removing a value from a list<String> in java throws java.lang.UnsupportedOperationException

Question

I want to remove specific elements from my List. I don't want to do this while iterating through the list. I want to specify the value which has to be deleted. In javadocs I found the function List.remove(Object 0) This is my code :

         String str="1,2,3,4,5,6,7,8,9,10";
         String[] stra=str.split(",");
         List<String> a=Arrays.asList(stra);
         a.remove("2");
         a.remove("3");

But I get an Exception : java.lang.UnsupportedOperationException

NPE · Accepted Answer · 2012-05-17 13:10:34Z

26

The problem is that Arrays.asList() returns a list that doesn't support insertion/removal (it's simply a view onto stra).

To fix, change:

List<String> a = Arrays.asList(stra);

to:

List<String> a = new ArrayList<String>(Arrays.asList(stra));

This makes a copy of the list, allowing you to modify it.

edited May 17, 2012 at 13:10

answered May 17, 2012 at 12:59

NPE

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3 Comments

Ashwin Over a year ago

It works. thanks What was wrong with my method. List<String> a=Arrays.asList(stra); The exception was not thrown in this line.

NPE Over a year ago

@Ashwin: That line was fine. However, the list produced by Arrays.asList() is read-only, so you can't modify it. You have to make a copy first.

Ashwin Over a year ago

okay.. with my method was the list just accessing the string[] values directly without making a copy of its own? So if any changes were allowed they would affect the String[] also right?

Eshan Sudharaka · Accepted Answer · 2012-05-17 14:05:13Z

3

http://docs.oracle.com/javase/6/docs/api/java/util/Arrays.html#asList%28T...%29

See this. Arrays.asList returns a fixed list. Which is an immutable one. By its definition you cant modify that object once it creates.Thats why it is throwing unsupported exception.

answered May 17, 2012 at 14:05

Eshan Sudharaka

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6 Comments

Ashwin Over a year ago

A string is also immutable. But when you modify a string it gets allocated a new memory location and internally a new string is created. So effectively you can modify an immutable string. So I don't think here the reason is that the list returned is immutable. I think it is because when you are doing Arrays.asList() it just references the string[](does not copy the string[] and make a list).

Eshan Sudharaka Over a year ago

If you have a look into the inner class call AraayList in Arrays class you will realize the fact that it cant be modify. Simply it use a private final E[] a; that is the reason you cant modify the returning list. In the case of string you cant modify a string once you created. But in string class API is developed in a manner it will handle modifications internally by creating a new object.

Ashwin Over a year ago

So you are saying that it creates a copy of the string[] but declares it as final. are you saying that it does not reference the string[] and stores a copy of its own(but as final)?

Eshan Sudharaka Over a year ago

@SafeVarargs public static <T> List<T> asList(T... a) { return new ArrayList<>(a); } private final E[] a; ArrayList(E[] array) { if (array==null) throw new NullPointerException(); a = array; } I copied the code from API.

Ashwin Over a year ago

okay.. so it does create a new arraylist object. But assigns it to a final e[]. I still don't realize the reason why it is declared as final in java. Thanks for the information:)

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