基于ArrayList初始化长度的作用及影响_开发_开发者

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 编程客栈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 Illega编程客栈lArgumentException("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)
      n开发者_Python开发ewCapacity = 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，

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
每次扩容后容量为oldCapacity + (oldCapacity >> 1)
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

分析：

在list.add()方法执行时，先调用ArrayList的：

/**
* 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); // 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：

建议初始化容量，减少系统初始化容量的耗时；
初始化android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。
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：

initialCapacityphp	time
未初始化	96
<=10	26
100	26
1000	23
10000	648
100000	24
1000000	18
10000000	609

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)

此时，list.size() = 0。

也就是说，该构造方法编程并不是将ArrayList()初始化为指定长度，而是指定了其内部的Object数组的长度，也就是其容量。

当我们调用size()时，返回的是其实际长度，而非容量大小。

对超出ArrayList长度的部分进行访问或赋值操作时也会造成访问越界，尽管它的容量大小足够。