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How to implement a Map with multiple keys?
Imagine a table with columns K1, K2 and D. I may need to search data by keys K1 or K2 or both. Is there any 'standard' solution in Java for this apart from creating several maps keeping references?
Consider using guava's Table.
Sample from the documentation
Table<Vertex, Vertex, Double> weightedGraph = HashTable.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
If you wanted to search by only (k1, k2) tuples, I would suggest a MultiKeyMap or, more likely, a wrapper object going into a standard Map. But if you want to search by both, or one, or the other -- it sounds like an embedded SQL engine might be better.
EDIT: depending on your data size, whether you also need a (k1, k2, k3) lookup, how many structures like this you'll need (a SQL engine is relatively heavy), you might be better off with Mairbek Khadikov's answer of a Guava Table.
If you are looking to search data by a range of keys then you can use navigable map.
Look at this code:
NavigableMap<String, String> orig = new TreeMap();
orig.put("1", "A");
orig.add("2", "B");
orig.add("3", "C");
orig.add("4", "D");
orig.add("5", "E");
//this submap will now contain "C", "C"
SortedMap submap = orig.subMap("2", "4");
I would wrap the Map in an object and provide some method that would search for data using both keys. After that, you could simply intersect the 2 result sets and return them. Nice and simple :) .
