1. The purpose of this post

This post would demo how to solve this exception when we iterate and remove an element from an arraylist :

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.AbstractList.remove(AbstractList.java:161)
    at java.util.AbstractList$Itr.remove(AbstractList.java:374)
    at java8.learn.collections.IterRemove.main(IterRemove.java:16)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

2. Environments

  • java 1.8+

3. How to reproduce this exception

List<String> all = Arrays.asList("a","b","c");
for(Iterator<String> iter = all.iterator();iter.hasNext();) {
    if(iter.next().equals("b")) {
        iter.remove();
    }
}
all.forEach(i-> System.out.println(i));

If we run it, we got the above error.

4. How to solve this exception

Change your code like this:

List<String> all = new ArrayList<>(Arrays.asList("a","b","c"));
for(Iterator<String> iter = all.iterator();iter.hasNext();) {
    if(iter.next().equals("b")) {
        iter.remove();
    }
}
all.forEach(i-> System.out.println(i));

rerun the code, then we get this:

a
c

Process finished with exit code 0

5. Why

The asList() method of java.util.Arrays class is used to return a fixed-size list backed by the specified array. This method acts as bridge between array-based and collection-based APIs, in combination with Collection.toArray().

So, if you want to change the result of the Arrays.asList, you must wrap it with a new ArrayList.