When should we use intern method of String on String literals

Question

According to String intern    intern method is supposed to return the String from the String pool if the String is found in String pool  otherwise a new string object will be added in String pool and the reference of this String is returned    So i tried this    String s1    Rakesh   String s2    Rakesh   String s3    Rakesh  intern     if   s1    s2        System out println  s1 and s2 are same        1     if   s1    s3        System out println  s1 and s3 are same         2      I was expecting that s1 and s3 are same will be printed as s3 is interned  and s1 and s2 are same will not be printed  But the result is  both lines are printed  So that means  by default String constants are interned  But if it is so  then why do we need the intern method  In other words when should we use this method

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Learn Java String Intern - once for all

Strings in java are immutable objects by design. Therefore, two string objects even with same value will be different objects by default. However, if we wish to save memory, we could indicate to use same memory by a concept called string intern.

The below rules would help you understand the concept in clear terms:

String class maintains an intern-pool which is initially empty. This pool must guarantee to contain string objects with only unique values.
All string literals having same value must be considered same memory-location object because they have otherwise no notion of distinction. Therefore, all such literals with same value will make a single entry in the intern-pool and will refer to same memory location.
Concatenation of two or more literals is also a literal. (Therefore rule #2 will be applicable for them)
Each string created as object (i.e. by any other method except as literal) will have different memory locations and will not make any entry in the intern-pool
Concatenation of literals with non-literals will make a non-literal. Thus, the resultant object will have a new memory location and will NOT make an entry in the intern-pool.
Invoking intern method on a string object, either creates a new object that enters the intern-pool or return an existing object from the pool that has same value. The invocation on any object which is not in the intern-pool, does NOT move the object to the pool. It rather creates another object that enters the pool.

Example:

String s1=new String (“abc”);
String s2=new String (“abc”);
If (s1==s2)  //would return false  by rule #4
If (“abc” == “a”+”bc” )  //would return true by rules #2 and #3
If (“abc” == s1 )  //would return false  by rules #1,2 and #4
If (“abc” == s1.intern() )  //would return true  by rules #1,2,4 and #6
If ( s1 == s2.intern() )      //wound return false by rules #1,4, and #6

Note: The motivational cases for string intern are not discussed here. However, saving of memory will definitely be one of the primary objectives.

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http   en wikipedia org wiki String interning  string interning is a method of storing only one copy of each distinct string value  which must be immutable  Interning strings makes some string processing tasks more time- or space-efficient at the cost of requiring more time when the string is created or interned  The distinct values are stored in a string intern pool

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String s1    Anish           String s2    Anish            String s3   new String  Anish                           When the intern method is invoked  if the pool already contains a            string equal to this String object as determined by the            method  then the string from the pool is            returned  Otherwise  this String object is added to the            pool and a reference to this String object is returned                      String s4   new String  Anish   intern            if  s1    s2                System out println  s1 and s2 are same                       if  s1    s3                System out println  s1 and s3 are same                       if  s1    s4                System out println  s1 and s4 are same                OUTPUT  s1 and s2 are same s1 and s4 are same

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I want to add my 2 cents on using    with interned strings   The first thing String equals does is this  object   So although there is some miniscule performance gain   you are not calling a method   from the maintainer point of view using    is a nightmare  because some interned strings have a tendency to become non-interned   So I suggest not to rely on special case of    for interned strings  but always use equals as Gosling intended   EDIT  interned becoming non-interned   V1 0 public class MyClass     private String reference val            private boolean hasReferenceVal   final String   strings           for   String s   strings               if   s    reference val                   return true                     return false         private void makeCall              final String   interned strings          init with interned values              if   hasReference  interned strings                                       In version 2 0 maintainer decided to make hasReferenceVal public  without going into much detail that it expects an array of interned strings   V2 0 public class MyClass     private String reference val            public boolean hasReferenceVal   final String   strings           for   String s   strings               if   s    reference val                   return true                     return false         private void makeCall              final String   interned strings          init with interned values              if   hasReference  interned strings                                       Now you have a bug  that may be very hard to find  because in majority of cases array contains literal values  and sometimes a non-literal string is used   If equals were used instead of    then hasReferenceVal would have still continue to work   Once again  performance gain is miniscule  but maintenance cost is high

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By using heap object reference if we want to get corresponding string constant pool object reference  then we should go for intern    String s1   new String  Rakesh    String s2   s1 intern    String s3    Rakesh    System out println s1    s2      false System out println s2    s3      true   Pictorial View   Step 1   Object with data  Rakesh  get created in heap and string constant pool  Also s1 is always pointing to heap object   Step 2   By using heap object reference s1  we are trying to get corresponding string constant pool object referenc s2  using intern    Step 3  Intentionally creating a object with data  Rakesh  in string constant pool  referenced by name s3  As      operator meant for reference comparison   Getting false for s1  s2   Getting true for s2  s3  Hope this help

