Thursday, 8 September 2011


Guava part 1 - MultiMaps

Next up on my tour of Guava, is the BiMap, another useful collection type. It's pretty simple really, a BiMap is simply a two way map.

Inverting a Map

A normal java map is a set of keys and values, and you can look up values by key, very useful, eg lets say I wanted to create a (very rudimentary) British English to American English dictionary:
Map<String,String> britishToAmerican = Maps.newHashMap();
But what if you want an American to British dictionary? Well you could write some code to invert the map:
    // Generic method to reverse map.
    public %lt;S,T> Map<T,S> getInverseMap(Map<S,T> map) {
  Map<T,S> inverseMap = new HashMap<T,S>();
  for(Entry<S,T> entry: map.entrySet()) {
   inverseMap.put(entry.getValue(), entry.getKey());
  return inverseMap;
It'll do the job, but there's several complications you might need to think about.
  • How do we handle duplicate values in the original map? At the moment they'll be silently overwritten in the reverse map.
  • What if we want to put a new entry in the reversed map? We'd also have to update the original map! This could get annoying.


Well, guess what? This is the sort of situation a BiMap is designed for! And here's how you might use it.

BiMap<String,String> britishToAmerican = HashBiMap.create();

// Initialise and use just like a normal map

System.out.println(britishToAmerican.get("aubergine")); // eggplant

BiMap<String,String> americanToBritish = britishToAmerican.inverse();

System.out.println(americanToBritish.get("eggplant")); // aubergine
System.out.println(americanToBritish.get("zucchini")); // courgette
Pretty simple really, but there's a few things to notice.

Enforcing uniqueness

Firstly the BiMap enforces uniqueness of it's values, and will give you an illegal argument exception if you try to insert a duplicate value, ie
britishToAmerican.put("sweet","dessert"); // IllegalArgumentException.
If you need to add a values that has already been added there's a forcePut method that will overwrite the entry with the duplicate value.
britishToAmerican.forcePut("sweet","dessert");  // Overwrites the previous entry
System.out.println(britishToAmerican.get("sweet")); // dessert
System.out.println(britishToAmerican.get("pudding")); // null

The inverse method

The other crucial thing to understand is the inverse method, this returns the inverse BiMap, ie the a map with the keys and values switched round.

Now this inverse map, isn't just a new map, such as my earlier reverseMap method might have created. It's actually a view of the of the original map. This means that any subsequent changes to the inverse method will affect the original map!
americanToBritish.put("potato chips","crisps");
System.out.println(britishToAmerican.containsKey("crisps")); // true
System.out.println(britishToAmerican.get("crisps")); // potato chips

So that's the BiMap, like I said pretty simple. As usual there are several implementations available, and as ever I recommend taking a look at the full API documentation:

Next up, Multisets!

Guava part 3 - Multisets


Sebastian Kusnier said...

You have a typo in your samples:
egglant -> eggplant

vatsal mevada said...

Quite helpful ...

Anthony Fedor said...

A bimap (or "bidirectional map") is a map that preserves the uniqueness of its values as well as that of its keys. This constraint enables bimaps to support an "inverse view", which is another bimap containing the same entries as this bimap but with reversed keys and values.

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