Object[] elementData; // non-private to simplify nested class access

    
    private static final Object[] EMPTY_ELEMENTDATA = {};    

    
    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);
        }
    }

    
    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);
    }

public static void main(String[] args) {
    final int count = 200 * 10000;
    List<Integer> list = new ArrayList<>();
    long begin = System.currentTimeMillis();
    for(int i = 0; i < count ; i++) {
        list.add(i);
    }
    System.out.println("没有设置ArrayList初始容量： " + (System.currentTimeMillis() - begin) + " ms");

    List<Integer> list2 = new ArrayList<>(10);
    long begin2 = System.currentTimeMillis();
    for(int i = 0; i < count ; i++) {
        list2.add(i);
    }
    System.out.println("设置了ArrayList初始容量： " + (System.currentTimeMillis() - begin2) + " ms");
}

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 static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {// 第一次add的时候，都会走这一步
        return Math.max(DEFAULT_CAPACITY, minCapacity);//初始化容量小于默认值10都会取10，反之取自定义的容量
    }
    return minCapacity;
}

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

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

 private void grow(int minCapacity) {//minCapacity是当前容量，比如，默认容量下，add一次后就是10+1
    // 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);
}

public static void main(String[] args) {
    List<Integer> list = new ArrayList<>(10);
    list.set(0, 666);
}