feat: 후위 연산식을 계산하는 클래스 구현

Author: wonu606

calculator/src/main/java/com/wonu606/calculator/calculator/Calculator.java

@@ -0,0 +1,8 @@
+package com.wonu606.calculator.calculator;
+
+import java.util.List;
+
+public interface Calculator {
+
+    double calculate(List<String> expression);
+}

calculator/src/main/java/com/wonu606/calculator/calculator/PostfixCalculator.java

@@ -0,0 +1,42 @@
+package com.wonu606.calculator.calculator;
+
+import com.wonu606.calculator.model.Operator;
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.List;
+import java.util.Objects;
+
+public class PostfixCalculator implements Calculator {
+
+    @Override
+    public double calculate(List<String> postfixExpression) {
+        Deque<Double> operandStack = new ArrayDeque<>();
+
+        for (String token : postfixExpression) {
+            handleToken(operandStack, token);
+        }
+
+        return operandStack.pop();
+    }
+
+    private void handleToken(Deque<Double> operandStack, String token) {
+        if (isOperator(token)) {
+            Operator operator = Objects.requireNonNull(Operator.get(token));
+            performOperation(operandStack, operator);
+        } else {
+            operandStack.push(Double.valueOf(token));
+        }
+    }
+
+    private void performOperation(Deque<Double> operandStack, Operator operator) {
+        double secondOperand = operandStack.pop();
+        double firstOperand = operandStack.pop();
+
+        double result = operator.apply(firstOperand, secondOperand);
+        operandStack.push(result);
+    }
+
+    private boolean isOperator(String token) {
+        return Operator.get(token) != null;
+    }
+}