一、问题代码

    话不多说，直接上问题代码：

package com.pajk.recsys.dk.test;import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;import com.pajk.recsys.utils.CommonUtils;public class CommonTest {public static List<String> UnsupportedOperationExceptionTest(List<String> source){source.add("12312");return source;}public static void main(String args[]){String str = "123,456,7899";String[] items = str.trim().split(",");List<String> realAdd = Arrays.asList(items);realAdd.add("123123123");List<String> xxx = UnsupportedOperationExceptionTest(realAdd);System.out.println(xxx);}}

上述代码抛出异常：

Exception in thread "main" java.lang.UnsupportedOperationExceptionat java.util.AbstractList.add(Unknown Source)at java.util.AbstractList.add(Unknown Source)at com.pajk.recsys.dk.test.CommonTest.main(CommonTest.java:20)

问题出在Arrays.asList(items)。

二、Arrays.asList() 源码

private static final class ArrayList<E> extends AbstractList<E>implements Serializable, RandomAccess{// We override the necessary methods, plus others which will be much// more efficient with direct iteration rather than relying on iterator()./*** Compatible with JDK 1.4.*/private static final long serialVersionUID = -2764017481108945198L;/*** The array we are viewing.* @serial the array*/private final E[] a;/*** Construct a list view of the array.* @param a the array to view* @throws NullPointerException if a is null*/ArrayList(E[] a){// We have to explicitly check.if (a == null)throw new NullPointerException();this.a = a;}
....public static <T> List<T> asList(final T... a){return new Arrays.ArrayList(a);}

由上边代码可知， asList返回一个final的,固定长度的ArrayList类，并不是java.util.ArrayList, 所以直接利用它无法执行改变list长度的操作， 比如 add、remove等。

三、修改方法

List<String> realAdd = new ArrayList(Arrays.asList(items));