Why doesn't Java have a putIfAbsent(key, supplier) method in Map?

Question

I just recently found myself wanting a version of putIfAbsent(...) in java.util.Map that you can supply some kind of factory method to, to instantiate an Object if it's not already present. This would simplify a lot of code.

Here is my modified interface:

import java.util.Map;
import java.util.function.Supplier;

/**
 * Extension of the Map Interface for a different approach on having putIfAbsent
 * 
 * @author Martin Braun
 */
public interface SupplierMap<K, V> extends Map<K, V> {

    public default V putIfAbsent(K key, Supplier<V> supplier) {
        V value = this.get(key);
        if(value == null) {
            this.put(key, value = supplier.get());
        }
        return value;
    }

}

Now my question: Is there another (easier) way of doing this or have I just overlooked something in the Java API?

Answer 1

Isn't computeIfAbsent what you want?

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

Implementation is something like:

if (map.get(key) == null) {
    V newValue = mappingFunction.apply(key);
    if (newValue != null) {
         map.put(key, newValue);
    }
}

So it's not exactly the Supplier<V> signature you've posted but close to that. Having key as argument in the mapping function definitely makes sense.

Answer 2

computeIfAbsent is not a 1:1 replacement for putIfAbsent because constraints for the returned value do not match. While putIfAbsent returns null when a new entry is created, computeIfAbsent always return the assigned value.

A default implementation as suggested above with a call to get and then put works, but it requires two lookups in the map, which destroys the idea of a performant in-situ replacement.