What is Java String interning

Question

What is String Interning in Java  when I should use it  and why

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http   docs oracle com javase 7 docs api java lang String html intern    Basically doing String intern   on a series of strings will ensure that all strings having same contents share same memory  So if you have list of names where  john  appears 1000 times  by interning you ensure only one  john  is actually allocated memory   This can be useful to reduce memory requirements of your program  But be aware that the cache is maintained by JVM in permanent memory pool which is usually limited in size compared to heap so you should not use intern if you don t have too many duplicate values     More on memory constraints of using intern       On one hand  it is true that you can remove String duplicates by   internalizing them  The problem is that the internalized strings go to   the Permanent Generation  which is an area of the JVM that is reserved   for non-user objects  like Classes  Methods and other internal JVM   objects  The size of this area is limited  and is usually much smaller   than the heap  Calling intern   on a String has the effect of moving   it out from the heap into the permanent generation  and you risk   running out of PermGen space    --  From  http   www codeinstructions com 2009 01 busting-javalangstringintern-myths html    From JDK 7  I mean in HotSpot    something has changed      In JDK 7  interned strings are no longer allocated in the permanent generation of the Java heap  but are instead allocated in the main part of the Java heap  known as the young and old generations   along with the other objects created by the application  This change will result in more data residing in the main Java heap  and less data in the permanent generation  and thus may require heap sizes to be adjusted  Most applications will see only relatively small differences in heap usage due to this change  but larger applications that load many classes or make heavy use of the String intern   method will see more significant differences    -- From Java SE 7 Features and Enhancements  Update  Interned strings are stored in main heap from Java 7 onwards  http   www oracle com technetwork java javase jdk7-relnotes-418459 html jdk7changes

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There are some  catchy interview  questions  such as why you get equals  if you execute the below piece of code   String s1    testString   String s2    testString   if s1    s2  System out println  equals       If you want to compare  Strings you should use equals    The above will print equals because the testString is already interned for you by the compiler  You can intern the strings yourself using intern method as is shown in previous answers

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String interning is an optimization technique by the compiler  If you have two identical string literals in one compilation unit then the code generated ensures that there is only one string object created for all the instance of that literal characters enclosed in double quotes  within the assembly   I am from C  background  so i can explain by giving a example from that   object obj    Int32   string str1    Int32   string str2   typeof int  Name    output of the following comparisons   Console WriteLine obj    str1      true Console WriteLine str1    str2      true     Console WriteLine obj    str2      false      Note1 Objects are compared by reference   Note2 typeof int  Name is evaluated by reflection method so it does not gets evaluated at compile time  Here these comparisons are made at compile time   Analysis of the Results  1  true because they both contain same literal and so the code generated will have only one object referencing  Int32   See Note 1   2  true because the content of both the value is checked which is same   3  FALSE because str2 and obj does not have the same literal  See Note 2

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Java interning   method basically makes sure that if String object is present in SCP  If yes then it returns that object and if not then creates that objects in SCP and return its references  for eg  String s1 new String  abc            String s2  abc           String s3  abc    s1  s2   false  because 1 object of s1 is stored in heap and other in scp but this objects doesn t have explicit reference  and s2 in scp s2  s3   true  now if we do intern on s1 s1 s1 intern       JVM checks if there is any string in the pool with value    abc    is present  Since there is a string object in the pool with value    abc     its reference is returned  Notice that we are calling s1   s1 intern    so the s1 is now referring to the string pool object having value    abc     At this point  all the three string objects are referring to the same object in the string pool  Hence s1  s2 is returning true now

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Since strings are objects and since all objects in Java are always stored only in the heap space  all strings are stored in the heap space  However  Java keeps strings created without using the new keyword in a special area of the heap space  which is called  quot string pool quot   Java keeps the strings created using the new keyword in the regular heap space  The purpose of the string pool is to maintain a set of unique strings  Any time you create a new string without using the new keyword  Java checks whether the same string already exists in the string pool  If it does  Java returns a reference to the same String object and if it does not  Java creates a new String object in the string pool and returns its reference  So  for example  if you use the string  quot hello quot  twice in your code as shown below  you will get a reference to the same string  We can actually test this theory out by comparing two different reference variables using the    operator as shown in the following code  String str1    quot hello quot   String str2    quot hello quot   System out println str1    str2     prints true  String str3   new String  quot hello quot    String str4   new String  quot hello quot     System out println str1    str3     prints false System out println str3    str4     prints false      operator is simply checks whether two references point to the same object or not and returns true if they do  In the above code  str2 gets the reference to the same String object which was created earlier  However  str3 and str4 get references to two entirely different String objects  That is why str1    str2 returns true but str1    str3 and str3    str4 return false   In fact  when you do new String  quot hello quot    two String objects are created instead of just one if this is the first time the string  quot hello quot  is used in the anywhere in program - one in the string pool because of the use of a quoted string  and one in the regular heap space because of the use of new keyword  String pooling is Java s way of saving program memory by avoiding the creation of multiple String objects containing the same value  It is possible to get a string from the string pool for a string created using the new keyword by using String s intern method  It is called  quot interning quot  of string objects  For example  String str1    quot hello quot   String str2   new String  quot hello quot    String str3   str2 intern      get an interned string obj  System out println str1    str2     prints false System out println str1    str3     prints true  OCP Java SE 11 Programmer  Deshmukh

