[Step1] 문자열 계산기 구현

src/main/java/calculator/StringCalculator.java

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+package calculator;
+
+public class StringCalculator {
+
+    public static int calculate(String input) {
+        validateNotBlank(input);
+
+        String[] values = input.split(" ");
+        return calculate(values);
+    }
+
+    private static void validateNotBlank(String input) {
+        if (input == null || input.isEmpty()) {
+            throw new IllegalArgumentException("Input cannot be null or empty");
+        }
+    }
+
+    private static int calculate(String[] values) {
+        int result = Integer.parseInt(values[0]);
+
+        for (int i = 1; i < values.length; i++) {
+            String operation = values[i];
+            i++;
+            int number = Integer.parseInt(values[i]);
+            result = calculate(result, operation, number);
+        }
+
+        return result;
+    }
+
+    private static int calculate(int number1, String operation, int number2) {
+        switch (operation) {
+            case "+":
+                return number1 + number2;
+            case "-":
+                return number1 - number2;
+            case "*":
+                return number1 * number2;
+            case "/":
+                return number1 / number2;
+            default:
+                throw new IllegalArgumentException("Invalid operation: " + operation);
+        }
+    }
+}

src/test/java/calculator/StringCalculatorTest.java

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+package calculator;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.NullAndEmptySource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+public class StringCalculatorTest {
+
+    @Test
+    public void add() {
+        int result = StringCalculator.calculate("1 + 2 + 3");
+        assertThat(result).isEqualTo(6);
+    }
+
+    @Test
+    public void subtract() {
+        int result = StringCalculator.calculate("10 - 2 - 3");
+        assertThat(result).isEqualTo(5);
+    }
+
+    @Test
+    public void multiply() {
+        int result = StringCalculator.calculate("2 * 3 * 4");
+        assertThat(result).isEqualTo(24);
+    }
+
+    @Test
+    public void divide() {
+        int result = StringCalculator.calculate("10 / 2");
+        assertThat(result).isEqualTo(5);
+    }
+
+    @ParameterizedTest
+    @NullAndEmptySource
+    public void blankInput(String input) {
+        assertThatIllegalArgumentException().isThrownBy(() -> StringCalculator.calculate(input));
+    }
+
+    @Test
+    public void invalidOperation() {
+        assertThatIllegalArgumentException().isThrownBy(() -> StringCalculator.calculate("1 % 2"));
+    }
+}