Improve camelization in DefaultCodegen

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen;
+ package org.openapitools.codegen;
 
 import com.github.benmanes.caffeine.cache.Cache;
 import com.github.benmanes.caffeine.cache.Caffeine;
@@ -5910,7 +5910,13 @@ public class DefaultCodegen implements CodegenConfig {
      * @return camelized string
      */
     protected String removeNonNameElementToCamelCase(final String name, final String nonNameElementPattern) {
-        String result = Arrays.stream(name.split(nonNameElementPattern))
+        String[] splitString = name.split(nonNameElementPattern);
+
+        if (splitString.length > 0) {
+            splitString[0] = lowerStartOfWord(splitString[0]);
+        }
+
+        String result = Arrays.stream(splitString)
                 .map(StringUtils::capitalize)
                 .collect(Collectors.joining(""));
         if (result.length() > 0) {
@@ -5919,6 +5925,29 @@ public class DefaultCodegen implements CodegenConfig {
         return result;
     }
 
+    /**
+     * Puts the first letters to lowercase. If the word starts with multiple capital letters, all of them
+     * will be converted to lowercase.
+     * @param name string to be changed
+     * @return string starting with lowercase
+     */
+    private String lowerStartOfWord(final String name) {
+        final StringBuilder result = new StringBuilder();
+        boolean isStartingBlock = true;
+        for (int i = 0; i < name.length(); i++) {
+            char current = name.charAt(i);
+            if (isStartingBlock && Character.isUpperCase(current)) {
+                current = Character.toLowerCase(current);
+            } else {
+                isStartingBlock = false;
+            }
+
+            result.append(current);
+        }
+
+        return result.toString();
+    }
+
     @Override
     public String apiFilename(String templateName, String tag) {
         String suffix = apiTemplateFiles().get(templateName);

modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java

@@ -4722,4 +4722,15 @@ public class DefaultCodegenTest {
 
         Assert.assertTrue(codegen.cliOptions.contains(expected));
     }
+
+    @Test
+    public void testRemoveNonNameElementToCamelCase() {
+        final DefaultCodegen codegen = new DefaultCodegen();
+
+        final String alreadyCamelCase = "aVATRate";
+        Assert.assertEquals(codegen.removeNonNameElementToCamelCase(alreadyCamelCase), alreadyCamelCase);
+
+        final String startWithCapitals = "DELETE_Invoice";
+        Assert.assertEquals(codegen.removeNonNameElementToCamelCase(startWithCapitals), "deleteInvoice");
+    }
 }