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 package org.antlr.runtime.misc;
 
 import java.util.*;

Sometimes we need to map a key to a value but key is two pieces of data. This nested hash table saves creating a single key each time we access map; avoids mem creation.
 
 public class DoubleKeyMap<Key1, Key2, Value> {
	Map<Key1, Map<Key2, Value>> data = new LinkedHashMap<Key1, Map<Key2, Value>>();
	public Value put(Key1 k1, Key2 k2, Value v) {
		Map<Key2, Value> data2 = .get(k1);
		Value prev = null;
		if ( data2==null ) {
			data2 = new LinkedHashMap<Key2, Value>();
			.put(k1data2);
		}
		else {
			prev = data2.get(k2);
		}
		data2.put(k2v);
		return prev;
	}
	public Value get(Key1 k1, Key2 k2) {
		Map<Key2, Value> data2 = .get(k1);
		if ( data2==null ) return null;
		return data2.get(k2);
	}
	public Map<Key2, Value> get(Key1 k1) { return .get(k1); }

Get all values associated with primary key
	public Collection<Value> values(Key1 k1) {
		Map<Key2, Value> data2 = .get(k1);
		if ( data2==null ) return null;
		return data2.values();
	}

get all primary keys
	public Set<Key1> keySet() {
		return .keySet();
	}

get all secondary keys associated with a primary key
	public Set<Key2> keySet(Key1 k1) {
		Map<Key2, Value> data2 = .get(k1);
		if ( data2==null ) return null;
		return data2.keySet();
	}
	public Collection<Value> values() {
		Set<Value> s = new HashSet<Value>();
		for (Map<Key2, Value> k2 : .values()) {
			for (Value v : k2.values()) {
				s.add(v);
			}
		}
		return s;
	}
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