gistfile1.java

import java.util.HashMap;

public class EmptyCollectionMap extends HashMap {
	Class collectionType;

	public EmptyCollectionMap(Class collectionType) {
		super();
		this.collectionType = collectionType;
	}

	public Object get(Object key) {
		Object collection;
		if (containsKey(key))
			collection = super.get(key);
		else {
			try {
				collection = collectionType.newInstance();
			} catch (Exception ex) {
				throw new RuntimeException(ex);
			}
			put(key, collection);
		}
		return collection;
	}
}