An unmodifiable set of 2 elements, with predictable iteration order.

Set2.java

import java.util.*;

/**
 * A set of 2 elements. Unfortunately, {@link Set#of(Object, Object)}
 * does not guarantee stable iteration order across JVM
 * instances.
 */
public final class Set2<T> extends AbstractSet<T> {

    private final T e0;
    private final T e1;


    Set2(T e0, T e1) {
        this.e0 = Objects.requireNonNull(e0);
        this.e1 = Objects.requireNonNull(e1);
    }


    @Override
    public Iterator<T> iterator() {
        return new Iterator<T>() {
            int idx = 0;


            @Override
            public boolean hasNext() {
                return idx < 2;
            }


            @Override
            public T next() {
                switch (idx++) {
                case 0: return e0;
                case 1: return e1;
                default:
                    throw new NoSuchElementException();
                }
            }
        };
    }


    @Override
    public int size() {
        return 2;
    }
}