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TheCodeAlchemist
TheCodeAlchemist

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You can’t sort Immutable Java Lists?

#programming #beginners #java #tutorial

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I was working on a sorting demo using Comparable and Comparator when I first noticed this.

So, if you create a new List by calling List.of(…) which returns an immutable list then you can’t sort this list in a straightforward manner.

Suppose, we have a list of integers.

List<Integer> nums = List.of(1, 4, 2, 3);
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And, if we try sorting this list via Collections.sort(..) or List.sort(..) then it throws an UnsupportedOperationExcecption.

nums.sort(null); //OR Collections.sort(nums); ---------------------------------- Exception in thread "main" java.lang.UnsupportedOperationException at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142) at java.base/java.util.ImmutableCollections$AbstractImmutableList.sort(ImmutableCollections.java:261) at java.base/java.util.Collections.sort(Collections.java:145) at SortingDemo.main(SortingDemo.java:13)
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So, how can we sort such list. Well, streams can do that.

List<Integer> sortedNums = nums.stream() .sorted() .collect(Collectors.toList());
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Top comments (3)

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alexts1993 profile image
Alex Ci •

You can't do a in place sort to sort a immutable list

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therealdumbprogrammer profile image
TheCodeAlchemist •

Could you please elaborate a bit more? I'm assuming by in-place you mean applying alogs like selection, insertion or heapsort not something JDK in-built, right?

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alexts1993 profile image
Alex Ci •

Sorry I made a mistake, I mean that we can't modify a immutable List only