Lambda expression using interface in one line

I'm learning about interfaces and I faced a problem which I cannot understand. It's about lambda expression which I learn that can be used in one line, but I've create a case, where this expression does not working and I don't know where I've made a mistake.

I have defined this interface:

public interface MathOperations {
    int add(int a, int b);
}

Next I have defined class in which I want to test this interface with adder method.

public class App {
    public static void main(String[] args) {
        int x;
        x = adder((a, b) -> return a+b);
    }

    public static int adder(MathOperations mo){
        return mo.add(3, 5);
    }
}

Unfortunately the line with the x assignment doesn't work and I cannot figure out where I've made a mistake. The compiler does not recognize the a and b variable in the return statement. I know that I can make this assignment with brackets but I'm curious if I can do this in one line.

@FunctionalInterface
interface MathOperations {
    int add(int a, int b);
}

Here are two ways to do what you want.

• To create a lambda, you need to specify the interface type and provide the definition of what to do with a and b.
• then you can invoke the lambda and get the value.

MathOperations compute = (a,b)->a+b;
x = compute.add(2,3);
System.out.println(x);

The above does essentially the following behind the scenes.

• Here an anonymous class is defined using the interface and instantiated using the new keyword. (the class contains the implemented method).
• Implementing the interface was standard until the concept of FuntionalInterfaces and lambdas were introduced in Java 8.
• Then call the adder method supplying the instance mo just created or call the method with the previously created compute lambda.

MathOperations mo =new MathOperations() {
                          public int add(int a,int b) {
                              return a+b;
                          }};

 System.out.println(adder(mo));
 System.out.println(adder(compute));      
    

output of above three print statements

5
8
8

public static int adder(MathOperations mo){
    return mo.add(3, 5);
}

Note: Imo, a more versatile interface can be created by following examples from the API functional interfaces already defined. The name MathOperation is fine but the add method is too specific (and functional interfaces may only contain one abstract method). So instead of having an add method, have a compute or similar method and let the name of the lambda dictate its operation.

MathOperation add = (a,b)->a+b;
int sum = add.compute(10,20); // 30
MathOperation sub = (a,b)->a-b;
int diff = sub.compute(10,20) // -10