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A command line utility and Ant plugin to wrap, build, or examine bundles.
There is a newer version: 2.4.0
package aQute.lib.collections;

import java.util.*;


public class MultiMap extends HashMap> {
	private static final long	serialVersionUID	= 1L;
	final boolean				noduplicates;
	final Class< ? >			keyClass;
	final Class< ? >			valueClass;

	final Set				EMPTY				= Collections.emptySet();

	public MultiMap() {
		noduplicates = false;
		keyClass = Object.class;
		valueClass = Object.class;
	}

	public MultiMap(Class keyClass, Class valueClass, boolean noduplicates) {
		this.noduplicates = noduplicates;
		this.keyClass = keyClass;
		this.valueClass = valueClass;
	}

	public MultiMap(Map> other) {
		this();
		for ( java.util.Map.Entry> e : other.entrySet()) {
			addAll(e.getKey(), e.getValue());
		}
	}
	public MultiMap(MultiMap other) {
		keyClass = other.keyClass;
		valueClass  = other.valueClass;
		noduplicates = other.noduplicates;
		for ( java.util.Map.Entry> e : other.entrySet()) {
			addAll(e.getKey(), e.getValue());
		}
	}

	@SuppressWarnings("unchecked")
	public boolean add(K key, V value) {
		assert keyClass.isInstance(key);
		assert valueClass.isInstance(value);

		List set = get(key);
		if (set == null) {
			set = new ArrayList();
			if (valueClass != Object.class) {
				set = Collections.checkedList(set, (Class) valueClass);
			}
			put(key, set);
		} else {
			if (noduplicates) {
				if (set.contains(value))
					return false;
			}
		}
		return set.add(value);
	}

	@SuppressWarnings("unchecked")
	public boolean addAll(K key, Collection< ? extends V> value) {
		assert keyClass.isInstance(key);
		List set = get(key);
		if (set == null) {
			set = new ArrayList();
			if (valueClass != Object.class) {
				set = Collections.checkedList(set, (Class) valueClass);
			}
			put(key, set);
		} else if (noduplicates) {
			boolean r = false;
			for (V v : value) {
				assert valueClass.isInstance(v);
				if (!set.contains(v))
					r |= set.add(v);
			}
			return r;
		}
		return set.addAll(value);
	}

	public boolean remove(K key, V value) {
		assert keyClass.isInstance(key);
		assert valueClass.isInstance(value);

		List set = get(key);
		if (set == null) {
			return false;
		}
		boolean result = set.remove(value);
		if (set.isEmpty())
			remove(key);
		return result;
	}

	public boolean removeAll(K key, Collection value) {
		assert keyClass.isInstance(key);
		List set = get(key);
		if (set == null) {
			return false;
		}
		boolean result = set.removeAll(value);
		if (set.isEmpty())
			remove(key);
		return result;
	}

	public Iterator iterate(K key) {
		assert keyClass.isInstance(key);
		List set = get(key);
		if (set == null)
			return EMPTY.iterator();
		return set.iterator();
	}

	public Iterator all() {
		return new Iterator() {
			Iterator>	master	= values().iterator();
			Iterator			current	= null;

			public boolean hasNext() {
				if (current == null || !current.hasNext()) {
					if (master.hasNext()) {
						current = master.next().iterator();
						return current.hasNext();
					}
					return false;
				}
				return true;
			}

			public V next() {
				return current.next();
			}

			public void remove() {
				current.remove();
			}

		};
	}

	public Map flatten() {
		Map map = new LinkedHashMap();
		for (Map.Entry> entry : entrySet()) {
			List v = entry.getValue();
			if (v == null || v.isEmpty())
				continue;

			map.put(entry.getKey(), v.get(0));
		}
		return map;
	}

	public MultiMap transpose() {
		MultiMap inverted = new MultiMap();
		for (Map.Entry> entry : entrySet()) {
			K key = entry.getKey();

			List value = entry.getValue();
			if (value == null)
				continue;

			for (V v : value)
				inverted.add(v, key);
		}

		return inverted;
	}

}