Java 数组转换成List，然后执行add或remove抛异常UnsupportedOperationException问题的解决

我就是我 2022-09-25 13:26 89阅读 0赞

**目录**

分析

解决方法

参考

--------------------

### 分析 ###

在使用Arrays.asList()后调用add，remove这些method时出现java.lang.UnsupportedOperationException异常。这是由于Arrays.asList() 返回java.util.Arrays$ArrayList， 而不是ArrayList。Arrays$ArrayList和ArrayList都是继承AbstractList，remove，add等method在AbstractList中是默认throw UnsupportedOperationException而且不作任何操作。ArrayList override这些method来对list进行操作，但是Arrays$ArrayList没有override remove()，add()等，所以throw UnsupportedOperationException。如下代码所示：

package CloudPanSys_Test;
    
    import java.util.Arrays;
    import java.util.List;
    
    public class Spilt {
    
    	public static void main(String[] args) {
    		String path = "/root/m/123.txt";
    		path = getClientPath(path);
    
    	}
    
    	public static String getClientPath(String svrPath) {
    
    		StringBuilder newpath = new StringBuilder();
    		String[] svrPaths = svrPath.split("/");
    		List<String> list = Arrays.asList(svrPaths);
    
    		list.remove(0);//执行抛出异常
    		for (String string : list) {
    			newpath.append("/" + string);
    		}
    		return newpath.toString();
    	}
    
    }

运行后，抛出异常如下：

![Center][]

## 解决方法 ##

使用Iterator，或者转换为ArrayList

List list = Arrays.asList(svrPaths);
    List<String> list2 = new ArrayList<String>(list);
    list2.remove(0);

## 参考 ##

When you call Arrays.asList it does not return a java.util.ArrayList. It returns a java.util.Arrays$ArrayList which is an immutable list. You cannot add to it and you cannot remove from it.  
If you want a mutable list built from your array you will have to loop over the array yourself and add each element into the list in turn.  
Even then your code won't work because you'll get an IndexOutOfBoundsException as you remove the elements from the list in the for loop. There are two options: use an Iterator which allows you to remove from the list as you iterate over it (my recommendation as it makes the code easier to maintain) or loop backwards over the loop removing from the last one downwards (harder to read).  
You are using AbstractList. ArrayList and Arrays$ArrayList are both types of AbstractList. That's why you get UnsupportedOperationException: Arrays$ArrayList does not override remove(int) so the method is called on the superclass, AbstractList, which is what is throwing the exception because this method is not implemented on that class (the reason being to allow you to build immutable subclasses).

[Center]: /images/20220717/44592b4566f340668b1934d4e88cf672.png