Java的ArrayList的IndexOutOfBoundsException异常,尽管给人的初始容量 [英] Java ArrayList IndexOutOfBoundsException despite giving an initial capacity
问题描述
当我这样做
ArrayList<Integer> arr = new ArrayList<Integer>(10);
arr.set(0, 1);
Java的给我
Java gives me
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.set(Unknown Source)
at HelloWorld.main(HelloWorld.java:13)
有一个简单的方法可以让我pre-保留ArrayList的大小,然后立即使用指标,就像数组?
Is there an easy way I can pre-reserve the size of ArrayList and then use the indices immediately, just like arrays?
推荐答案
下面是从的ArrayList
来源:
构造:
public ArrayList(int initialCapacity)
{
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
this.elementData = new Object[initialCapacity];
}
您名为设置(INT,E)
:
public E set(int index, E element)
{
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
设置
通话 rangeCheck(INT)
:
private void rangeCheck(int index)
{
if (index >= size) {
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
}
这可能是微妙的,但是当你调用构造函数,尽管初始化一个对象[]
,你没有初始化尺寸
。因此,从 rangeCheck
,你得到的 IndexOutOfBoundsException异常
,因为尺寸
0,而不是使用设置(INT,E)
,您可以使用添加(E五)
(增加电子
类型电子
到列表的末尾,你的情况:添加(1 )
),这不会发生。或者,如果它适合你,你可以在另一种答案建议初始化为0的所有元素。
It may be subtle, but when you called the constructor, despite initializing an Object[]
, you did not initialize size
. Hence, from rangeCheck
, you get the IndexOutOfBoundsException
, since size
is 0. Instead of using set(int, E)
, you can use add(E e)
(adds e
of type E
to the end of the list, in your case: add(1)
) and this won't occur. Or, if it suits you, you could initialize all elements to 0 as suggested in another answer.
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