# Use Factory Methods for Collections
# Description
Java 9 introduced factory methods for creating immutable collections.
This rule replaces the invocations of Collections.unmodifiable{List|Set|Map} with the corresponding factory method List.of (opens new window), Set.of (opens new window) and Map.ofEntries (opens new window) accordingly.
Requirements
Java 9
# Benefits
Remove verbosity and avoid creating intermediate collections only for the sake of initialization.
# Tags
# Code Changes
# Creating immutable List
Pre
List<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");
list = Collections.unmodifiableList(list);
Post
List<String> list = List.of("a", "b", "c");
# Creating immutable List with anonymous class
Pre
List<String> list = Collections.unmodifiableList(new ArrayList<String>() {{
add("a");
add("b");
add("c");
}});
Post
List<String> list = List.of("a", "b", "c");
# Creating immutable List form array
Pre
List<String> list = Collections.unmodifiableList(Arrays.asList("a", "b", "c"));
Post
List<String> list = List.of("a", "b", "c");
# Creating immutable Set
Pre
Set<String> set = new HashSet<>();
set.add("a");
set.add("b");
set.add("c");
set = Collections.unmodifiableSet(set);
Post
Set<String> set = Set.of("a", "b", "c");
# Creating immutable Map
Pre
Map<String, String> map = new HashMap<>();
map.put("1", "one");
map.put("2", "two");
map.put("3", "three");
map = Collections.unmodifiableMap(map);
Post
Map<String, String> map = Map.ofEntries(entry("1", "one"), entry("2", "two"), entry("3", "three"));
🛠️ Auto-refactor Available
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# Properties
| Property | Value |
|---|---|
| Rule ID | CollectionsFactoryMethods |
| First seen in jSparrow version | 3.6.0 |
| Minimum Java version | 9 |
| Remediation cost | 5 min |