String equals versus

Question

This code separates a string into tokens and stores them in an array of strings  and then compares a variable with the first home     why isn t it working   public static void main String   aArguments  throws IOException        String usuario    Jorman       String password    14988611        String strDatos    Jorman 14988611       StringTokenizer tokens   new StringTokenizer strDatos            int nDatos   tokens countTokens        String   datos   new String nDatos       int i   0       while  tokens hasMoreTokens              String str   tokens nextToken            datos i    str          i                 System out println  usuario        if   datos 0     usuario             System out println  WORKING

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If you are going to compare any assigned value of the string i.e. primitive string, both "==" and .equals will work, but for the new string object you should use only .equals, and here "==" will not work.

Example:

String a = "name";

String b = "name";

if(a == b) and (a.equals(b)) will return true.

But

String a = new String("a");

In this case if(a == b) will return false

So it's better to use the .equals operator...

User · Answer

The  equals   will check if the two strings have the same value and return the boolean value where as the    operator checks to see if the two strings are the same object

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a  b  Compares references  not values  The use of    with object references is generally limited to the following    Comparing to see if a reference is null  Comparing two enum values  This works because there is only one object for each enum constant  You want to know if two references are to the same object    a  equals  b    Compares values for equality  Because this method is defined in the Object class  from which all other classes are derived  it s automatically defined for every class  However  it doesn t perform an intelligent comparison for most classes unless the class overrides it  It has been defined in a meaningful way for most Java core classes  If it s not defined for a  user  class  it behaves the same as

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It will also work if you call intern   on the string before inserting it into the array  Interned strings are reference-equal      if and only if they are value-equal  equals      public static void main  String    aArguments  throws IOException    String usuario    Jorman   String password    14988611    String strDatos  Jorman 14988611   StringTokenizer tokens new StringTokenizer strDatos        int nDatos tokens countTokens    String   datos new String nDatos   int i 0   while tokens hasMoreTokens          String str tokens nextToken        datos i   str intern                    i         System out println  usuario    if datos 0   usuario           System out println   WORKING

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tests for reference equality    equals   tests for value equality   Consequently  if you actually want to test whether two strings have the same value you should use  equals    except in a few situations where you can guarantee that two strings with the same value will be represented by the same object eg  String interning       is for testing whether two strings are the same Object      These two have the same value new String  test   equals  test      gt  true          but they are not the same object new String  test       test     gt  false          neither are these new String  test      new String  test      gt  false          but these are because literals are interned by     the compiler and thus refer to the same object  test      test     gt  true      concatenation of string literals happens at compile time resulting in same objects  test      te     st      gt  true     but  substring   is invoked at runtime  generating distinct objects  test       test  substring 1     gt  false   It is important to note that    is much cheaper than equals    a single pointer comparision instead of a loop   thus  in situations where it is applicable  i e  you can guarantee that you are only dealing with interned strings  it can present an important performance improvement  However  these situations are rare

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I know this is an old question but here s how I look at it  I find very useful      Technical explanations  In Java  all variables are either primitive types or references    If you need to know what a reference is   Object variables  are just pointers to objects  So with Object something        something is really an address in memory  a number        compares the exact values  So it compares if the primitive values are the same  or if the references  addresses  are the same  That s why    often doesn t work on Strings  Strings are objects  and doing    on two string variables just compares if the address is same in memory  as others have pointed out   equals   calls the comparison method of objects  which will compare the actual objects pointed by the references  In the case of Strings  it compares each character to see if they re equal     The interesting part   So why does    sometimes return true for Strings  Note that Strings are immutable  In your code  if you do  String foo    hi   String bar    hi     Since strings are immutable  when you call  trim   or something  it produces a new string  not modifying the original object pointed to in memory   you don t really need two different String  hi   objects  If the compiler is smart  the bytecode will read to only generate one String  hi   object  So if you do   if  foo    bar        right after  they re pointing to the same object  and will return true  But you rarely intend this  Instead  you re asking for user input  which is creating new strings at different parts of memory  etc  etc   Note  If you do something like baz   new String bar  the compiler may still figure out they re the same thing  But the main point is when the compiler sees literal strings  it can easily optimize same strings   I don t know how it works in runtime  but I assume the JVM doesn t keep a list of  live strings  and check if a same string exists   eg if you read a line of input twice  and the user enters the same input twice  it won t check if the second input string is the same as the first  and point them to the same memory   It d save a bit of heap memory  but it s so negligible the overhead isn t worth it  Again  the point is it s easy for the compiler to optimize literal strings   There you have it    a gritty explanation for    vs   equals   and why it seems random

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You should use string equals to compare two strings for equality  not operator    which just compares the references

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The    operator is a simple comparison of values  For object references the  values  are the  references   So x    y returns true if x and y reference the same object

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Use the string equals Object other  function to compare strings  not the    operator   The function checks the actual contents of the string  the    operator checks whether the references to the objects are equal   Note that string constants are usually  interned  such that two constants with the same value can actually be compared with     but it s better not to rely on that   if  usuario equals datos 0                  NB  the compare is done on  usuario  because that s guaranteed non-null in your code  although you should still check that you ve actually got some tokens in the datos array otherwise you ll get an array-out-of-bounds exception

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operator compares the reference of an object in Java  You can use string s equals method    String s    Test   if s equals  Test          System out println  Equal

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The    operator checks if the two references point to the same object or not   equals   checks for the actual string content  value     Note that the  equals   method belongs to class Object  super class of all classes   You need to override it as per you class requirement  but for String it is already implemented and it checks whether two strings have the same value or not   Case1  String s1    Stack Overflow   String s2    Stack Overflow   s1    s1          true s1 equals s2      true Reason  String literals created without null are stored in the string pool in the permgen area of the heap  So both s1 and s2 point to the same object in the pool  Case2  String s1   new String  Stack Overflow    String s2   new String  Stack Overflow    s1    s2          false s1 equals s2      true Reason  If you create a String object using the  new  keyword a separate space is allocated to it on the heap

