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Thread: Java String Pool - Beginner Question

  1. #1
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    Question Java String Pool - Beginner Question

    Hi.

    I have a question about string pool.
    There is a code:

    public static void main(String[] args) {
            String s1 = new String("ABC");
            String s2 = new String("ABC");
            String s3 = "ABC";
            String s4 = "ABC";
            final String s5 = "AB";
            String s6 = "C";
     
            System.out.println(s1 == s2);
            System.out.println(s2 == s3);
            System.out.println(s3 == s4);
            System.out.println(s4 == s5 + s6);
            System.out.println(s4 == s5 + "C");
    }

    General rules as I understand are that literals (like "ABC") go to String Poll, objects (like new String()) go to Java Heap.
    In this way,
    first println has "false"
    2 - "false"
    3 - "true"
    4 - "false", because object plus literal goes Java Heap
    5 - "true"

    I have question for you about fifth println.
    If I removed word final, I would get result false.

    Why? How do strings behave in fifth line?
    What exactly word final does?
    As I know final variables go to something special memory space. For example global cache. I am not sure.

    OK
    Can you please explain me about fifth println with final and without final?
    I will thankful for doc references, because I couldn't find anything.
    Last edited by Norm; January 28th, 2022 at 02:23 PM. Reason: Added / to end code tag

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  3. #2
    Super Moderator Norm's Avatar
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    Default Re: Java String Pool - Beginner Question

    Sorry, I have never studied the implementation of String pools. I think that any code that relies on knowing what String is in the pool and what String in not in the pool is weak/bad code. You should always use the equals method to compare String objects.
    If you don't understand my answer, don't ignore it, ask a question.

  4. #3
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    Default Re: Java String Pool - Beginner Question

    Thanks for your answer.

    Yes, I understand that I must use equals method for comparing two strings.
    My question is more about Java theory. Not about practical issues.

    I tried find something in Java docs by myself, but I didn't.
    I decided to ask my question for more experienced people.

  5. #4
    Super Moderator Norm's Avatar
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    Default Re: Java String Pool - Beginner Question

    Take a look in the java specs: http://docs.oracle.com/javase/specs/
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Java String Pool - Beginner Question

    local final String is constant like literal String
    two literal constants concatenated by + ,can be stored in the pool
    as compile-time optimalisation like
    String s7 = "AB"+"C";
    System.out.println(s4 == s7); //true

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