public class MemoFunction<T, U> implements Function<T, U> {

    public final Map<T, U> cache = new ConcurrentHashMap<>();
    private final Function<T, U> function;

    public MemoFunction(Function<T, U> function) {
        this.function = function;
    }

    public Function<T, U> cache() {
        return input -> cache.computeIfAbsent(input, function::apply);
    }

    @Override
    public U apply(T t) {
        return function.apply(t);
    }
}