如何初始化ArrayList

最近遇到一个问题

ArrayList list = new ArrayList(5);
list.add(2,"zs");

而居然出现了如下的异常

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 2, Size: 0
at java.util.ArrayList.rangeCheckForAdd(Unknown Source)
at java.util.ArrayList.add(Unknown Source)
at 集合.MyImplement.main(MyImplement.java:18)

后来看了看ArrayList的源码

	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);
}
}

initialCapacity只是指定了他的elementData数组的初始化容量的大小,而size()方法返回的是size属性的大小,两者并无关系

	public int size() {
return size;
}

/**
* 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;
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;

此时size还是0,所以此时getSize()的值也就为0

重点来了

要想解决怎么办呢,就要用到ArrayList的另一个构造函数了

ArrayList list = new ArrayList(Arrays.asList(new Object[5]));
System.out.println(list.size());

此时结果为

5
Process finished with exit code 0

这是因为size = elementData.length 会把此时的数组长度赋值给size,所以得到的值就会为new Object[5]的长度

public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}

最后

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