Enthuware

the question explains
1. replace(char oldChar, char newChar) method returns the same String object if both the parameters are same, i.e. if there is no change.
Thus, "String" == "String".replace('g', 'g') will return true.

Why can't "String".replace('g', 'g') return new another "String" object?

API Document explains
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. Otherwise, a String object is returned that represents a character sequence identical to the character sequence represented by this String object, except that every occurrence of oldChar is replaced by an occurrence of newChar.

'g' occurs in "String".
It does not explain the case where oldChar is same with newChar, that is, the character sequence of result is same with the character sequence of this String.
I think that the result of "String" == "String".replace('g', 'g') depends on JVM.

And the question explains
2. replace(CharSequence oldSeq, CharSequence newSeq) method returns a new String object even if there is no change after replacement.
Thus, "String" == "String".replace("g", "g") will return false.

on Eclipse Adoptium JVM
"String" == "String".replace('g', 'g') return true.
"String" == "String".replace("g", "g") return true. replace(char,char) is called internally.
"String" == "String".replace("ng", "ng") return false.

I think that the result depends on JVM.

You are right. The JavaDoc does not clearly specify what happens when both the params are the same. But no, it doesn't depend on the JVM.
It depends on how the replace methods are coded.

The explanation is saying that if both the params are the same then the same reference is returned. Although this is true for replace(char , char), I agree that this fact should not be relied upon because it is not specified in the JavaDoc.

The explanation has been updated.

thank you for your feedback!