Java lambda - type cast of object in return statement?

Question

I have following code in my spring cloud application :

@EnableBinding(Source.class)
@SpringBootApplication
public class dataflow_producer {

static SimpleDateFormat dateFormatter = new SimpleDateFormat("hh:mm:ss"); 

public static void main(String[] args) {
    SpringApplication.run(dataflow_producer.class, args);

}

@Bean
@InboundChannelAdapter(value = Source.OUTPUT)
public MessageSource<String> timerMessageSource() {       
    return ()-> new GenericMessage<String>(dateFormatter.format(new Date()));
}
}

My question is, how this statement works ? :

return ()-> new GenericMessage<String>(dateFormatter.format(new Date()));

Does it retype object like so ? :

return (MessageSource<String>)new GenericMessage<String>(dateFormatter.format(new Date()));

Can you explain meaning of return ()-> ? Why is it valid in this case ? Input is Object of type GenericMessage<T>, but output is Object of type MessageSource<T> ?

Can mentioned return statement be rewritten to non-lambda code ?

Answer 1

I don't know Spring.

Though, in Java 8. When you got a SAM interface (Single abstract method). Called for exemple "SAMInterface".

public interface SAMInterface {
    void theSAMMethod();
}

If you need to return an object of type "SAMInterface", these code are equivalent:

SAMInterface toReturn = () -> println("Hello World!");

and

SAMInterface toReturn = new SAMInterface() {
    public void theSAMMethod() {
      println("Hello World!");
    }
}

So if MessageSource<T> interface has exactly 1 method to redefine. You can use lambda :)