What is difference between mutable and immutable String in java

Question

As per my knowledge   a mutable string can be changed  and an immutable string cannot be changed   Here I want to change the value of String like this   String str  Good   str str   Morning     and other way is   StringBuffer str  new StringBuffer  Good    str append   Morning      In both the cases I am trying to alter the value of str  Can anyone tell me  what is difference in both case and give me clear picture of mutable and immutable objects

User · Accepted Answer

Case 1   String str    Good   str   str     Morning     In the above code you create 3 String Objects     Good  it goes into the String Pool     Morning  it goes into the String Pool as well   Good Morning  created by concatenating  Good  and   Morning   This guy goes on the Heap     Note  Strings are always immutable  There is no  such thing as a mutable String  str is just a reference which eventually points to  Good Morning   You are actually  not working on 1 object  you have 3 distinct String Objects     Case 2   StringBuffer str   new StringBuffer  Good     str append   Morning      StringBuffer contains an array of characters  It is not same as a String  The above code adds characters to the existing array  Effectively  StringBuffer is mutable  its String representation isn t

User · Answer

When you say str  you should be careful what you mean    do you mean the variable str  or do you mean the object referenced by str    In your StringBuffer example you are not altering the value of str  and in your String example you are not altering the state of the String object   The most poignant way to experience the difference would be something like this   static void change String in       in   in     changed      static void change StringBuffer in      in append   changed       public static void main String   args       StringBuffer sb   new StringBuffer  value       String str    value      change sb      change str      System out println  StringBuffer    sb      System out println  String    str

User · Answer

A mutable variable is one whose value may change in place  whereas in an immutable variable change of value will not happen in place  Modifying an immutable variable will rebuild the same variable

User · Answer

In Java  all strings are immutable Can t change   When you are trying to modify a String  what you are really doing is creating a new one   Following ways we can create the string object   Using String literal  String str  java    Using new keyword  String str   new String  java     Using character array  char   helloArray      h    e    l    l    o           String helloString   new String helloArray         coming to String immutability   simply means unmodifiable or unchangeable  Let s take one example  I m initializing the value to the String literal s  String s  kumar     Below I m going to display the decimal representation of the location address using hashcode    System out println s hashCode       Simply printing the value of a String s  System out println  value   s     Okay  this time I m inittializing value  kumar  to s1  String s1  kumar        what you think is this line  takes new location in the memory        Okay let s check by displaying hashcode of the s1 object which we created  System out println s1 hashCode       okay  let s check below code  String s2 new String  Kumar        System out println s2 hashCode         why this gives the different address      Okay  check this below code at last  String s3 new String  KUMAR        System out println s3 hashCode         again different address       YES  if you see Strings  s  and  s1  having the same hashcode because the value hold by  s   amp   s1  are same that is  kumar    Let s consider String  s2  and  s3  these two  Strings hashcode appears to be different in the sense  they both stored in a different location because you see their values are different    since s and s1 hashcode is same because those values are same and storing in the same location   Example 1   Try below code and analyze line by line  public class StringImmutable   public static void main String   args         String s  java       System out println s hashCode         String s1  javA       System out println s1 hashCode         String s2 new String  Java        System out println s2 hashCode         String s3 new String  JAVA        System out println s3 hashCode           Example 2  Try below code and analyze line by line  public class StringImmutable       public static void main String   args             String s  java           s concat   programming        s can not be changed  immutablity          System out println  value of s   s           System out println   hashcode of s   s hashCode              String s1  java           String s2 s concat   programming         s1 can not be changed  immutablity  rather creates object s2         System out println  value of s1   s1           System out println   hashcode of s1   s1 hashCode                System out println  value of s2   s2           System out println   hashcode of s2   s2 hashCode                Okay  Let s look what is the difference between mutable and immutable   mutable it change  vs  immutable  it can t change   public class StringMutableANDimmutable       public static void main String   args                 it demonstrates immutable concept         String s  java           s concat   programming        s can not be changed  immutablity          System out println  value of s       s            System out println   hashcode of s      s hashCode     n n                 it demonstrates mutable concept         StringBuffer s1  new StringBuffer  java            s1 append   programming        s can be changed  mutablity          System out println  value of s1       s1            System out println   hashcode of s1      s1 hashCode                 Any further questions   please write on

