How do I compare strings in Java

Question

I ve been using the    operator in my program to compare all my strings so far  However  I ran into a bug  changed one of them into  equals   instead  and it fixed the bug   Is    bad  When should it and should it not be used  What s the difference

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All objects are guaranteed to have a  equals   method since Object contains a method   equals    that returns a boolean  It is the subclass  job to override this method if a further defining definition is required  Without it  i e  using     only memory addresses are checked between two objects for equality  String overrides this  equals   method and instead of using the memory address it returns the comparison of strings at the character level for equality    A key note is that strings are stored in one lump pool so once a string is created it is forever stored in a program at the same address  Strings do not change  they are immutable  This is why it is a bad idea to use regular string concatenation if you have a serious of amount of string processing to do  Instead you would use the StringBuilder classes provided  Remember the pointers to this string can change and if you were interested to see if two pointers were the same    would be a fine way to go  Strings themselves do not

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tests object references   equals   tests the string values     Sometimes it looks as if    compares values  because Java does some behind-the-scenes stuff to make sure identical in-line strings are actually the same object   For example     String fooString1   new String  foo    String fooString2   new String  foo        Evaluates to false fooString1    fooString2      Evaluates to true fooString1 equals fooString2       Evaluates to true  because Java uses the same object  bar      bar     But beware of nulls        handles null strings fine  but calling  equals   from a null string will cause an exception     String nullString1   null  String nullString2   null      Evaluates to true System out print nullString1    nullString2       Throws a NullPointerException System out print nullString1 equals nullString2      So if you know that fooString1 may be null  tell the reader that by writing  System out print fooString1    null  amp  amp  fooString1 equals  bar       The following are shorter  but it   s less obvious that it checks for null   System out print  bar  equals fooString1         bar  is never null System out print Objects equals fooString1   bar         Java 7 required

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The    operator check if the two references point to the same object or not   equals   check 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    Case 1  String s1    Stack Overflow   String s2    Stack Overflow   s1    s2         true s1 equals s2     true   Reason  String literals created without null are stored in the String pool in the permgen area of heap  So both s1 and s2 point to same object in the pool  Case 2  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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You can also use the compareTo   method to compare two Strings  If the compareTo result is 0  then the two strings are equal  otherwise the strings being compared are not equal    The    compares the references and does not compare the actual strings  If you did create every string using new String somestring  intern   then you can use the    operator to compare two strings  otherwise equals   or compareTo methods can only be used

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compares Object references    equals   compares String values   Sometimes    gives illusions of comparing String values  as in following cases   String a  Test   String b  Test   if a  b      gt  true   This is because when you create any String literal  the JVM first searches for that literal in the String pool  and if it finds a match  that same reference will be given to the new String  Because of this  we get    a  b       true                         String Pool      b ----------------- gt   test   lt -----------------a   However     fails in the following case   String a  test   String b new String  test    if  a  b      gt  false   In this case for new String  test   the statement new String will be created on the heap  and that reference will be given to b  so b will be given a reference on the heap  not in String pool   Now a is pointing to a String in the String pool while b is pointing to a String on the heap  Because of that we get   if a  b       false                   String Pool       test   lt -------------------- a                     Heap       test   lt -------------------- b   While  equals   always compares a value of String so it gives true in both cases   String a  Test   String b  Test   if a equals b       gt  true  String a  test   String b new String  test    if a equals b       gt  true   So using  equals   is always better

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I think that when you define a String you define an object  So you need to use  equals    When you use primitive data types you use    but with String  and any object  you must use  equals

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Java have a String pool under which Java manages the memory allocation for the String objects  See String Pools in Java  When you check  compare  two objects using the    operator it compares the address equality into the string-pool  If the two String objects have the same address references then it returns true  otherwise false  But if you want to compare the contents of two String objects then you must override the equals method   equals is actually the method of the Object class  but it is Overridden into the String class and a new definition is given which compares the contents of object   Example      stringObjectOne equals stringObjectTwo     But mind it respects the case of String  If you want case insensitive compare then you must go for the equalsIgnoreCase method of the String class    Let s See   String one      HELLO    String two      HELLO    String three   new String  HELLO     String four     hello     one    two       TRUE one    three     FALSE one    four      FALSE  one equals two                 TRUE one equals three               TRUE one equals four                FALSE one equalsIgnoreCase four      TRUE

