MostlySingleton.java

package org.checkerframework.dataflow.util;

import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Set;

/**
 * A set that is more efficient than HashSet for 0 and 1 elements.
 */
final public class MostlySingleton<T> implements Set<T> {
    private enum State {
        EMPTY, SINGLETON, ANY
    }
    private State state = State.EMPTY;
    private T value;
    private HashSet<T> set;

    @Override
    public int size() {
        switch (state) {
        case EMPTY:
            return 0;
        case SINGLETON:
            return 1;
        case ANY:
            return set.size();
        default:
            throw new AssertionError();
        }
    }

    @Override
    public boolean isEmpty() {
        return size() == 0;
    }

    @Override
    public boolean contains(Object o) {
        switch (state) {
        case EMPTY:
            return false;
        case SINGLETON:
            return Objects.equals(o, value);
        case ANY:
            return set.contains(o);
        default:
            throw new AssertionError();
        }
    }

    @Override
    @SuppressWarnings("fallthrough")
    public boolean add(T e) {
        switch (state) {
        case EMPTY:
            state = State.SINGLETON;
            value = e;
            return true;
        case SINGLETON:
            state = State.ANY;
            set = new HashSet<T>();
            set.add(value);
            value = null;
            // fallthrough
        case ANY:
            return set.add(e);
        default:
            throw new AssertionError();
        }
    }

    @Override
    public Iterator<T> iterator() {
        switch (state) {
        case EMPTY:
            return Collections.emptyIterator();
        case SINGLETON:
            return new Iterator<T>() {
                private boolean hasNext = true;

                @Override
                public boolean hasNext() {
                    return hasNext;
                }

                @Override
                public T next() {
                    if (hasNext) {
                        hasNext = false;
                        return value;
                    }
                    throw new NoSuchElementException();
                }

                @Override
                public void remove() {
                    throw new UnsupportedOperationException();
                }
            };
        case ANY:
            return set.iterator();
        default:
            throw new AssertionError();
        }
    }

    @Override
    public Object[] toArray() {
        throw new UnsupportedOperationException();
    }

    @Override
    public <S> S[] toArray(S[] a) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(Object o) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean containsAll(Collection<?> c) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean addAll(Collection<? extends T> c) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean retainAll(Collection<?> c) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean removeAll(Collection<?> c) {
        throw new UnsupportedOperationException();
    }

    @Override
    public void clear() {
        throw new UnsupportedOperationException();
    }
}