Utilities & patterns: interview questions & scenarios

Model answer

import java.util.*;

public class Main {
    public static void main(String[] args) {
        String[] raw = {"new", "paid"};

        List<String> fixed = Arrays.asList(raw);
        List<String> mutable = new ArrayList<>(Arrays.asList(raw));
        List<String> immutable = List.of("new", "paid");

        fixed.set(1, "settled");
        mutable.add("refunded");

        System.out.println(Arrays.toString(raw));
        System.out.println(mutable);

        try {
            fixed.add("shipped");
        } catch (UnsupportedOperationException ex) {
            System.out.println("fixed add throws");
        }

        try {
            immutable.set(0, "created");
        } catch (UnsupportedOperationException ex) {
            System.out.println("immutable set throws");
        }

        try {
            List.of("ok", null);
        } catch (NullPointerException ex) {
            System.out.println("List.of rejects null");
        }
    }
}

It prints:

[new, settled]
[new, paid, refunded]
fixed add throws
immutable set throws
List.of rejects null

Arrays.asList(raw) is a fixed-size list backed by the array. set works because it replaces an existing array slot, and the array sees the change. add cannot work because the array length cannot grow.

new ArrayList<>(Arrays.asList(raw)) copies the references into a normal resizable list. List.of(...) creates an unmodifiable, null-rejecting factory list, so add, remove and set all throw.

Model answer

I first ask where the list came from. UnsupportedOperationException usually means the runtime implementation does not support that operation, even though the variable type is List.

Common sources are:

The debugging move is boring and effective: log or inspect orders.getClass().getName() at the boundary, then search the construction path. The fix should match intent. If this is internal working state, copy into a mutable list. If this is an API return value, keep it unmodifiable and change the caller.