Testing Kotlin With Spock, Part 2 - Enum Instance Method

The enum class with an instance method in Kotlin is quite similar to its Java version, but they look a bit different in the bytecode. Let's see the difference by writing some tests using Spock.

What Do We Want to Test?

Let's see the code that we want to test:

enum class EnumWithInstanceMethod {
    PLUS {
        override fun sign(): String = "+"
    },
    MINUS {
        override fun sign(): String = "-"
    };

    abstract fun sign(): String
}

Obviously, it can be written in a better way (e. g., using an enum instance variable), but this example shows the case we want to test in the simplest way.

How to Test it With Spock?

The Simplest Test (That Does Not Work)

First, we can write the test like we would with a Java enum:

def "should use enum method like in java"() {
    expect:
        EnumWithInstanceMethod.MINUS.sign() == '-'
}

The test fails:

Condition failed with Exception:

EnumWithInstanceMethod.MINUS.sign() == '-'
                             |
                             groovy.lang.MissingMethodException: No signature of method: static com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod$MINUS.sign() is applicable for argument types: () values: []
                             Possible solutions: sign(), sign(), is(java.lang.Object), find(), with(groovy.lang.Closure), find(groovy.lang.Closure)


    at com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethodTest.should use enum method like in java(EnumWithInstanceMethodTest.groovy:11)
Caused by: groovy.lang.MissingMethodException: No signature of method: static com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod$MINUS.sign() is applicable for argument types: () values: []
Possible solutions: sign(), sign(), is(java.lang.Object), find(), with(groovy.lang.Closure), find(groovy.lang.Closure)
    ... 1 more

Interesting... why is Groovy telling us that we are trying to call a static method? Maybe we are not using the enum instance but something else? Let's create a test where we pass the enum instance to a method:

static String consume(EnumWithInstanceMethod e) {
    return e.sign()
}

def "should pass enum as parameter"() {
    expect:
        consume(EnumWithInstanceMethod.MINUS) == '-'
}

Error message:

Condition failed with Exception:

consume(EnumWithInstanceMethod.MINUS) == '-'
|
groovy.lang.MissingMethodException: No signature of method: static com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethodTest.consume() is applicable for argument types: (java.lang.Class) values: [class com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod$MINUS]
Possible solutions: consume(com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod)


    at com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethodTest.should pass enum as parameter(EnumWithInstanceMethodTest.groovy:29)
Caused by: groovy.lang.MissingMethodException: No signature of method: static com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethodTest.consume() is applicable for argument types: (java.lang.Class) values: [class com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod$MINUS]
Possible solutions: consume(com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod)
    ... 1 more

Now we see that we passed the class com.github.alien11689.testingkotlinwithspock.EnumWithInstanceMethod$MINUS, not the enum instance.

But it Works in Java...

Analogous code in JUnit works perfectly and the test passes:

@Test
public void shouldReturnSign() {
    assertEquals("-", EnumWithInstanceMethod.MINUS.sign());
}

Java can access Kotlin's instance method without problems, so maybe something is wrong with Groovy...

But the Java enum with instance method, e. g.

public enum EnumWithInstanceMethodInJava {
    PLUS {
        public String sign() {
            return "+";
        }
    },
    MINUS {
        public String sign() {
            return "-";
        }
    };

    public abstract String sign();
}

works correctly in the Spock test:

def "should use enum method"() {
    expect:
        EnumWithInstanceMethodInJava.MINUS.sign() == '-'
}

What's the Difference?

We can spot the difference just by looking at the compiled classes:

$ tree build/classes/main/
build/classes/main/
└── com
    └── github
        └── alien11689
            └── testingkotlinwithspock
                ├── AdultValidator.class
                ├── EnumWithInstanceMethod.class
                ├── EnumWithInstanceMethodInJava$1.class
                ├── EnumWithInstanceMethodInJava$2.class
                ├── EnumWithInstanceMethodInJava.class
                ├── EnumWithInstanceMethod$MINUS.class
                ├── EnumWithInstanceMethod$PLUS.class
                ├── Error.class
                ├── Ok.class
                ├── ValidationStatus.class
                └── Validator.class

Java generates anonymous classes (EnumWithInstanceMethodInJava$1 and EnumWithInstanceMethodInJava$2) for the enum instances, but Kotlin names those classes after the enum instances names (EnumWithInstanceMethod$MINUS and EnumWithInstanceMethod$PLUS).

How does it tie into the problem with Groovy? Groovy does not need the .class when accessing class in the code, so when we try to access EnumWithInstanceMethod.MINUS, Groovy converts it to EnumWithInstanceMethod.MINUS.class, not the instance of the enum. The same problem does not occur in Java code since there is no EnumWithInstanceMethodInJava$MINUS class.

Solution

Knowing the difference, we can solve the problem of accessing Kotlin's enum instance in our Groovy code.

The first solution is accessing the enum instance with valueOf method:

def "should use enum method working"() {
    expect:
        EnumWithInstanceMethod.valueOf('MINUS').sign() == '-'
}

The second way is to tell Groovy explicitly that we want to access the static field which is the instance of enum:

def "should use enum method"() {
    expect:
        EnumWithInstanceMethod.@MINUS.sign() == '-'
}

You can choose either solution depending on the style of your code and your preferences.

Show Me the Code

The code is available here.