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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.

Because,

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 java.io.Serializable, 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

Community
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overexchange
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2 Answers2

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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.

ALXGTV
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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.

Esben Skov Pedersen
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