学步园

错误信息如下：

Exception in thread "main" java.lang.IndexOutOfBoundsException: Source does not fit in dest
at java.util.Collections.copy(Unknown Source)

错误源代码：

import java.util.ArrayList;
import java.util.Collections;

public class test_4 {
	public static void main(String args[]) {
		ArrayList<Integer> List = new ArrayList<Integer>();
		for (int i = 0; i < 10; i++)
			List.add(i); 
		ArrayList<Integer> newList = new ArrayList<Integer>(100);
		Collections.copy(newList, List);
		System.out.println(List.toString());
		System.out.println(List);
		System.out.println(newList);
		List.set(2, 8);
		System.out.println(newList);
	}
}

看下API文档中对于copy方法的介绍如下：

public static <T> void copy(List<? super T> dest,
                            List<? extends T> src)

Type Parameters:

T - the class of the objects in the lists

Parameters:

dest - The destination list.

src - The source list.

Throws:

IndexOutOfBoundsException -
if the destination list is too small to contain the entire source List.

UnsupportedOperationException -
if the destination list's list-iterator does not support the set operation.

错误的方式打印出来的newList.size()==0！！！可是我们明明将其设置为了100！Why?通过仔细观察，发现100并不是指的是newList的size,而是其容量（打个比方，你有一个桶，它的容量很大，但是它是空的！！！）。笔者认识到该错误后，对源码进行了修改，得到了正确的方式，修改如下：

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

public class test_4 {
	public static void main(String args[]) {
		ArrayList<Integer> List = new ArrayList<Integer>();
		for (int i = 0; i < 10; i++)
			List.add(i); 
		ArrayList<Integer> newList = new ArrayList<Integer>(Arrays.asList(new Integer[List.size()]));
		Collections.copy(newList, List);
		System.out.println(List.toString());
		System.out.println(List);
		System.out.println(newList);
		List.set(2, 8);
		System.out.println(newList);
	}
}