memory address?


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Thread: memory address?

  1. #1
    Join Date
    Oct 2004
    Posts
    1

    Unhappy memory address?

    Hi

    Here is the code:

    import java.io.*;
    import java.util.*;

    public class test{


    public static void main(String args[]) throws IOException
    {

    BufferedReader stdin = new BufferedReader( new InputStreamReader ( System.in ));

    String mystr = new String("1");

    string(mystr);

    System.out.println(mystr);



    }

    public static void string(String str){

    str = new String("2");



    }

    }

    I just wonder why mystring value is not "2"??
    There might be somehow the two variables mystring and str has different memory address, but I don't know why? How can I make "mystring" changed to "2"??

    Thanks

  2. #2
    Join Date
    Feb 2004
    Posts
    808
    you cant. string is immutable.

    i'll explain what goes on, line by line. it will help to think of java objects as some object, like a ball. it will help to think of the named variable you give as being like a piece of paper tied to the object with string.. a tag, that you have a hold of

    now, yu make a new string, and somewhere in memory comes the word "1", tied to it is a label tag, mystr:

    mystr-------("1")

    you call a method with it, passing in the label. now, your method has the parameter named as String str.. and just like String myStr, this also attaches a label to our object in memory.. note it attaches to the object, NOT your tag:

    mystr-------("1")--------str

    not:

    str------mystr------("1")

    not that it would matter but.. you need to be clear that one object now has 2 labels (see above above)


    then you make a new object and reattach the str tag to the new:

    Code:
    mystr--------("1")           str------("2")
    then the method ends, str goes out of scope and the label is destroyed:

    Code:
    mystr--------("1")                      ("2")

    the new string "2" is now orphaned and will be garbage collected; it's dead

    and mystr is still attached to "1"



    this cannot be violated.. string cannot be changed once created..
    only a method that works on the object will show a change. as soon as you use the "new" keyword, you are not changing the object; youre making another
    The 6th edict:
    "A thing of reference thing can hold either a null thing or a thing to any thing whose thing is assignment compatible with the thing of the thing" - ArchAngel, www.dictionary.com et al.
    JAR tutorial GridBag tutorial Inherited Shapes Inheritance? String.split(); FTP?

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