Java - Unique Enum Field values

September 24, 2024

When you have a Java Enum with a field, a requirement can be that the field value needs to be unique. Using a utility method it can be easy to create a test for this. Given an Enum class with a field (using some Lombok):

@Getter
@RequiredArgsConstructor
@Accessors(fluent = true)
public enum Sides {  
    LEFT("left"),  
    RIGHT("right");  
  
    final String label;
}

You can enforce uniqueness using a unit test:

  
import io.vavr.collection.HashMap;  
import io.vavr.collection.Vector;  
import org.junit.jupiter.api.Test;  
  
import java.util.function.Function;  
  
import static org.junit.jupiter.api.Assertions.assertEquals;  
  
class SidesTest {  
    @Test  
    void unique() {  
        assertUnique(Sides.values(), Sides::label);  
    }  
  
    private static <T> void assertUnique(T[] values, Function<T, String> fieldValue) {  
        assertEquals(  
            HashMap.empty(),  
            Vector.of(values)  
                .groupBy(fieldValue)  
                .filter(it -> it._2().size() > 1));  
    }  
}

The test will automatically validate that all fields have distinct values, even if the Enum expands or changes in the future. Failures will report the enum values that have the same property. If we misconfigure the enum above ( RIGHT("left")) we see which value occurred multiple times, and in which Enum values:

Expected :HashMap()
Actual   :LinkedHashMap((left, Vector(LEFT, RIGHT)))