Create mutable collections (Map.of and List.of)

Question

Looks like Map.of() and List.of() both create immutable collections. That's useful, but I am looking for a way to create mutable collections sometimes using a factory method.

When I try: HashMap.of() I get this error:

Static method may be invoked on containing interface class only

Answer 1

HashMap has a constructor that can take another Map , so you can create a HashMap by passing it whatever Map.of(...) produces:

HashMap<String, String> map = new HashMap<>(Map.of("k1", "v1", "k2", "v2"));

Likewise for ArrayList and List.of(...) .

Answer 2

I use a custom method inside of the utils class. Please try, it works correctly.

public static Map<String, Object> mutableMap(Object... args) {
        return IntStream.iterate(0, i -> i < args.length, i -> i + 2)
                        .filter(i -> i + 1 < args.length)
                        .boxed()
                        .collect(toMap(i -> (String) args[i], i -> args[i + 1], (a, b) -> b, HashMap::new));
}

Also, you can add validation on paired number of arguments OR add logic to replace the 'null' value according to your requires.