DefaultMethod.java

interface SomeInterfaceJava8 {
   Integer customCalc(int a, int b);
   default Integer calc(int a, int b) {
       return (a + b);
   }
}

class SomeImplementationJava8 implements SomeInterfaceJava8 {
   @Override
   public Integer customCalc(int a, int b) {
       return calc(a, b) + 1;
   }
}

class AnotherImplementationJava8 implements SomeInterfaceJava8 {
   @Override
   public Integer customCalc(int a, int b) {
       return calc(a, b) + 2;
   }
}