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If you have a List<String> strings instance, would you keep writing:
Collections.unmodifiableList(strings)
or switch to:
List.of(strings.toArray(new String[strings.size()]))
What's the initial impact in performance (memory- and runtime-wise) of instantion? Is there a runtime benefit in the List.of variant?
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The unmodifiableList() method of Java Collections class is used to get an unmodifiable view of the specified list. If any attempt occurs to modify the returned list whether direct or via its iterator, results in an UnsupportedOperationException.
The unmodifiableList() method of java. util. Collections class is used to return an unmodifiable view of the specified list. This method allows modules to provide users with “read-only” access to internal lists.
The unmodifiable view of the specified ArrayList can be obtained by using the method java. util. Collections. unmodifiableList().
The solution to this problem is quite simple and is highlighted in the following code. final List<String> modifiable = new ArrayList<>(); modifiable. add("Java"); modifiable. add("is"); // Here we are creating a new array list final List<String> unmodifiable = Collections.
It is the list for which an unmodifiable view is to be returned. The unmodifiableList () method returns an unmodifiable view of the specified list. System.out.print ("Unmodifiable List after adding element to the list:");
The unmodifiableList () method of Java Collections class is used to get an unmodifiable view of the specified list. If any attempt occurs to modify the returned list whether direct or via its iterator, results in an UnsupportedOperationException.
Collections unmodifiableList () method returns an unmodifiable “read-only” view of the specified list. Any attempt to modify the returned list directly or via its iterator will result in an UnsupportedOperationException. However, any changes made to the original list will be reflected in the unmodifiable list.
If you need unmodifiable and immutable list or in other words unmodifiable copy of the source list without any dependency to other libraries try this: Collections.unmodifiableList(Collections.list(Collections.enumeration(sourceList)))
This is not really a good comparison because these approaches do different things:
Collections::unmodifiable... creates an unmodifiable view. It is not immutable because it changes if you're changing the original, backing collection (list in your example)....::of on the other hand, creates an immutable copy. Changing the original list will not affect it.From a performance view it is obvious that creation of the unmodifiable wrapper is cheaper because it only creates one instance with a single field. The new factory methods will create at least one object, maybe backed by an array (if you have three or more elements), that it needs to copy into.
Access could be faster on the new immutable collections but that would have to be benchmarked.
But correctness trumps performance. What do you need? If you need an immutable copy, use the new methods (or Guava's Immutable..., which I would prefer). If you need something immutable, use unmodifiable... and throw away the original (and make sure it stays like that). If you need a view that your caller can not edit, use unmodifiable....
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According to JEP 269 (Convenience Factory Methods for Collections):
Goals
Provide static factory methods on the collection interfaces that will create compact, unmodifiable collection instances. The API is deliberately kept minimal.
Non-Goals
It is not a goal to provide a fully-general "collection builder" facility that, for example, lets the user control the collection implementation or various characteristics such as mutability, expected size, loading factor, concurrency level, and so forth.
It is not a goal to support high-performance, scalable collections with arbitrary numbers of elements. The focus is on small collections.
It is not a goal to provide unmodifiable collection types. That is, this proposal does not expose the characteristic of unmodifiability in the type system, even though the proposed implementations are actually unmodifiable.
It is not a goal to provide "immutable persistent" or "functional" collections.
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