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Java automatically interns String literals  This means that in many cases  the    operator appears to work for Strings in the same way that it does for ints or other primitive values   Since interning is automatic for String literals  the intern   method is to be used on Strings constructed with new String    Using your example   String s1    Rakesh   String s2    Rakesh   String s3    Rakesh  intern    String s4   new String  Rakesh    String s5   new String  Rakesh   intern     if   s1    s2        System out println  s1 and s2 are same        1     if   s1    s3        System out println  s1 and s3 are same         2     if   s1    s4        System out println  s1 and s4 are same         3     if   s1    s5        System out println  s1 and s5 are same         4      will return   s1 and s2 are same s1 and s3 are same s1 and s5 are same   In all the cases besides of s4 variable  a value for which was explicitly created using new operator and where intern method was not used on it s result  it is a single immutable instance that s being returned JVM s string constant pool   Refer to JavaTechniques  String Equality and Interning  for more information

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you should make out two period time which are compile time and runtime time for example     example 1   test      test     -- gt  true   test      te     st     -- gt  true    example 2   test       test  substring 1     -- gt  false  test       test  substring 1  intern      -- gt  true   in the one hand in the example 1 we find the results are all return true because in the compile time the jvm will put the  test  to the pool of literal strings if the jvm find  test  exists then it will use the exists one in example 1 the  test  strings are all point to the same memory address so the example 1 will return true  in the other hand in the example 2 the method of substring   execute in the runtime time  in the case of  test       test  substring 1  the pool will create two string object  test  and   test  so they are different reference objects so this case will return false in the case of  test       test  substring 1  intern   the method of intern   will put the    test  substring 1   to the pool of literal strings so in this case they are same reference objects so will return true

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public static void main String   args           TODO Auto-generated method stub     String s1    test       String s2   new String  test        System out println s1  s2                  false     System out println s1  s2 intern          true -- gt  because this time compiler is checking from string constant pool

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Interned Strings avoid duplicate Strings  Interning saves RAM at the expense of more CPU time to detect and replace duplicate Strings  There is only one copy of each String that has been interned  no matter how many references point to it  Since Strings are immutable  if two different methods incidentally use the same String  they can share a copy of the same String  The process of converting duplicated Strings to shared ones is called interning String intern   gives you the address of the canonical master String  You can compare interned Strings with simple     which compares pointers  instead of equals which compares the characters of the String one by one  Because Strings are immutable  the intern process is free to further save space  for example  by not creating a separate String literal for  pot  when it exists as a substring of some other literal such as  hippopotamus    To see more http   mindprod com jgloss interned html

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string intern   method is used to create an exact copy of heap string object in string constant pool  The string objects in the string constant pool are automatically interned but string objects in heap are not  The main use of creating interns is to save the memory space and to perform faster comparison of string objects    Source   What is string intern in java

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As you said  that string intern   method will first find from the String pool  if it finds  then it will return the object that points to that  or will add a new String into the pool       String s1    Hello       String s2    Hello       String s3    Hello  intern        String s4   new String  Hello         System out println s1    s2    true     System out println s1    s3    true     System out println s1    s4 intern      true   The s1 and s2 are two objects pointing to the String pool  Hello   and using  Hello  intern   will find that s1 and s2  So  s1    s3  returns true  as well as to the s3 intern

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String p1    example   String p2    example   String p3    example  intern    String p4   p2 intern    String p5   new String p3   String p6   new String  example    String p7   p6 intern     if  p1    p2      System out println  p1 and p2 are the same    if  p1    p3      System out println  p1 and p3 are the same    if  p1    p4      System out println  p1 and p4 are the same    if  p1    p5      System out println  p1 and p5 are the same    if  p1    p6      System out println  p1 and p6 are the same    if  p1    p6 intern        System out println  p1 and p6 are the same when intern is used    if  p1    p7      System out println  p1 and p7 are the same      When two strings are created independently  intern   allows you to compare them and also it helps you in creating a reference in the string pool if the reference didn t exist before   When you use String s   new String hi   java creates a new instance of the string  but when you use String s    hi   java checks if there is an instance of word  hi  in the code or not and if it exists  it just returns the reference   Since comparing strings is based on reference  intern   helps in you creating a reference and allows you to compare the contents of the strings   When you use intern   in the code  it clears of the space used by the string referring to the same object and just returns the reference of the already existing same object in memory   But in case of p5 when you are using    String p5   new String p3     Only contents of p3 are copied and p5 is created newly  So it is not interned   So the output will be   p1 and p2 are the same p1 and p3 are the same p1 and p4 are the same p1 and p6 are the same when intern is used p1 and p7 are the same

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On a recent project  some huge data structures were set up with data that was read in from a database  and hence not String constants literals  but with a huge amount of duplication  It was a banking application  and things like the names of a modest set  maybe 100 or 200  corporations appeared all over the place  The data structures were already large  and if all those corp names had been unique objects they would have overflowed memory  Instead  all the data structures had references to the same 100 or 200 String objects  thus saving lots of space   Another small advantage of interned Strings is that    can be used  successfully   to compare Strings if all involved strings are guaranteed to be interned  Apart from the leaner syntax  this is also a performance enhancement  But as others have pointed out  doing this harbors a great risk of introducing programming errors  so this should be done only as a desparate measure of last resort   The downside is that interning a String takes more time than simply throwing it on the heap  and that the space for interned Strings may be limited  depending on the Java implementation  It s best done when you re dealing with a known reasonable number of Strings with many duplications

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String literals and constants are interned by default  That is   foo      foo   declared by the String literals   but new String  foo      new String  foo

[java] When should we use intern method of String on String literals

Learn Java String Intern - once for all

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