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Thread: Immutable String

  1. #1
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    Default Immutable String

    Hello, I am a beginner and can not understand this:

    String s = "Skakal pes pres oves";
    s = s.substring(0, 6);
    System.out.println(s);
    String concat = s + " pes";
    System.out.println(concat);
    System.out.println();

    It is an example code from a site. From what I know is, that the content of an immutable string can not be changed. But doesn't the s = s.substring(0, 6) changed it? Or am I misunderstanding this?


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    Default Re: Immutable String

    The literal String "Skakal pes pres oves" is stored in memory.
    The String variable s references the literal String in memory.
    When you assign a new value to the s variable, you do not alter the previously referenced literal String, but rather you create a new literal String in memory and reassign what the s variable is referencing. The previous reference still exists in memory. You now have two literal Strings in memory, but only the second one has a reference pointing to it.

    **Note: I think the substring method actually does something a bit different than what I described, but your question was more about reassigning a String variable than the nuts and bolts of what the substring method does**
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    Default Re: Immutable String

    You can also used the hashcode() method to check it. each object has space in memory and can access it through memory address(I think that is
    where the variable pointing).
    print the hashcode() of your variable every before and after altering its value. you will notice that the hashcode() of the variable changes every alteration since that variable points to different memory address every alteration (because of being immutable of String)

    you can also do some experiment in StringBuilder in java, StringBuilder is mutable unlike String. print hashcode() of your variable of StringBuilder every
    before and after alteration, you'll see the difference between two.

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    Default Re: Immutable String

    You can also used the hashcode() method to check it. each object has space in memory and can access it through memory address(I think that is
    where the variable pointing).
    print the hashcode() of your variable every before and after altering its value. you will notice that the hashcode() of the variable changes every alteration since that variable points to different memory address every alteration (because of being immutable of String)

    you can also do some experiment in StringBuilder in java, StringBuilder is mutable unlike String. print hashcode() of your variable of StringBuilder every
    before and after alteration, you'll see the difference between two.
    Not exactly. From the API of the hashcode() method for String:

    The hash code for a String object is computed as

    s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]


    using int arithmetic, where s[i] is the ith character of the string, n is the length of the string, and ^ indicates exponentiation. (The hash value of the empty string is zero.)
    In other words, the hashcode for this class doesn't directly refer to memory - doing so would be pretty catastrophic in terms of data structures that rely on the hashcode of an object (such as Maps or Sets). That being said, see the API for Object, whose hashcode implementation may be an integer representation of memory - however classes may often override this default implementation.

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    Default Re: Immutable String

    Thanks for the correction

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    Default Re: Immutable String

    To add to aussiemcgr's reply, take a look at String (Java Platform SE 6): "Returns a new string that is a substring of this string".

    To digress slightly, as a general tip, sometimes the API documentation may not clearly describe what a method really does, in which case it can be helpful to look into the actual code itself. Oracle's JDK comes with a file called src.zip that you can look up the code implementation for methods such as substring. On top of that, if you're using Eclipse, you can associate rt.jar (under JRE System Library) to src.zip so that you can "open declaration (F3)" a library method to jump straight into its code in src.zip.

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    Default Re: Immutable String

    The code for the substring method actually isn't that nice. What the substring method "really" does is create a new String object which references the same literal String in memory, but adjusts some pointer indexes or something. It's not as cut-and-dry as you'd expect it to be.
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    Default Re: Immutable String

    Quote Originally Posted by aussiemcgr View Post
    The code for the substring method actually isn't that nice. What the substring method "really" does is create a new String object which references the same literal String in memory, but adjusts some pointer indexes or something. It's not as cut-and-dry as you'd expect it to be.
    Indeed. Sorry for the self-promotion, but I wrote an article a while back that describes when to construct a new String object, and goes into detail about what goes on under the hood of this class, using the Reflection API to demonstrate this behavior.

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