Java陷阱(一)——ArrayList.asList，

Java陷阱(一)——ArrayList.asList，

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.UnsupportedOperationException
    at 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));