BAEL-8928-Convert-camel-to-snake-case (#18168)

* BAEL-8928-Convert-camel-to-snake-case

* BAEL-8928-Convert-camel-to-snake-case

* BAEL-8928-Convert-camel-to-snake-case

---------

Co-authored-by: tienvn <[email protected]>

Author: vunamtien

core-java-modules/core-java-string-conversions-4/src/main/java/com/baeldung/cameltosnakecase/CamelToSnakeCaseConverter.java

@@ -0,0 +1,23 @@
+package com.baeldung.cameltosnakecase;
+
+public class CamelToSnakeCaseConverter {
+
+    public static String convertCamelCaseToSnake(String input) {
+        StringBuilder result = new StringBuilder();
+        for (char c : input.toCharArray()) {
+            if (Character.isUpperCase(c)) {
+                result.append("_").append(Character.toLowerCase(c));
+            } else {
+                result.append(c);
+            }
+        }
+        return result.toString();
+    }
+
+    public static String convertCamelCaseToSnakeRegex(String input) {
+        return input
+            .replaceAll("([A-Z])(?=[A-Z])", "$1_")
+            .replaceAll("([a-z])([A-Z])", "$1_$2")
+            .toLowerCase();
+    }
+}

core-java-modules/core-java-string-conversions-4/src/test/java/com/baeldung/cameltosnakecase/CamelToSnakeCaseConverterUnitTest.java

@@ -0,0 +1,52 @@
+package com.baeldung.cameltosnakecase;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class CamelToSnakeCaseConverterUnitTest {
+
+    @Test
+    public void whenConvertNormalCamelCase_thenGetCorrectSnakeCase() {
+        String input = "convertCamelCase";
+        String expected = "convert_camel_case";
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnake(input));
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnakeRegex(input));
+    }
+
+    @Test
+    public void whenConvertNotNormalCamelCase_thenGetCorrectSnakeCase() {
+        String input = "convertCCamelCase";
+        String expected = "convert_c_camel_case";
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnake(input));
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnakeRegex(input));
+    }
+
+    @Test
+    public void whenConvertAlreadySnakeCase_thenGetUnchangedSnakeCase() {
+        String input = "snake_case";
+        String expected = "snake_case";
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnake(input));
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnakeRegex(input));
+    }
+
+    @Test
+    public void whenConvertAllLowerCaseString_thenGetUnchangedString() {
+        String input = "snakecase";
+        String expected = "snakecase";
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnake(input));
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnakeRegex(input));
+    }
+
+    @Test
+    public void whenConvertOtherEdgeCases_thenGetCorrectSnakeCases() {
+        // Blank string
+        Assertions.assertEquals("", CamelToSnakeCaseConverter.convertCamelCaseToSnake(""));
+        Assertions.assertEquals("", CamelToSnakeCaseConverter.convertCamelCaseToSnakeRegex(""));
+
+        // Special character
+        String input = "sn@keCase#";
+        String expected = "sn@ke_case#";
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnake(input));
+        Assertions.assertEquals(expected, CamelToSnakeCaseConverter.convertCamelCaseToSnakeRegex(input));
+    }
+}