User · Answer

In Java  all strings are immutable  When you are trying to modify a String  what you are really doing is creating a new one  However  when you use a StringBuilder  you are actually modifying the contents  instead of creating a new one

User · Answer

Java Strings are immutable    In your first example  you are changing the reference to the String  thus assigning it the value of two other Strings combined  str     Morning     On the contrary  a StringBuilder or StringBuffer can be modified through its methods

User · Answer

Mutable means you will save the same reference to variable and change its contents but immutable you can not change contents but you will declare new reference contains the new and the old value of the variable  Ex Immutable -  String  String x    value0ne     adresse one x     valueTwo     an other adresse  adresse two  adresse on the heap memory change   Mutable -  StringBuffer - StringBuilder StringBuilder sb   new StringBuilder    sb append  valueOne       adresse One sb append  valueTwo       adresse One  sb still in the same adresse i hope this comment helps

User · Answer

What is difference between mutable and immutable String in java   immutable exist  mutable don t

User · Answer

I modified the code of william with a output comments for better understandable     static void changeStr String in           in   in   changed         System out println  fun   in     value changed            static void changeStrBuf StringBuffer in          in append   changed        value changed            public static void main String   args           StringBuffer sb   new StringBuffer  value           String str    value          changeStrBuf sb          changeStr str          System out println  StringBuffer    sb     value changed        System out println  String    str            value          In above code   look at the value of str in both main   and changeStr     even though u r changing the value of str in changeStr   it is affecting only to that function but in the main function the value is not changed   but it not in the case of StringBuffer     In StringBuffer changed value is affected as a global     hence String is immutable and StringBuffer is mutable     In Simple    whatever u changed to String Object will affecting only to that function By going to String Pool  but not Changed

User · Answer

Mutable    nbsp  nbsp  nbsp  nbsp  - Changeable Immutable  - Unchangeable      String in Java is immutable  However what does it mean to be mutable in programming context is the first question  Consider following class   public class Dimension       private int height       private int width       public Dimenstion                public void setSize int height  int width            this height   height          this width   width             public getHeight             return height             public getWidth             return width            Now after creating the instance of Dimension we can always update it s attributes  Note that if any of the attribute  in other sense state  can be updated for instance of the class then it is said to be mutable  We can always do following   Dimension d   new Dimension    d setSize 10  20     Dimension changed d setSize 10  200     Dimension changed d setSize 100  200     Dimension changed   Let s see in different ways we can create a String in Java   String str1    Hey    String str2    Jack   String str3   new String  Hey Jack     String str4   new String new char     H    e    y          String str5   str1   str2  str1    Hi              So    str1 and str2 are String literals which gets created in String constant pool str3  str4 and str5 are String Objects which are placed in Heap memory str1    Hi    creates  Hi   in String constant pool and it s totally different reference than  Hey   which str1 referencing earlier    Here we are creating the String literal or String Object  Both are different  I would suggest you to read following post to understand more about it    Difference between string object and string literal   In any String declaration  one thing is common  that it does not modify but it gets created or shifted to other   String str    Good      Create the String literal in String pool str   str     Morning      Create String with concatenation of str    Morning                                  - Step 1   Concatenate  Good   and   Morning  with StringBuilder         - Step 2   assign reference of created  Good Morning  String Object to str      How String became immutable     It s non changing behaviour  means  the value once assigned can not be updated in any other way  String class internally holds data in character array  Moreover  class is created to be immutable  Take a look at this strategy for defining immutable class    Shifting the reference does not mean you changed it s value  It would be mutable if you can update the character array which is behind the scene in String class  But in reality that array will be initialized once and throughout the program it remains the same      Why StringBuffer is mutable     As you already guessed  StringBuffer class is mutable itself as you can update it s state directly  Similar to String it also holds value in character array and you can manipulate that array by different methods i e  append  delete  insert etc  which directly changes the character value array

[java] What is difference between mutable and immutable String in java

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