Many people are posting Pair code that is usable as a key in a Map...If you're trying to use a pair as a hashing key (a common idiom), be sure to check out Guava's Table<R,C,V>: http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Table. They give the following example usage, for graph edges:

Table<Vertex, Vertex, Double> weightedGraph = HashBasedTable.create();
weightedGraph.put(v1, v2, 4);
weightedGraph.put(v1, v3, 20);
weightedGraph.put(v2, v3, 5);

weightedGraph.row(v1); // returns a Map mapping v2 to 4, v3 to 20
weightedGraph.column(v3); // returns a Map mapping v1 to 20, v2 to 5

There are array, tree, and other implementations for both dense and sparse uses, with the option of specifying your own intermediate map classes.

Many people are posting Pair code that is usable as a key in a Map...If you're trying to use a pair as a hashing key (a common idiom), be sure to check out Guava's Table<R,C,V>: http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Table. They give the following example usage, for graph edges:

Table<Vertex, Vertex, Double> weightedGraph = HashBasedTable.create();
weightedGraph.put(v1, v2, 4);
weightedGraph.put(v1, v3, 20);
weightedGraph.put(v2, v3, 5);

weightedGraph.row(v1); // returns a Map mapping v2 to 4, v3 to 20
weightedGraph.column(v3); // returns a Map mapping v1 to 20, v2 to 5

A Table maps two keys to a single value, and provides efficient lookups for both types of keys alone as well. I've started using this data structure instead of a Map<Pair<K1,K2>, V> in many parts of my code. There are array, tree, and other implementations for both dense and sparse uses, with the option of specifying your own intermediate map classes.

If you're trying to use a pair as a hashing key (a common idiom), be sure to check out Guava's Table<R,C,V>: http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Table. They give the following example usage, for graph edges:

Table<Vertex, Vertex, Double> weightedGraph = HashBasedTable.create();
weightedGraph.put(v1, v2, 4);
weightedGraph.put(v1, v3, 20);
weightedGraph.put(v2, v3, 5);

weightedGraph.row(v1); // returns a Map mapping v2 to 4, v3 to 20
weightedGraph.column(v3); // returns a Map mapping v1 to 20, v2 to 5

There are array, tree, and other implementations for both dense and sparse uses, with the option of specifying your own intermediate map classes.

Many people are posting Pair code that is usable as a key in a Map...If you're trying to use a pair as a hashing key (a common idiom), be sure to check out Guava's Table<R,C,V>: http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Table. They give the following example usage, for graph edges:

Table<Vertex, Vertex, Double> weightedGraph = HashBasedTable.create();
weightedGraph.put(v1, v2, 4);
weightedGraph.put(v1, v3, 20);
weightedGraph.put(v2, v3, 5);

weightedGraph.row(v1); // returns a Map mapping v2 to 4, v3 to 20
weightedGraph.column(v3); // returns a Map mapping v1 to 20, v2 to 5

There are array, tree, and other implementations for both dense and sparse uses, with the option of specifying your own intermediate map classes.