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It s good to notice that in some cases use of      operator can lead to the expected result  because the way how java handles strings - string literals are interned   see String intern    during compilation - so when you write for example  hello world  in two classes and compare those strings with      you could get result  true  which is expected according to specification  when you compare same strings  if they have same value  when the first one is string literal  ie  defined through  i am string literal   and second is constructed during runtime ie  with  new  keyword like new String  i am string literal    the     equality  operator returns false  because both of them are different instances of the String class    Only right way is using  equals   -  datos 0  equals usuario      says only if two objects are the same instance of object  ie  have same memory address   Update  01 04 2013 I updated this post due comments below which are somehow right  Originally I declared that interning  String intern  is side effect of JVM optimization  Although it certainly save memory resources  which was what i meant by  optimization   it is mainly feature of language

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Someone said on a post higher up that     is used for int and for checking nulls  It may also be used to check for Boolean operations and char types   Be very careful though and double check that you are using a char and not a String  for example       String strType    a       char charType    a     for strings you would then check This would be correct      if strType equals  a           do something   but       if charType equals  a           do something else   would be incorrect  you would need to do the following      if charType     a            do something else

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Meet Jorman  Jorman is a successful businessman and has 2 houses      But others don t know that   Is it the same Jorman   When you ask neighbours from either Madison or Burke streets  this is the only thing they can say     Using the residence alone  it s tough to confirm that it s the same Jorman  Since they re 2 different addresses  it s just natural to assume that those are 2 different persons   That s how the operator    behaves  So it will say that datos 0   usuario is false  because it only compares the addresses   An Investigator to the Rescue  What if we sent an investigator  We know that it s the same Jorman  but we need to prove it  Our detective will look closely at all physical aspects  With thorough inquiry  the agent will be able to conclude whether it s the same person or not  Let s see it happen in Java terms   Here s the source code of String s equals   method     It compares the Strings character by character  in order to come to a conclusion that they are indeed equal   That s how the String equals method behaves  So datos 0  equals usuario  will return true  because it performs a logical comparison

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Generally  equals is used for Object comparison  where you want to verify if two Objects have an identical value      for reference comparison  are the two Objects the same Object on the heap   amp  to check if the Object is null  It is also used to compare the values of primitive types

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Let s analyze the following Java  to understand the identity and equality of Strings   public static void testEquality        String str1    Hello world        String str2    Hello world         if  str1    str2          System out print  str1    str2 n        else         System out print  str1    str2 n         if str1 equals str2           System out print  str1 equals to str2 n        else         System out print  str1 doesn t equal to str2 n         String str3   new String  Hello world         String str4   new String  Hello world          if  str3    str4          System out print  str3    str4 n        else         System out print  str3    str4 n         if str3 equals str4           System out print  str3 equals to str4 n        else         System out print  str3 doesn t equal to str4 n        When the first line of code String str1    Hello world   executes  a string  Hello world   is created  and the variable str1 refers to it  Another string  Hello world   will not be created again when the next line of code executes because of optimization  The variable str2 also refers to the existing   Hello world     The operator    checks identity of two objects  whether two variables refer to same object   Since str1 and str2 refer to same string in memory  they are identical to each other  The method equals checks equality of two objects  whether two objects have same content   Of course  the content of str1 and str2 are same   When code String str3   new String  Hello world    executes  a new instance of string with content  Hello world   is created  and it is referred to by the variable str3  And then another instance of string with content  Hello world   is created again  and referred to by str4  Since str3 and str4 refer to two different instances  they are not identical  but their content are same   Therefore  the output contains four lines   Str1    str2  Str1 equals str2  Str3    str4  Str3 equals str4

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Instead of  datos 0     usuario   use   datos 0  equals usuario       compares the reference of the variable where  equals   compares the values which is what you want

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Melkhiah66 You can use equals method instead of      method to check the equality  If you use intern   then it checks whether the object is in pool if present then returns equal else unequal  equals method internally uses hashcode and gets you the required result    public class Demo     public static void main String   args                    String str1    Jorman 14988611       String str2   new StringBuffer  Jorman   append   14988611   toString        String str3   str2 intern        System out println  str1    str2      str1    str2                gives false     System out println  str1    str3      str1    str3                gives true     System out println  str1 equals str2      str1 equals str2        gives true     System out println  str1 equals str3      str1 equals str3        gives true

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equals   function is a method of Object class which should be overridden by programmer  String class overrides it to check if two strings are equal i e  in content and not reference       operator checks if the references of both the objects are the same    Consider the programs  String abc    Awesome    String xyz    abc   if abc    xyz       System out println  Refers to same string      Here the abc and xyz  both refer to same String  Awesome   Hence the expression  abc    xyz  is true   String abc    Hello World   String xyz    Hello World    if abc    xyz      System out println  Refers to same string    else     System out println  Refers to different strings     if abc equals xyz        System out prinln  Contents of both strings are same    else      System out prinln  Contents of strings are different      Here abc and xyz are two different strings with the same content  Hello World   Hence here the expression  abc    xyz  is false where as  abc equals xyz   is true   Hope you understood the difference between    and  lt Object gt  equals     Thanks

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Use Split rather than tokenizer it will surely provide u exact output for E g   string name  Harry   string salary  25000   string namsal  Harry 25000   string   s namsal split       for int i 0 i lt s length i      System out println s i      if s 0  equals  Harry      System out println  Task Complete        After this I am sure you will get better results

[java] String.equals versus ==

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