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Update for Java 8 or plus  In Java 8  PermGen  Permanent Generation  space is removed and replaced by Meta Space  The String pool memory is moved to the heap of JVM    Compared with Java 7  the String pool size is increased in the heap  Therefore  you have more space for internalized Strings  but you have less memory for the whole application   One more thing  you have already known that when comparing 2  referrences of  objects in Java       is used for comparing the reference of object   equals  is used for comparing the contents of object   Let s check this code   String value1    70   String value2    70   String value3   new Integer 70  toString      Result   value1    value2          ---  true  value1    value3          ---  false  value1 equals value3      ---  true  value1    value3 intern   ---  true  That s why you should use  equals  to compare 2 String objects  And that s is how intern   is useful

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JLS  JLS 7 3 10 5 defines it and gives a practical example      Moreover  a string literal always refers to the same instance of class String  This is because string literals - or  more generally  strings that are the values of constant expressions    15 28  - are  interned  so as to share unique instances  using the method String intern       Example 3 10 5-1  String Literals      The program consisting of the compilation unit    7 3     package testPackage  class Test       public static void main String   args            String hello    Hello   lo    lo           System out print  hello     Hello                   System out print  Other hello    hello                  System out print  other Other hello    hello                  System out print  hello      Hel   lo                    System out print  hello      Hel  lo                   System out println hello      Hel  lo  intern             class Other   static String hello    Hello           and the compilation unit   package other  public class Other   public static String hello    Hello           produces the output   true true true true false true    JVMS  JVMS 7 5 1 says says that interning is implemented magically and efficiently with a dedicated CONSTANT String info struct  unlike most other objects which have more generic representations       A string literal is a reference to an instance of class String  and is derived from a CONSTANT String info structure    4 4 3  in the binary representation of a class or interface  The CONSTANT String info structure gives the sequence of Unicode code points constituting the string literal       The Java programming language requires that identical string literals  that is  literals that contain the same sequence of code points  must refer to the same instance of class String  JLS   3 10 5   In addition  if the method String intern is called on any string  the result is a reference to the same class instance that would be returned if that string appeared as a literal  Thus  the following expression must have the value true     a     b     c   intern       abc        To derive a string literal  the Java Virtual Machine examines the sequence of code points given by the CONSTANT String info structure          If the method String intern has previously been called on an instance of class String containing a sequence of Unicode code points identical to that given by the CONSTANT String info structure  then the result of string literal derivation is a reference to that same instance of class String    Otherwise  a new instance of class String is created containing the sequence of Unicode code points given by the CONSTANT String info structure  a reference to that class instance is the result of string literal derivation  Finally  the intern method of the new String instance is invoked       Bytecode  Let s decompile some OpenJDK 7 bytecode to see interning in action   If we decompile   public class StringPool       public static void main String   args            String a    abc           String b    abc           String c   new String  abc            System out println a           System out println b           System out println a    c             we have on the constant pool    2   String              32      abc        32   Utf8               abc   and main    0  ldc            2             String abc  2  astore 1  3  ldc            2             String abc  5  astore 2  6  new            3             class java lang String  9  dup 10  ldc            2             String abc 12  invokespecial  4             Method java lang String   lt init gt    Ljava lang String  V 15  astore 3 16  getstatic      5             Field java lang System out Ljava io PrintStream  19  aload 1 20  invokevirtual  6             Method java io PrintStream println  Ljava lang String  V 23  getstatic      5             Field java lang System out Ljava io PrintStream  26  aload 2 27  invokevirtual  6             Method java io PrintStream println  Ljava lang String  V 30  getstatic      5             Field java lang System out Ljava io PrintStream  33  aload 1 34  aload 3 35  if acmpne     42 38  iconst 1 39  goto          43 42  iconst 0 43  invokevirtual  7             Method java io PrintStream println  Z V   Note how    0 and 3  the same ldc  2 constant is loaded  the literals  12  a new string instance is created  with  2 as argument  35  a and c are compared as regular objects with if acmpne   The representation of constant strings is quite magic on the bytecode    it has a dedicated CONSTANT String info structure  unlike regular objects  e g  new String  the struct points to a CONSTANT Utf8 info Structure that contains the data  That is the only necessary data to represent the string    and the JVMS quote above seems to say that whenever the Utf8 pointed to is the same  then identical instances are loaded by ldc   I have done similar tests for fields  and    static final String s    abc  points to the constant table through the ConstantValue Attribute non-final fields don t have that attribute  but can still be initialized with ldc   Conclusion  there is direct bytecode support for the string pool  and the memory representation is efficient   Bonus  compare that to the Integer pool  which does not have direct bytecode support  i e  no CONSTANT String info analogue

[java] What is Java String interning?

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