This question already has an answer here:

This answer gives an indication that class String is declared final for thread safety, which does not convince.

This answer does not convince when it says: And so, you make the classes final. There can be no mutable String, because its an immutable class and its final.


below field with final modifier,

/** The value is used for character storage. */
    private final char value[];

in class String would suffice/indicate that data stored in the created object is suppose to be immutable and thread safe.

So, class String being final has nothing to do with immutability or thread safety.

But it is not clear, why class String is declared final?

Because, as per below class definition, it has final modifier:

public final class String
    implements, Comparable<String>, CharSequence
{ .... }

So, class String cannot be inherited.

Despite field value[] in class String is declared final, additionaly, What is the necessity for class String also to be final?

Note: Answers to this question will give an idea behind this design decision

marked as duplicate by maple_shaft Jun 19 '15 at 17:52

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

You answered your question in your question.

Class String cannot be inherited because it has a final modifier in the class definition. The final modifier does just that in a class definition, declares a class which cannot be inherited from, it has nothing to do with immutability in this case.

The rationale behind this is that a programmer may subclass String therefore have an object which "looks like" a String (i.e. can be passed to methods which take a String argument) however is mutable whereas String is expected to be immutable.

  • I understand that class String being final has nothing to do with immutability. But why class String is declared final? Of course classes that are final cannot be inherited. But what is the necessity for class String to be final? – overexchange Jun 17 '15 at 8:20
  • Edited my response – ALXGTV Jun 17 '15 at 8:35
  • Can you be more precise on, how subclass designer would treat as mutable? I am not clear with your updated response. – overexchange Jun 17 '15 at 8:39
  • @overexchange If String would not be final, I could subclass it and use another mutable (!!) field to store the raw char[], overriding all methods to use that value, which makes it possible to pass an object which is both recognised by the type checker as a String and mutable. – 11684 Jun 17 '15 at 10:44
  • 1
    Consider the nefarious String subclasses too: class AlwaysTrueString extends String { public boolean equals(Object o) { return 1; }} and then pass it into a method somewhere that looks like boolean passwordMatch(String password) This would be bad. Being able to reason how String works is important. Having to worry if each String that you dealt with to or from other sources might be designed nefariously makes it much harder for me to write simple code and reason about how that code works. – user40980 Jun 17 '15 at 13:26

It doesn't matter that value[] is final when none of the methods on String are not. If string class were not final, I could derive and override all the methods and in effect have a mutable string.

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