Search code examples
javadictionarygetnullput

Java map.get(key) - automatically do put(key) and return if key doesn't exist?

I am sick of the following pattern:

value = map.get(key);
if (value == null) {
    value = new Object();
    map.put(key, value);
}

This example only scratches the surface of the extra code to be written when you have nested maps to represent a multi-dimensional structure.

I'm sure something somewhere exists to avoid this, but my Googling efforts yielded nothing relevant. Any suggestions?

Solution

  • The

    java.util.concurrent.ConcurrentMap

    and from Java 8

    Java.util.Map

    has

    putIfAbsent(K key, V value)

    which returns the existing value, and if that is null inserts given value. So if no value exists for key returns null and inserts the given value, otherwise returns existing value

    If you need lazy evaluation of the value there is

    computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)