How to convert lambda for swap param to a method

Question

I'm trying to understand how lambda is work. In the process i found this example.

public class Calculator {
   interface IntegerMath {
      int operation(int a, int b);

      default IntegerMath swap() {
         return (a, b) -> operation(b, a);
      }
   }

   private static int apply(int a, int b, IntegerMath op) {
      return op.operation(a, b);
   }

   public static void main(String[] args) {
      IntegerMath addition =  (a, b) -> a + b;
      Math subtraction = (a, b) -> a - b;

      System.out.println(apply(5, 10, subtraction));
      System.out.println(apply(5, 10, subtraction.swap()));
   }
}

I'm trying to convert the swap method implementation from lambda to normal method.

But i cant change that, because if i did that i should write the implementation of operation method and create an infinity recursive.

public class Calculator {
   interface IntegerMath {
      int operation(int a, int b);

      default IntegerMath swap() {
         return new IntegerMath() {
            @Override
            public int operation(int a, int b) {
               return operation(b, a);
            }
         };
      }
   }
}

Can i not use lambda in swap method in interface IntegerMath?

Could you try to rephrase the question please? It's not really clear what you're asking. Your fist code block is not valid Java. Should it be `public interface Calculator`? — mypetlion, Jul 29 '19 at 18:30
You are returning different things in each implementation. swap() returns a *function* that can be later called. Your implementation of swap() simply won't work because it will infinitely recurse and wants to return a *value* not a function. — Ryan Schaefer, Jul 29 '19 at 18:30
@RyanSchaefer you are right. can i not use lambda in method swap in IntegerMath interface — Naufal Muntaaza, Jul 29 '19 at 18:51
Possible duplicate of [Calling outer class function from inner class](https://stackoverflow.com/questions/2808501/calling-outer-class-function-from-inner-class) — shmosel, Jul 29 '19 at 19:03

shmosel · Answer 1 · 2019-07-29T19:51:58.397

2

If you're asking how to call the outer operation() from within your anonymous implementation, you can do it like this:

return IntegerMath.this.operation(b, a);

edited Jul 29 '19 at 19:51

answered Jul 29 '19 at 18:34

shmosel

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sorry, i have fixed my question. i don't think that works? – Naufal Muntaaza Jul 29 '19 at 18:49
Why don't you think so? Did you try it? – shmosel Jul 29 '19 at 18:55
i'm try it. but i don't exactly understand your answer, maybe i'm wrong. where i can do that? – Naufal Muntaaza Jul 29 '19 at 19:00
How can you do what? – shmosel Jul 29 '19 at 19:01
sorry i'm not. i just cant explaining my problem clear enough. check my edited question. i want to change the swap method in IntegerMath interface without using lambda. – Naufal Muntaaza Jul 29 '19 at 19:11
I understood the question. But I don't understand what problem you're having with the answer. – shmosel Jul 29 '19 at 19:15
in example code i can call subtraction.swap() and the output from that two is different, so the swap method is work using lambda. but what if i don't want to use lambda. in confuse how to implement your answer in swap method. because when i use your answer to replace the lambda it doesn't work. or i have to create anonymous class that implement operation and use your answer but it show warning infinite recursion – Naufal Muntaaza Jul 29 '19 at 19:24

How to convert lambda for swap param to a method

1 Answers1