I'll provide in description

Question

I can't understand the steps used in main method please explain!! Or tell me the concept used

interface test{
int a(int b,int c);
static String show(){
return "hello";
}
}
public class Main{
public static void main (String[] args) {
test r=(p,q)->p+q;
System.out.println(r.a(1,2)+test.show());
}
}

Answer 1

interface which has only one method to implement, is called functional interface and
can be implemented by lambda expression :
test r = (p, q) -> p + q;

same as
class TestImpl implements test {
int a (int a, int b) {
return a + b;
}
}

Answer 2

All of this is Java 8 features, you might want to read a little bit about it in order to understand how it is implemented.
Concepts you need to particularly look into include-
1) Functional interface
2) Static methods in interface
3) Lambda expressions