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The    operator checks to see if the two strings are exactly the same object   The  equals   method will check if the two strings have the same value

User · Answer

Yes     is bad for comparing Strings  any objects really  unless you know they re canonical       just compares object references    equals   tests for equality  For Strings  often they ll be the same but as you ve discovered  that s not guaranteed always

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Yea  it s bad        means that your two string references are exactly the same object  You may have heard that this is the case because Java keeps sort of a literal table  which it does   but that is not always the case  Some strings are loaded in different ways  constructed from other strings  etc   so you must never assume that two identical strings are stored in the same location   Equals does the real comparison for you

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tests for reference equality  whether they are the same object    equals   tests for value equality  whether they are logically  quot equal quot    Objects equals   checks for null before calling  equals   so you don t have to  available as of JDK7  also available in Guava   Consequently  if you want to test whether two strings have the same value you will probably want to use Objects equals       These two have the same value new String  quot test quot   equals  quot test quot      -- gt  true          but they are not the same object new String  quot test quot       quot test quot     -- gt  false          neither are these new String  quot test quot      new String  quot test quot      -- gt  false          but these are because literals are interned by     the compiler and thus refer to the same object  quot test quot      quot test quot     -- gt  true          string literals are concatenated by the compiler    and the results are interned   quot test quot      quot te quot     quot st quot     -- gt  true         but you should really just call Objects equals   Objects equals  quot test quot   new String  quot test quot       -- gt  true Objects equals null   quot test quot      -- gt  false Objects equals null  null     -- gt  true  You almost always want to use Objects equals    In the rare situation where you know you re dealing with interned strings  you can use     From JLS 3 10 5  String Literals   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  quot interned quot  so as to share unique instances  using the method String intern   Similar examples can also be found in JLS 3 10 5-1  Other Methods To Consider String equalsIgnoreCase   value equality that ignores case  Beware  however  that this method can have unexpected results in various locale-related cases  see this question  String contentEquals   compares the content of the String with the content of any CharSequence  available since Java 1 5   Saves you from having to turn your StringBuffer  etc into a String before doing the equality comparison  but leaves the null checking to you

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String a   new String  foo    String b   new String  foo    System out println a    b      prints false System out println a equals b       prints true   Make sure you understand why   It s because the    comparison only compares references  the equals   method does a character-by-character comparison of the contents   When you call new for a and b  each one gets a new reference that points to the  foo  in the string table   The references are different  but the content is the same

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I agree with the answer from zacherates   But what you can do is to call intern   on your non-literal strings   From zacherates example          but they are not the same object new String  test       test     gt  false    If you intern the non-literal String equality is true   new String  test   intern       test     gt  true

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In Java  when the    operator is used to compare 2 objects  it checks to see if the objects refer to the same place in memory  In other words  it checks to see if the 2 object names are basically references to the same memory location  The Java String class actually overrides the default equals   implementation in the Object class     and it overrides the method so that it checks only the values of the strings  not their locations in memory  This means that if you call the equals   method to compare 2 String objects  then as long as the actual sequence of characters is equal  both objects are considered equal   The    operator checks if the two strings are exactly the same object    The  equals   method  check if the two strings have the same value

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String a   new String  foo    String b   new String  foo    System out println a    b      prints false System out println a equals b       prints true   Make sure you understand why   It s because the    comparison only compares references  the equals   method does a character-by-character comparison of the contents   When you call new for a and b  each one gets a new reference that points to the  foo  in the string table   The references are different  but the content is the same

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Java have a String pool under which Java manages the memory allocation for the String objects  See String Pools in Java  When you check  compare  two objects using the    operator it compares the address equality into the string-pool  If the two String objects have the same address references then it returns true  otherwise false  But if you want to compare the contents of two String objects then you must override the equals method   equals is actually the method of the Object class  but it is Overridden into the String class and a new definition is given which compares the contents of object   Example      stringObjectOne equals stringObjectTwo     But mind it respects the case of String  If you want case insensitive compare then you must go for the equalsIgnoreCase method of the String class    Let s See   String one      HELLO    String two      HELLO    String three   new String  HELLO     String four     hello     one    two       TRUE one    three     FALSE one    four      FALSE  one equals two                 TRUE one equals three               TRUE one equals four                FALSE one equalsIgnoreCase four      TRUE

