ArrayList初始化长度的作用及影响

平时写代码都直接写

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

由于公司做政府项目，对并发和响应没有太苛刻的要求，平时就没有考虑到这一块。今天看同事代码在new ArrayList<>()的时候带入初始容量，于是好奇百度一下，讲结果记录下来。

一、有无初始容量的区别

 / * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; / * Default initial capacity. */ private static final int DEFAULT_CAPACITY = 10; / * 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 / * Shared empty array instance used for empty instances. */ private static final Object[] EMPTY_ELEMENTDATA = { 
   }; / * Constructs an empty list with the specified initial capacity. * * @param initialCapacity the initial capacity of the list * @throws IllegalArgumentException if the specified initial capacity * is negative */ 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); } } / * 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); } 

以上是JDK1.8的ArrayList源码，可以看出，

1. 没有初始容量的话，在做数据操作的时候ArrayList会自己创建容量，JDK1.8默认为10
2. 每次扩容后容量为oldCapacity + (oldCapacity >> 1)
3. 容量最大值Integer.MAX_VALUE – 8

由此可以想到，如果存在上千上万数据量的操作，不初始容量和初始化了合适的容量，处理时间肯定不同，因为初始化和扩容是需要时间的。

测试代码如下：

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"); } 

输出：

没有设置ArrayList初始容量： 96 ms 设置了ArrayList初始容量： 26 ms 

/ * Appends the specified element to the end of this list. * * @param e element to be appended to this list * @return true (as specified by {@link Collection#add}) */ public boolean add(E e) { 
    ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } 

进入方法：

private void ensureCapacityInternal(int minCapacity) { 
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } 

再往下：

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; } 

扩容方法：

private void ensureExplicitCapacity(int minCapacity) { 
    modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } 

grow():

/ * 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) { 
   //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); } 

总结：

1. 建议初始化容量，减少系统初始化容量的耗时；
2. 初始化容量不是越大越好，跟系统配置相关，因为要开辟内存。如果能确定add的总数，以总数作为初始容量效率最高，但这种场景太少了。最佳的设置要兼顾内存空间和扩容次数，我也没有找到最优解，欢迎大佬补充。
3. 尽管不知道初始化多少最快，但是初始化比未初始化快，并且有限的数据量下，设置不同initialCapacity的差距不大。最终，我建议大家初始化容量，并且就写10（<=10都一样，看自己喜好）。

上例不同大小初始容量的耗时：

二、initialCapacity != list.size()

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

console:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at java.util.ArrayList.rangeCheck(ArrayList.java:657) at java.util.ArrayList.set(ArrayList.java:448) at top.chengsw.demo.test.ListTest.main(ListTest.java:25)