BAEL-2303 Updated the code as per the review comments.

core-java-8/src/main/java/com/baeldung/reducingIfElse/AddCommand.java

@@ -1,19 +1,17 @@
 package com.baeldung.reducingIfElse;
 
-public class AddCommand implements Command<Integer, Integer, Integer> {
+public class AddCommand implements Command {
 
     private int a;
     private int b;
 
+    public AddCommand(int a, int b) {
+        this.a = a;
+        this.b = b;
+    }
+
     @Override
     public Integer execute() {
         return a + b;
     }
-
-    @Override
-    public Command<Integer, Integer, Integer> takeInput(Integer a, Integer b) {
-        this.a = a;
-        this.b = b;
-        return this;
-    }
 }

core-java-8/src/main/java/com/baeldung/reducingIfElse/AddRule.java

@@ -15,7 +15,7 @@ public class AddRule implements Rule {
     }
 
     @Override
-    public int getResult() {
-        return result;
+    public Result getResult() {
+        return new Result(result);
     }
 }

core-java-8/src/main/java/com/baeldung/reducingIfElse/Calculator.java

@@ -77,8 +77,7 @@ public class Calculator {
         return targetOperation.apply(a, b);
     }
 
-    public int calculate(int a, int b, Command<Integer, Integer, Integer> command) {
-        return command.takeInput(a, b)
-            .execute();
+    public int calculate(Command command) {
+        return command.execute();
     }
 }

core-java-8/src/main/java/com/baeldung/reducingIfElse/Command.java

@@ -1,7 +1,5 @@
 package com.baeldung.reducingIfElse;
 
-public interface Command<A, B, R> {
-    R execute();
-
-    Command<A, B, R> takeInput(A a, B b);
+public interface Command {
+    Integer execute();
 }

core-java-8/src/main/java/com/baeldung/reducingIfElse/Result.java

@@ -0,0 +1,13 @@
+package com.baeldung.reducingIfElse;
+
+public class Result {
+    int value;
+
+    public Result(int value) {
+        this.value = value;
+    }
+
+    public int getValue() {
+        return value;
+    }
+}

core-java-8/src/main/java/com/baeldung/reducingIfElse/Rule.java

@@ -4,5 +4,5 @@ public interface Rule {
 
     boolean evaluate(Expression expression);
 
-    int getResult();
+    Result getResult();
 }

core-java-8/src/main/java/com/baeldung/reducingIfElse/RuleEngine.java

@@ -2,6 +2,7 @@ package com.baeldung.reducingIfElse;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Optional;
 import java.util.stream.Collectors;
 
 public class RuleEngine {
@@ -12,9 +13,12 @@ public class RuleEngine {
         rules.add(new AddRule());
     }
 
-    public List<Rule> process(Expression expression) {
-        return rules.stream()
+    public Result process(Expression expression) {
+
+        Rule rule = rules.stream()
             .filter(r -> r.evaluate(expression))
-            .collect(Collectors.toList());
+            .findFirst()
+            .orElseThrow(() -> new IllegalArgumentException("Expression does not matches any Rule"));
+        return rule.getResult();
     }
 }

core-java-8/src/test/java/com/baeldung/reduceIfelse/CalculatorUnitTest.java

@@ -0,0 +1,32 @@
+package com.baeldung.reduceIfelse;
+
+import com.baeldung.reducingIfElse.AddCommand;
+import com.baeldung.reducingIfElse.Calculator;
+import com.baeldung.reducingIfElse.Operator;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class CalculatorUnitTest {
+
+    @Test
+    public void whenCalculateUsingStringOperator_thenReturnCorrectResult() {
+        Calculator calculator = new Calculator();
+        int result = calculator.calculate(3, 4, "add");
+        assertEquals(7, result);
+    }
+
+    @Test
+    public void whenCalculateUsingEnumOperator_thenReturnCorrectResult() {
+        Calculator calculator = new Calculator();
+        int result = calculator.calculate(3, 4, Operator.valueOf("ADD"));
+        assertEquals(7, result);
+    }
+
+    @Test
+    public void whenCalculateUsingCommand_thenReturnCorrectResult() {
+        Calculator calculator = new Calculator();
+        int result = calculator.calculate(new AddCommand(3, 7));
+        assertEquals(10, result);
+    }
+}

core-java-8/src/test/java/com/baeldung/reduceIfelse/RuleEngineUnitTest.java

@@ -2,12 +2,10 @@ package com.baeldung.reduceIfelse;
 
 import com.baeldung.reducingIfElse.Expression;
 import com.baeldung.reducingIfElse.Operator;
-import com.baeldung.reducingIfElse.Rule;
+import com.baeldung.reducingIfElse.Result;
 import com.baeldung.reducingIfElse.RuleEngine;
 import org.junit.Test;
 
-import java.util.List;
-
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 
@@ -17,11 +15,9 @@ public class RuleEngineUnitTest {
     public void whenNumbersGivenToRuleEngine_thenReturnCorrectResult() {
         Expression expression = new Expression(5, 5, Operator.ADD);
         RuleEngine engine = new RuleEngine();
-        List<Rule> rules = engine.process(expression);
+        Result result = engine.process(expression);
 
-        assertNotNull(rules);
-        assertEquals(1, rules.size());
-        assertEquals(10, rules.get(0)
-            .getResult());
+        assertNotNull(result);
+        assertEquals(10, result.getValue());
     }
 }