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I agree with the answer from zacherates   But what you can do is to call intern   on your non-literal strings   From zacherates example          but they are not the same object new String  test       test     gt  false    If you intern the non-literal String equality is true   new String  test   intern       test     gt  true

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compares object references in Java  and that is no exception for String objects   For comparing the actual contents of objects  including String   one must use the equals method   If a comparison of two String objects using    turns out to be true  that is because the String objects were interned  and the Java Virtual Machine is having multiple references point to the same instance of String  One should not expect that comparing one String object containing the same contents as another String object using    to evaluate as true

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Yes     is bad for comparing Strings  any objects really  unless you know they re canonical       just compares object references    equals   tests for equality  For Strings  often they ll be the same but as you ve discovered  that s not guaranteed always

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tests for reference equality  whether they are the same object    equals   tests for value equality  whether they are logically  quot equal quot    Objects equals   checks for null before calling  equals   so you don t have to  available as of JDK7  also available in Guava   Consequently  if you want to test whether two strings have the same value you will probably want to use Objects equals       These two have the same value new String  quot test quot   equals  quot test quot      -- gt  true          but they are not the same object new String  quot test quot       quot test quot     -- gt  false          neither are these new String  quot test quot      new String  quot test quot      -- gt  false          but these are because literals are interned by     the compiler and thus refer to the same object  quot test quot      quot test quot     -- gt  true          string literals are concatenated by the compiler    and the results are interned   quot test quot      quot te quot     quot st quot     -- gt  true         but you should really just call Objects equals   Objects equals  quot test quot   new String  quot test quot       -- gt  true Objects equals null   quot test quot      -- gt  false Objects equals null  null     -- gt  true  You almost always want to use Objects equals    In the rare situation where you know you re dealing with interned strings  you can use     From JLS 3 10 5  String Literals   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  quot interned quot  so as to share unique instances  using the method String intern   Similar examples can also be found in JLS 3 10 5-1  Other Methods To Consider String equalsIgnoreCase   value equality that ignores case  Beware  however  that this method can have unexpected results in various locale-related cases  see this question  String contentEquals   compares the content of the String with the content of any CharSequence  available since Java 1 5   Saves you from having to turn your StringBuffer  etc into a String before doing the equality comparison  but leaves the null checking to you

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If you re like me  when I first started using Java  I wanted to use the      operator to test whether two String instances were equal  but for better or worse  that s not the correct way to do it in Java   In this tutorial I ll demonstrate several different ways to correctly compare Java strings  starting with the approach I use most of the time  At the end of this Java String comparison tutorial I ll also discuss why the      operator doesn t work when comparing Java strings   Option 1  Java String comparison with the equals method Most of the time  maybe 95  of the time  I compare strings with the equals method of the Java String class  like this   if  string1 equals string2     This String equals method looks at the two Java strings  and if they contain the exact same string of characters  they are considered equal   Taking a look at a quick String comparison example with the equals method  if the following test were run  the two strings would not be considered equal because the characters are not the exactly the same  the case of the characters is different    String string1    foo   String string2    FOO    if  string1 equals string2            this line will not print because the        java string equals method returns false      System out println  The two strings are the same        But  when the two strings contain the exact same string of characters  the equals method will return true  as in this example   String string1    foo   String string2    foo       test for equality with the java string equals method if  string1 equals string2            this line WILL print     System out println  The two strings are the same        Option 2  String comparison with the equalsIgnoreCase method  In some string comparison tests you ll want to ignore whether the strings are uppercase or lowercase  When you want to test your strings for equality in this case-insensitive manner  use the equalsIgnoreCase method of the String class  like this   String string1    foo   String string2    FOO        java string compare while ignoring case  if  string1 equalsIgnoreCase string2              this line WILL print      System out println  Ignoring case  the two strings are the same         Option 3  Java String comparison with the compareTo method  There is also a third  less common way to compare Java strings  and that s with the String class compareTo method  If the two strings are exactly the same  the compareTo method will return a value of 0  zero   Here s a quick example of what this String comparison approach looks like   String string1    foo bar   String string2    foo bar       java string compare example if  string1 compareTo string2     0           this line WILL print     System out println  The two strings are the same        While I m writing about this concept of equality in Java  it s important to note that the Java language includes an equals method in the base Java Object class  Whenever you re creating your own objects and you want to provide a means to see if two instances of your object are  equal   you should override  and implement  this equals method in your class  in the same way the Java language provides this equality comparison behavior in the String equals method    You may want to have a look at this      equals    compareTo    and compare

