feat: StringAddCalculator 추가

src/main/java/StringAddCalculator.java

@@ -0,0 +1,41 @@
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class StringAddCalculator {
+    private static final Pattern pattern = Pattern.compile("//(.)\n(.*)");
+
+    static int splitAndSum(String text) {
+        if (text == null || text.isEmpty()) {
+            return 0;
+        }
+        String[] tokens = splitText(text);
+        int[] numbers = convertStringToIntList(tokens);
+        return sumNumbers(numbers);
+    }
+
+    private static String[] splitText(String text) {
+        Matcher m = pattern.matcher(text);
+        if (m.find()) {
+            return m.group(2).split(m.group(1));
+        }
+        return text.split("[,:]");
+    }
+
+    private static int[] convertStringToIntList(String[] numbers) {
+        int[] converted = new int[numbers.length];
+        for (int i = 0; i < numbers.length; i++) {
+            converted[i] = Integer.parseInt(numbers[i]);
+        }
+        return converted;
+    }
+
+    private static int sumNumbers(int[] numbers) {
+        int sum = 0;
+        for (int number : numbers) {
+            if (number < 0)
+                throw new RuntimeException();
+            sum += number;
+        }
+        return sum;
+    }
+}

src/test/java/StringAddCalculatorTest.java

@@ -0,0 +1,51 @@
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class StringAddCalculatorTest {
+    @Test
+    public void splitAndSum_null_또는_빈문자() {
+        int result = StringAddCalculator.splitAndSum(null);
+        assertThat(result).isEqualTo(0);
+
+        result = StringAddCalculator.splitAndSum("");
+        assertThat(result).isEqualTo(0);
+    }
+
+    @Test
+    public void splitAndSum_숫자하나() throws Exception {
+        int result = StringAddCalculator.splitAndSum("1");
+        assertThat(result).isEqualTo(1);
+    }
+
+    @Test
+    public void splitAndSum_쉼표구분자() throws Exception {
+        int result = StringAddCalculator.splitAndSum("1,2");
+        assertThat(result).isEqualTo(3);
+    }
+
+    @Test
+    public void splitAndSum_쉼표_또는_콜론_구분자() throws Exception {
+        int result = StringAddCalculator.splitAndSum("1,2:3");
+        assertThat(result).isEqualTo(6);
+    }
+
+    @Test
+    public void splitAndSum_custom_구분자() throws Exception {
+        int result = StringAddCalculator.splitAndSum("//;\n1;2;3");
+        assertThat(result).isEqualTo(6);
+    }
+
+    @Test
+    public void splitAndSum_negative() throws Exception {
+        assertThatThrownBy(() -> StringAddCalculator.splitAndSum("-1,2,3"))
+                .isInstanceOf(RuntimeException.class);
+    }
+
+    @Test
+    public void splitAndSum_custom_구분자_negative() throws Exception {
+        assertThatThrownBy(() -> StringAddCalculator.splitAndSum("//;\n-1;2;3"))
+                .isInstanceOf(RuntimeException.class);
+    }
+}