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1. Overview

In this quick article, we'll take a look at how to invert a Map in Java. The idea is to create a new instance of Map<V, K> for a given map of type Map<K, V>. In addition, we'll also see how to handle the case where there are duplicate values present in the source map.

Please refer to our other article to learn more about the HashMap class itself.

2. Defining the Problem

Let's consider we have a Map with a few Key-Value pairs:

Map<String, Integer> map = new HashMap<>();
map.put("first", 1);
map.put("second", 2);

The original Map would store the items like:

{first=1, second=2}

Instead, we'd like to invert the keys into values and vice versa into a new Map object. The result would be:

{1=first, 2=second}

3. Using a Traditional for Loop

First, let's see how to invert a Map using a for loop:

public static <V, K> Map<V, K> invertMapUsingForLoop(Map<K, V> map) {
    Map<V, K> inversedMap = new HashMap<V, K>();
    for (Entry<K, V> entry : map.entrySet()) {
        inversedMap.put(entry.getValue(), entry.getKey());
    }
    return inversedMap;
}

Here, we're iterating through the entrySet() of the Map object. After that, we add the original Value as a new Key and the original Key as the new Value into the inversedMap object. In other words, we copy the contents of the map by replacing the keys with values and values with keys. Further, this is suitable for Java versions before 8, though we should note that this approach only works if the source map's values are unique.

4. Using Stream API to Invert a Map

Java 8 provides convenient methods from the Stream API to invert a Map in a more functional style. Let's have a look at a few of them.

4.1. Collectors.toMap()

We can use Collectors.toMap() if we don't have any duplicate values in the source map:

public static <V, K> Map<V, K> invertMapUsingStreams(Map<K, V> map) {
    Map<V, K> inversedMap = map.entrySet()
        .stream()
        .collect(Collectors.toMap(Entry::getValue, Entry::getKey));
    return inversedMap;
}

First, the entrySet() is converted into a stream of objects. Subsequently, we used Collectors.toMap() to collect the Key and Value into the inversedMap object.

Let's consider that the source map contains duplicate values. In such cases, we can use a mapping function to apply custom rules to the input elements:

public static <K, V> Map<V, K> invertMapUsingMapper(Map<K, V> sourceMap) {
    return sourceMap.entrySet()
        .stream().collect(
            Collectors.toMap(Entry::getValue, Entry::getKey, (oldValue, newValue) -> oldValue) 
        );
}

In this method, the last argument to Collectors.toMap() is a mapping function. Using this, we can customize which key should be added in case there are duplicates. In the above example, we retain the first value as key if the source map contains duplicate values. However, we can retain only one key if the values repeat.

4.2. Collectors.groupingBy()

Sometimes, we may need all the keys even if the source map contains duplicate values. Alternatively, Collectors.groupingBy() provides better control for handling duplicate values.

For instance, let's consider we have the following KeyValue pair:

{first=1, second=2, two=2}

Here, the value “2” is repeated twice for different keys. In these cases, we can use the groupingBy() method to implement a cascaded “group by” operation on the Value objects:

private static <V, K> Map<V, List<K>> invertMapUsingGroupingBy(Map<K, V> map) {
    Map<V, List<K>> inversedMap = map.entrySet()
        .stream()
        .collect(Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toList())));
    return inversedMap;
}

To explain a bit, the Collectors.mapping() function performs a reduction operation on the values associated with the given key using the specified collector. The groupingBy() collector collects duplicate values into a List, resulting in a MultiMap. The output now will be:

{1=[first], 2=[two, second]}

5. Conclusion

In this article, we quickly reviewed several built-in ways to invert a HashMap with examples. Also, we saw how to handle duplicate values when we invert a Map object.

Meanwhile, a few external libraries provide additional features on top of the Map interface. We've previously demonstrated how to invert a Map using Google Guava BiMap and Apache BidiMap.

As always, the code for these examples is available over on GitHub.

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