Reduce a Map where the keys are values in some entries

Dr. Mza :

I am trying to make a new Map from a another Map where some of the values are keys in other entries.

Example:

HashMap<String,String> testMap = new HashMap<>();
testMap.put("a","b");
testMap.put("b","d");
testMap.put("d","e");
testMap.put("e","f");
testMap.put("k","r");

I need as result a new Map with this format:

a->f
b->f
d->f
e->f
k->r

producedMap.put("a","f");
producedMap.put("b","f");
producedMap.put("d","f");
producedMap.put("e","f");
producedMap.put("k","r");

My code is that, but seems not give the true result.

    public HashMap<String,String> getMatched(HashMap<String,String> correpondanceMap){

    Collection<String> correpondanceKeys = correpondanceMap.keySet();
    HashMap<String,String> newCorrepondanceMap= new HashMap<>();
    correpondanceMap.entrySet().forEach(entry->{
        if (correpondanceKeys.contains(entry.getValue())){
            String newValue = entry.getValue();
            String keyOfnewValue = correpondanceMap
                    .entrySet()
                    .stream()
                    .filter(entriii -> newValue.equals(entry.getValue()))
                    .map(Map.Entry::getKey).limit(1).collect(Collectors.joining());


            newCorrepondanceMap.put(keyOfnewValue,correpondanceMap.get(newValue));
        }
        else
        {
            newCorrepondanceMap.put(entry.getKey(),entry.getValue());
        }
    });

    newCorrepondanceMap.entrySet().forEach(entry-> System.out.println(entry.getKey() +"  -- > " +entry.getValue()));

    return newCorrepondanceMap;
}

Lino :

You can achieve that by some simple recursion logic in a helper function:

public static String findCorrespondingValue(Map<String, String> map, String key){
    if(map.containsKey(key)){
        return findCorrespondingValue(map, map.get(key));
    }
    return key;
}

As said the logic is very simple, we just check if for a given key a value exists in the given map

• if yes we then execute the function again, but this time with the value as the new key.
• if no mapping exists we can safely say that the key given is the last value in the chain

You can call the method like this:

Map<String, String> testMap = ... // setup testMap

Map<String, String> result = new HashMap<>();
for (final Entry<String, String> entry : testMap.entrySet()) {
    result.put(
        entry.getKey(), 
        findCorrespondingValue(testMap, entry.getValue())
    );
}

or if you happend to use java 8:

Map<String, String> result = testMap.entrySet().stream()
    .collect(Collectors.toMap(
        e -> e.getKey(),  // or just Map.Entry::getKey
        e -> findCorrespondingValue(e.getValue())
     ));

---

You of course would have to implement some kind of logic to find out if you have cyclic references. E.g:

a -> b
b -> f
f -> a

Which would currently just fail with a StackOverflowError.

---

You can make this also generic if you want to support multiple different types, not just String:

public static <T> T findCorrespondingValue(Map<? extends T, ? extends T> map, T key){
    if(map.containsKey(key)){
        return findCorrespondingValue(map, map.get(key));
    }
    return key;
}