java.lang.UnsupportedOperationException at java.util.AbstractList.add(Unknown Source)异常

在跑Junit测试时报出异常：
java.lang.UnsupportedOperationException

代码如下：

    public String poem(String input) {List<String> inputwords = Arrays.asList(input.split(" "));Graph<String> tempgraph = graph;ListIterator<String> it = inputwords.listIterator();while(it.hasNext()){System.out.println(inputwords.get(it.nextIndex()));if(graph.vertices().contains(inputwords.get(it.nextIndex()))){if(graph.vertices().contains(inputwords.get(it.nextIndex()+1).toString())){for(String temp:graph.targets(inputwords.get(it.nextIndex())).keySet()){if(graph.sources(inputwords.get(it.nextIndex()+1)).containsKey(temp)){System.out.println("add="+temp);it.next();it.add(temp);break;}}}}it.next();}String join = String.join(" ", inputwords);return join;}

原因

查找原因发现我在代码中使用了.asList()，
具体参见asList
调用Arrays.asList()生产的List的add、remove方法时报异常，这是由Arrays.asList() 返回的是Arrays的内部类ArrayList， 而不是java.util.ArrayList。Arrays的内部类ArrayList和java.util.ArrayList都是继承AbstractList，remove、add等方法AbstractList中是默认throw UnsupportedOperationException而且不作任何操作。java.util.ArrayList重新了这些方法而Arrays的内部类ArrayList没有重新，所以会抛出异常。

解决办法：

将该ArrayList复制给新建的一个list即可

修改后代码：

public String poem(String input) {List<String> arrList = Arrays.asList(input.split(" "));List<String> inputwords = new ArrayList<>(arrList);//此处为添加Graph<String> tempgraph = graph;ListIterator<String> it = inputwords.listIterator();while(it.hasNext()){System.out.println(inputwords.get(it.nextIndex()));if(graph.vertices().contains(inputwords.get(it.nextIndex()))){if(graph.vertices().contains(inputwords.get(it.nextIndex()+1).toString())){for(String temp:graph.targets(inputwords.get(it.nextIndex())).keySet()){if(graph.sources(inputwords.get(it.nextIndex()+1)).containsKey(temp)){System.out.println("add="+temp);it.next();it.add(temp);break;}}}}it.next();}String join = String.join(" ", inputwords);return join;}

Arrays.asList()的详细注意事项参见：
其中包含了对该错误的解释