Kotlin在Java项目中的高阶函数

Question

I have a function that has a function type as a formal parameter:

fun doSomething(code: () -> Boolean) = false //package function in TestKt.class

I've tried calling it in Java, passing in a lambda:

//Java class, in Java project
class Demo {
    public static void main(String[] args) {
        TestKt.doSomething(() -> false);
    }
}

But I get the error:

Cannot infer functional interface type

It works when the Java class is in a Kotlin project, but not in a Java project. I haven't had problems with anything else while using classes from Kotlin in my Java project, such as using kotlin methods typed with kotlin.Boolean and working with vararg parameters.

Question

How do I properly call the doSomething function from a Java project?

Answer 1

I threw your code into a module in my project and it built just fine, though this may be a side effect of me using multiple modules as the kotlin is always compiled to java before the java projects can touch it.

[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] mesh-parent ....................................... SUCCESS [1.718s]
[INFO] mesh-common ....................................... SUCCESS [13.141s]
[INFO] mesh-controller ................................... SUCCESS [8.217s]
[INFO] java-so-project ................................... SUCCESS [1.121s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 24.558s
[INFO] Finished at: Thu Jan 07 20:21:30 GMT 2016
[INFO] Final Memory: 55M/341M
[INFO] ------------------------------------------------------------------------

Process finished with exit code 0

If you are using maven (with mixed kotlin and java in a single module) then you may be required to add the following to your POM:

<plugin>
    <artifactId>kotlin-maven-plugin</artifactId>
    <groupId>org.jetbrains.kotlin</groupId>
    <version>${kotlin.version}</version>
    <executions>
        <execution>
            <phase>process-sources</phase>
            <goals> 
                <goal>compile</goal>
            </goals>
        </execution>
        <execution>
            <phase>process-test-sources</phase>
            <goals>
                <goal>test-compile</goal>
            </goals>
        </execution>
    </executions>
</plugin>

I'm not sure if such a solution exists for gradle. If you don't use a dependency manager then you should be able to work around this by invoking the kotlin compiler and then javac .

Without the workarounds mentioned I couldn't compile it either.

Edit: It appears Gradle requires no workaround as demonstrated here .

Answer 2

Using the following code

fun doSomething(method: () -> Boolean) = method()

And the following java

public class Test {
    public static void main(String[] args) {
        System.out.println(MainKt.doSomething(() -> true));
    }
}

It works just fine.

Answer 3

The environment must support the Kotlin API.

If the environment has restrictions on 3rd party libraries, and does not support the Kotlin API, you will not be able to use Kotlin in that environment.

---

I came back to this issue and finally found the problem. The environment I was working restricted third-party libraries, which prevented me from using the Kotlin API.

It's due to Kotlin using it's own types for primitives (such as Boolean ). At first, I thought those types would be compiled into Java primitive wrappers to ensure full interop. But apparently it still uses the Boolean from the Kotlin API.