如何将方法转换为 lambda 引用

Question

I'm writing some jUnit5 extensions to make it easier to test some code. The extension has these annotations:

@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Test
@ExtendWith({CustomJunit5Extension.class})
public @interface MyAnnotation {
   String jsonFile;
   Class<?> converter;
}

// test case
class Test {
   @MyAnnotation(converter = MyClass.class)
   void someTest();
}

// Some class which contains a converter method annotated with @JsonConverterMethod
public class MyClass {
    @JsonConverterMethod
    public static Car converter(String jsonLine);
}

// jUnit5 extension
public class CustomJunit5Extension implements ParameterResolver, AfterEachCallback, AfterAllCallback {

   // ultra simplified version
   public Object resolveParameter(ParameterContext pctx, ExtensionContext ectx) throws ParameterResolutionException {
      final MyAnnotation annotation = getAnnotation(pctx, ectx);
      final Method converterMethod = getMethodByAnnotation(annotation.converter(), JsonConverterMethod.class);

      @SuppressWarnings({"unchecked"})
      final var converter = needs to become a 
                            direct reference to MyClass::converter
                            without hardcoding it, because you can
                            have other classes providing different
                            jsonString to Object converters;

      final JsonCandleProvider provider = new JsonCandleProvider(resource, converter);

      return provider;
   }

}

Hence the question - how to convert a Method reference into a Lambda reference so I could pass it to the MyJsonProvider ? Or how would you achieve a similar result in another way maybe?

Answer 1

The method parameter needs to be a Functional interface of the type of the method reference.

MyMethod(() -> "Hello, World!");

Supplier<String> supplier = () -> "Hello, World!";
MyMethod(supplier);
MyMethod(supplier::get);
MyMethod2(supplier.get());


public static void MyMethod(Supplier<String> sup) {
    System.out.println(sup.get());
}

public static void MyMethod2(String value) {
    System.out.println(value);
}

prints

Hello, World!
Hello, World!
Hello, World!
Hello, World!