import java.time.LocalDate;
import java.util.EnumSet;
import java.util.Objects;
import java.util.Optional;
import java.util.function.Function;
import java.util.function.Supplier;

public class TestLambdaConversion {
	
	public static void main(String[] args) { 
		System.out.println(valueOfKey(Size.class, LocalDate.now())); 
	} 

	enum Size implements Supplier<LocalDate> { 
		S, M, L; 

		@Override 
		public LocalDate get() { 
			return LocalDate.now(); 
		} 

	} 

	public static <K, T extends Enum<T> & Supplier<K>> Optional<T> valueOfKey(Class<T> enumType, K key) { 
		return valueOf(enumType, key, T::get); 
	} 

	public static <K, T extends Enum<T>> Optional<T> valueOf(Class<T> enumType, K key, Function<T, K> keyExtractor) { 
		return EnumSet.allOf(enumType) 
				.stream() 
				.filter(t -> Objects.equals(keyExtractor.apply(t), key)) 
				.findFirst(); 
	} 

}