当SAM方法未在Java中返回值时，Lambda会返回值

Question

@FunctionalInterface
public interface Runnable {
    public abstract void run();
}

public class MethodReference1 {

    public static String ThreadStatus() {
        System.out.println( Thread.currentThread().getName() +  " is running...");
        return "threadname";
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> ThreadStatus());
        t1.start();
    }

}

In the above example using Java 8, ThreadStatus() returns a string but Runnable interface "run()" method doesnt return any value. But it still works (No compile/runtime error). I am wondering how it is working because as per the lamba specification, SAM should have exactly same signature.

If i flip the case where ThreadStatus method doesnt return any value and change the Functional interface method to return a value, i get a compile time error.

Can someone help me understand this?

Answer 1

This is because Thread t1 = new Thread(() -> ThreadStatus()); is equivalent to:

Thread t1 = new Thread(new Runnable() {
    @Override
    public void run() {
        ThreadStatus();
    }
});

So you're not returning anything from run() . run() is simply instructed to invoke the ThreadStatus() method and ignore the return value.