Collections: List.of is immutable and rejects nulls

The Java 9 factory methods return space-efficient immutable lists. They throw on modification and on null elements, which catches bugs early.

Code

var fixed = List.of("a", "b", "c");
System.out.println(fixed);

try {
    fixed.add("d");
} catch (UnsupportedOperationException e) {
    System.out.println("List.of cannot be modified");
}
try {
    List.of("a", null);
} catch (NullPointerException e) {
    System.out.println("List.of rejects null elements");
}
System.out.println("Arrays.asList allows null: " + Arrays.asList("a", null));

Output

[a, b, c]
List.of cannot be modified
List.of rejects null elements
Arrays.asList allows null: [a, null]

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Published 2026-07-30