SimpleCache.java

import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;


public class SimpleCache<K, V> {

    private final Function<K, V> initialValue;
    private final ConcurrentHashMap<K, V> cache;

    public SimpleCache(Function<K, V> initialValue) {
        this.initialValue = initialValue;
        cache = new ConcurrentHashMap<>();
    }

    public V get(K key) {
        return cache.computeIfAbsent(key, initialValue);
    }
}

Use.java

public static void main(String[] args) {

    SimpleCache<String, String> cache = new SimpleCache<>(key -> "Traitement long...");

    String value = cache.get("key1");
}