Java invert map

Question

I need create inverse map - select unique values and for them find keys. Seems that only way is to iterate all key/value pairs, because entrySet returns set of so value not unique? Thanks.

Answer 1

The values in a map may not be unique. But if they are (in your case) you can do as you wrote in your question and create a generic method to convert it:

private static <V, K> Map<V, K> invert(Map<K, V> map) {

    Map<V, K> inv = new HashMap<V, K>();

    for (Entry<K, V> entry : map.entrySet())
        inv.put(entry.getValue(), entry.getKey());

    return inv;
}

Java 8:

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

Example of usage:

public static void main(String[] args) {

    Map<String, Integer> map = new HashMap<String, Integer>();

    map.put("Hello", 0);
    map.put("World!", 1);

    Map<Integer, String> inv = invert(map);

    System.out.println(inv); // outputs something like "{0=Hello, 1=World!}"
}

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Side note: the put(.., ..) method will return the the "old" value for a key. If it is not null you may throw a new IllegalArgumentException("Map values must be unique") or something like that.