refactor : 요구사항 1, 2 재구현 및 private() 생성자

- 아래 메서드들을 splitAndSum에 재구현
- emptyOrNull
- processOneNumberString

src/main/java/StringAddCalculator.java

@@ -6,15 +6,16 @@ public class StringAddCalculator {
     public static final String COMMA = ",";
     public static final String DELIMETER = ",|:";
     public static final String CUSTOMDELIMETER = "//(.)\n(.*)";
+    public static final int DEFAULTVALUE_OF_NULL_AND_EMPTYSTRING = 0;
 
-    public static int emptyOrNull(String input) {
-        if (input == null || input.isEmpty()) {
-            return 0;
-        }
-        return 1;
+    private StringAddCalculator() {
+        throw new RuntimeException();
     }
 
-    public static int processOneNumberString(String input) {
+    public static int splitAndSum(String input) {
+        if (input == null || input.isEmpty()) {
+            return DEFAULTVALUE_OF_NULL_AND_EMPTYSTRING;
+        }
         return Integer.parseInt(input);
     }
 
@@ -66,5 +67,4 @@ private static int[] toInt(String[] splited) {
         }
         return numbers;
     }
-
 }

src/test/java/StringAddCalculatorTest.java

@@ -14,15 +14,15 @@ class StringAddCalculatorTest {
     @NullSource
     @DisplayName("빈 문자열 또는 null이면 0을 반환")
     void whenGivenNullOrEmpty(String input) {
-        int result = StringAddCalculator.emptyOrNull(input);
+        int result = StringAddCalculator.splitAndSum(input);
 
         assertThat(result).isEqualTo(0);
     }
 
     @Test
     @DisplayName("숫자 하나를 입력할 경우 해당 숫자를 반환한다")
     void inputOneNumberStringReturnToInteger() {
-        int result = StringAddCalculator.processOneNumberString("1");
+        int result = StringAddCalculator.splitAndSum("1");
 
         assertThat(result).isEqualTo(1);
     }