docs: update interpreter

interpreter/README.md

@@ -99,6 +99,11 @@ public class PlusExpression extends Expression {
 Now, we are able to show the interpreter pattern in action parsing some simple math.
 
 ```java
+@Slf4j
+public class App {
+
+    public static void main(String[] args) {
+
         // the halfling kids are learning some basic math at school
         // define the math string we want to parse
         final var tokenString = "4 3 2 - 1 + *";
@@ -114,7 +119,8 @@ for (var s : tokenList) {
                 // the stack
                 var rightExpression = stack.pop();
                 var leftExpression = stack.pop();
-        LOGGER.info("popped from stack left: {} right: {}", leftExpression.interpret(),rightExpression.interpret());
+                LOGGER.info("popped from stack left: {} right: {}",
+                        leftExpression.interpret(), rightExpression.interpret());
                 var operator = getOperatorInstance(s, leftExpression, rightExpression);
                 LOGGER.info("operator: {}", operator);
                 var result = operator.interpret();
@@ -131,18 +137,39 @@ for (var s : tokenList) {
         }
         // in the end, the final result lies on top of the stack
         LOGGER.info("result: {}", stack.pop().interpret());
+    }
+
+    public static boolean isOperator(String s) {
+        return s.equals("+") || s.equals("-") || s.equals("*");
+    }
+
+    public static Expression getOperatorInstance(String s, Expression left, Expression right) {
+        return switch (s) {
+            case "+" -> new PlusExpression(left, right);
+            case "-" -> new MinusExpression(left, right);
+            default -> new MultiplyExpression(left, right);
+        };
+    }
+}
  ```
 
 Executing the program produces the following console output.
 
 ```
-popped from stack left: 1 right: 1
+13:33:15.437 [main] INFO com.iluwatar.interpreter.App -- push to stack: 4
-operator: +
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- push to stack: 3
-push result to stack: 2
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- push to stack: 2
-popped from stack left: 4 right: 2
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- popped from stack left: 3 right: 2
-operator: *
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- operator: -
-push result to stack: 8
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- push result to stack: 1
-result: 8
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- push to stack: 1
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- popped from stack left: 1 right: 1
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- operator: +
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- push result to stack: 2
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- popped from stack left: 4 right: 2
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- operator: *
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- push result to stack: 8
+13:33:15.440 [main] INFO com.iluwatar.interpreter.App -- result: 8
 ```
 
 ## Class diagram