Enthuware

"replace(char oldChar, char newChar) method returns the same String object if both the parameters are same, i.e. if there is no change." - Is the behavior of this method documented in any official Oracle documentation? Could you please provide a source from which to find out such details of how the standard Java API works?

You can get such details from JavaDoc API documentation. For example, JavaDoc API documentation for replace method says:

If the character oldChar does not occur in the character sequence represented by this String object, then a reference to this String object is returned.

I have read the JavaDoc API documentation for the replace method and in my opinion it does not explain this method's behavior
"String".replace('g','g')=="String" - the character 'g' occur in the character sequence "String", so the quoted piece of documentation doesn't explain the behavior of the method.

Sorry, my mistake. You are right. That statement doesn't explain the behavior. Let me see if I can find a valid reference.
Paul.