Enum List Removal Issue

Question

Can anyone help me to get the issue ?

public static void main(String[] args) 
{
    List<TestEnum> list1 = new ArrayList<TestEnum>();
    list1.add(TestEnum.ONE);
    list1.add(TestEnum.TWO);
    list1.add(TestEnum.THREE);

    System.out.println(list1);
    System.out.println(list1.remove(TestEnum.TWO));
    System.out.println(list1);

    System.out.println("-----------------------");

    TestEnum[] xx = new TestEnum[]{TestEnum.ONE, TestEnum.TWO, TestEnum.THREE};
    List<TestEnum> list2 = Arrays.asList(xx);

    System.out.println(list2);
    System.out.println(list2.remove(TestEnum.TWO));
    System.out.println(list2);
}

Below is the Result -

[ONE, TWO, THREE]
true
[ONE, THREE]
-----------------------
[ONE, TWO, THREE]
Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.remove(Unknown Source)
at java.util.AbstractList$Itr.remove(Unknown Source)
at java.util.AbstractCollection.remove(Unknown Source)
at Test.main(Test.java:24)

Please help - Why this is happening, I have also checked hashcode ?

list returned by `Arrays.asList()` doesn't support `.remove()` — Alex Salauyou, Apr 01 '16 at 11:44
Possible duplicate of [Difference between Arrays.asList(array) vs new ArrayList(Arrays.asList(ia)) in java](http://stackoverflow.com/questions/16748030/difference-between-arrays-aslistarray-vs-new-arraylistintegerarrays-aslist) — jhamon, Apr 01 '16 at 11:44

Alex Salauyou · Accepted Answer · 2016-04-01T11:51:18.360

List returned by Arrays.asList() has fixed size--the size of array it is backed by. It doesn't support operations that may lead to size change: add(), addAll(), remove(), removeAll(), retainAll(), clear().

You may use the following idiom to create a list of modifiable size:

List<TestEnum> = new ArrayList<>(Arrays.asList(xx));

this will create an ordinary ArrayList initialized by elements copied from result of Arrays.asList().

score 2 · Answer 2 · answered Apr 01 '16 at 11:45

2

Arrays.asList returns instance of immutable list, on which remove cannot be called. You have to wrap it in new list

new ArrayList(Arrays.asList(xx))

answered Apr 01 '16 at 11:45

sidgate

14,650
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`Arrays.asList()` is not immutable since you can modify it calling `.set()` – Alex Salauyou Apr 03 '19 at 14:33

score 2 · Answer 3 · answered Apr 01 '16 at 11:55

If you take a look into the method asList of the Arrays class, you will see that a new ArrayList<>() is returned. But the object which is instantied there is another private ArrayList class from inside the Arrays class. This class extends AbstractList like the normal ArrayList does, but it does not override the remove method. And this is the original implementation of the remove method.

/**
 * {@inheritDoc}
 *
 * <p>This implementation always throws an
 * {@code UnsupportedOperationException}.
 *
 * @throws UnsupportedOperationException {@inheritDoc}
 * @throws IndexOutOfBoundsException     {@inheritDoc}
 */
public E remove(int index) {
    throw new UnsupportedOperationException();
}

justAbit · Answer 4 · 2016-04-01T11:52:37.880

1

Arrays.asList returns a special ArrayList object which does not support any operation which would modify it's size.

From the documentation of this method:

This method also provides a convenient way to create a fixed-size list initialized to contain several elements.

edited Apr 01 '16 at 11:52

answered Apr 01 '16 at 11:47

justAbit

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2

it actually does support modification methods, but only those that don't change the size. – Alex Salauyou Apr 01 '16 at 11:49

Enum List Removal Issue

4 Answers4