Fix Discounter examples

core-java/src/main/java/com/baeldung/strategy/Discounter.java

@@ -5,6 +5,10 @@ import java.util.function.UnaryOperator;
 
 public interface Discounter extends UnaryOperator<BigDecimal> {
 
+    default Discounter combine(Discounter after) {
+        return value -> after.apply(this.apply(value));
+    }
+
     static Discounter christmas() {
         return (amount) -> amount.multiply(BigDecimal.valueOf(0.9));
     }

core-java/src/test/java/com/baeldung/strategy/StrategyDesignPatternUnitTest.java

@@ -57,7 +57,7 @@ public class StrategyDesignPatternUnitTest {
 
         final Discounter combinedDiscounter = discounters
           .stream()
-          .reduce(v -> v, (d1, d2) -> (Discounter) d1.andThen(d2));
+          .reduce(v -> v, Discounter::combine);
 
         combinedDiscounter.apply(amount);
     }