What is String pool in Java

Question

I am confused about StringPool in Java  I came across this while reading the String chapter in Java  Please help me understand  in layman terms  what StringPool actually does

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When the JVM loads classes  or otherwise sees a literal string  or some code interns a string  it adds the string to a mostly-hidden lookup table that has one copy of each such string   If another copy is added  the runtime arranges it so that all the literals refer to the same string object   This is called  interning    If you say something like  String s    test   return  s     test      it ll return true  because the first and second  test  are actually the same object   Comparing interned strings this way can be much  much faster than String equals  as there s a single reference comparison rather than a bunch of char comparisons   You can add a string to the pool by calling String intern    which will give you back the pooled version of the string  which could be the same string you re interning  but you d be crazy to rely on that -- you often can t be sure exactly what code has been loaded and run up til now and interned the same string    The pooled version  the string returned from intern  will be equal to any identical literal   For example   String s1    test   String s2   new String  test         new String  guarantees a different object  System out println s1    s2       should print  false   s2   s2 intern    System out println s1    s2       should print  true

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This prints true  even though we don t use equals method  correct way to compare strings       String s    a     bc       String t    ab     c       System out println s    t     When compiler optimizes your string literals  it sees that both s and t have same value and thus you need only one string object  It s safe because String is immutable in Java  As result  both s and t point to the same object and some little memory saved   Name  string pool  comes from the idea that all already defined string are stored in some  pool  and before creating new String object compiler checks if such string is already defined

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I don t think it actually does much  it looks like it s just a cache for string literals   If you have multiple Strings who s values are the same  they ll all point to the same string literal in the string pool    String s1    Arul     case 1  String s2    Arul     case 2    In case 1  literal s1 is created newly and kept in the pool  But in case 2  literal s2 refer the s1  it will not create new one instead     if s1    s2  System out println  equal      Prints equal    String n1   new String  Arul     String n2   new String  Arul     if n1    n2  System out println  equal      No output      http   p2p wrox com java-espanol 29312-string-pooling html

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Let s start with a quote from the virtual machine spec      Loading of a class or interface that contains a String literal may create a new String object    2 4 8  to represent that literal  This may not occur if the a String object has already been created to represent a previous occurrence of that literal  or if the String intern method has been invoked on a String object representing the same string as the literal    This may not occur - This is a hint  that there s something special about String objects  Usually  invoking a constructor will always create a new instance of the class  This is not the case with Strings  especially when String objects are  created  with literals  Those Strings are stored in a global store  pool  - or at least the references are kept in a pool  and whenever a new instance of an already known Strings is needed  the vm returns a reference to the object from the pool  In pseudo code  it may go like that   1  a     one      -- gt  if pool hash  one       null      true            pool hash  one   -- gt   one          return pool hash  one     2  b     one     -- gt  if pool hash  one       null       false   one  already in pool         pool hash  one   -- gt   one         return pool hash  one       So in this case  variables a and b hold references to the same object  IN this case  we have  a    b   amp  amp   a equals b      true   This is not the case if we use the constructor   1  a     one  2  b    new String  one     Again   one  is created on the pool but then we create a new instance from the same literal  and in this case  it leads to  a    b   amp  amp   a equals b      false  So why do we have a String pool  Strings and especially String literals are widely used in typical Java code  And they are immutable  And being immutable allowed to cache String to save memory and increase performance  less effort for creation  less garbage to be collected    As programmers we don t have to care much about the String pool  as long as we keep in mind     a    b   amp  amp   a equals b   may be true or false  always use equals to compare Strings  Don t use reflection to change the backing char   of a String  as you don t know who is actualling using that String

[java] What is String pool in Java?

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