Remove duplicate values from HashMap in Java

Question

I have a map with duplicate values:

(\"A\", \"1\");
(\"B\", \"2\");
(\"C\", \"2\");
(\"D\", \"3\");
(\"E\", \"3\");

I would like to the map

Answer 1

Assuming that you use Java 8, it could be done using the Stream API with a Set that will store the existing values:

Map map = new HashMap<>();
map.put("A", "1");
...
System.out.printf("Before: %s%n", map);

// Set in which we keep the existing values
Set existing = new HashSet<>();
map = map.entrySet()
    .stream()
    .filter(entry -> existing.add(entry.getValue()))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
System.out.printf("After: %s%n", map);     

Output:

Before: {A=1, B=2, C=2, D=3, E=3}
After: {A=1, B=2, D=3}

NB: Strictly speaking a predicate of a filter is not supposed to be stateful, it should be stateless as mentioned into the javadoc in order to ensure that the result remain deterministic and correct even if we use a parallel stream. However here, I assume that you don't intend to use a parallel stream such that this approach remains valid.