interface.dart

interface MathOperators {
  int add(int a, int b);
  int subtract(int a, int b);
}

int calculate(MathOperators operators, int a, int b) {
  return operators.add(a, operators.subtract(a, b));
}

class IntegerMath {
  int add(int a, int b) => a + b;
  int subtract(int a, int b) => a - b;
}

class DoubleMath {
  double add(double a, double b) => a + b;
  double subtract(double a, double b) => a - b;
}

class OnlyAdd {
  int add(int a, int b) => a + b;
}

// Okay becouse IntegerMath satisfies the interface MathOperators.
// Note that IntegerMath does not explicitly implement MathOperators, it may be defined
// in the standard lib or in anothert package that does not know that MathOperators exist
calculate(new IntegerMath(), 5, 2);
// Not okay becouse the types of the methods is invalid
calculate(new DoubleMath(), 5, 2);
// Not okay becouse the method subtract is missing
calculate(new OnlyAdd(), 5, 2);