Select labmda parameter 'x'->Right click->Refactor->Rename...
New Name: a

Dialog will show error: Cannot rename. a already used. But in fact the code:
        IntegerMath addition = (a, b) -> a + b;
        IntegerMath subtraction = (a, b) -> a - b;

is correct.


public class Calculator {

    interface IntegerMath {

        int operation(int a, int b);
    }

    public int operateBinary(int a, int b, IntegerMath op) {
        return op.operation(a, b);
    }

    public static void main(String... args) {

        Calculator myApp = new Calculator();
        IntegerMath addition = (a, b) -> a + b;
        IntegerMath subtraction = (x, y) -> x - y; // Try to rename 'x' to 'a'
        System.out.println("40 + 2 = "
                + myApp.operateBinary(40, 2, addition));
        System.out.println("20 - 10 = "
                + myApp.operateBinary(20, 10, subtraction));
    }
}