What are the immutable collection factory methods introduced in Java 9?

Java 9 (JEP 269) introduced convenient static factory methods on List, Set, and Map for creating small, immutable collections without the verbosity of Arrays.asList() or Collections.unmodifiableList().

// Java 9+ — concise and immutable
List       list = List.of("a", "b", "c");           // immutable
Set       set  = Set.of(1, 2, 3);                    // immutable, no dups
Map map = Map.of("one", 1, "two", 2);        // up to 10 pairs
Map bigMap = Map.ofEntries(                    // more than 10 pairs
    Map.entry("a", 1),
    Map.entry("b", 2)
);

// Copying (Java 10+)
List copy = List.copyOf(mutableList);  // immutable copy
Set  sCopy = Set.copyOf(existingSet);

// Java 9 vs Arrays.asList
// Arrays.asList: fixed-size but MUTABLE (set() works, add() throws)
// List.of: IMMUTABLE (set(), add(), remove() all throw)

list.add("d");      // UnsupportedOperationException
list.set(0, "z");  // UnsupportedOperationException

// Null not permitted in List.of / Set.of / Map.of
List.of("a", null, "c"); // NullPointerException at creation

// Set.of does not allow duplicates
Set.of(1, 2, 2); // IllegalArgumentException

These factory-created collections are optimised for memory: small sets and maps use compact array-based implementations rather than hash tables when they have few elements. The iteration order of Set.of and Map.of is intentionally unspecified and may vary between JVM runs.

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