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Strings in Java are immutable  That means whenever you try to change modify the string you get a new instance  You cannot change the original string  This has been done so that these string instances can be cached  A typical program contains a lot of string references and caching these instances can decrease the memory footprint and increase the performance of the program   When using    operator for string comparison you are not comparing the contents of the string  but are actually comparing the memory address  If they are both equal it will return true and false otherwise  Whereas equals in string compares the string contents   So the question is if all the strings are cached in the system  how come    returns false whereas equals return true  Well  this is possible  If you make a new string like String str   new String  Testing   you end up creating a new string in the cache even if the cache already contains a string having the same content  In short  MyString     new String  MyString   will always return false   Java also talks about the function intern   that can be used on a string to make it part of the cache so  MyString     new String  MyString   intern   will return true   Note     operator is much faster than equals just because you are comparing two memory addresses  but you need to be sure that the code isn t creating new String instances in the code  Otherwise you will encounter bugs

User · Answer

If you re like me  when I first started using Java  I wanted to use the      operator to test whether two String instances were equal  but for better or worse  that s not the correct way to do it in Java   In this tutorial I ll demonstrate several different ways to correctly compare Java strings  starting with the approach I use most of the time  At the end of this Java String comparison tutorial I ll also discuss why the      operator doesn t work when comparing Java strings   Option 1  Java String comparison with the equals method Most of the time  maybe 95  of the time  I compare strings with the equals method of the Java String class  like this   if  string1 equals string2     This String equals method looks at the two Java strings  and if they contain the exact same string of characters  they are considered equal   Taking a look at a quick String comparison example with the equals method  if the following test were run  the two strings would not be considered equal because the characters are not the exactly the same  the case of the characters is different    String string1    foo   String string2    FOO    if  string1 equals string2            this line will not print because the        java string equals method returns false      System out println  The two strings are the same        But  when the two strings contain the exact same string of characters  the equals method will return true  as in this example   String string1    foo   String string2    foo       test for equality with the java string equals method if  string1 equals string2            this line WILL print     System out println  The two strings are the same        Option 2  String comparison with the equalsIgnoreCase method  In some string comparison tests you ll want to ignore whether the strings are uppercase or lowercase  When you want to test your strings for equality in this case-insensitive manner  use the equalsIgnoreCase method of the String class  like this   String string1    foo   String string2    FOO        java string compare while ignoring case  if  string1 equalsIgnoreCase string2              this line WILL print      System out println  Ignoring case  the two strings are the same         Option 3  Java String comparison with the compareTo method  There is also a third  less common way to compare Java strings  and that s with the String class compareTo method  If the two strings are exactly the same  the compareTo method will return a value of 0  zero   Here s a quick example of what this String comparison approach looks like   String string1    foo bar   String string2    foo bar       java string compare example if  string1 compareTo string2     0           this line WILL print     System out println  The two strings are the same        While I m writing about this concept of equality in Java  it s important to note that the Java language includes an equals method in the base Java Object class  Whenever you re creating your own objects and you want to provide a means to see if two instances of your object are  equal   you should override  and implement  this equals method in your class  in the same way the Java language provides this equality comparison behavior in the String equals method    You may want to have a look at this      equals    compareTo    and compare

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String a   new String  foo    String b   new String  foo    System out println a    b      prints false System out println a equals b       prints true   Make sure you understand why   It s because the    comparison only compares references  the equals   method does a character-by-character comparison of the contents   When you call new for a and b  each one gets a new reference that points to the  foo  in the string table   The references are different  but the content is the same

User · Answer

Yea  it s bad        means that your two string references are exactly the same object  You may have heard that this is the case because Java keeps sort of a literal table  which it does   but that is not always the case  Some strings are loaded in different ways  constructed from other strings  etc   so you must never assume that two identical strings are stored in the same location   Equals does the real comparison for you

User · Answer

equals   compares the data in a class  assuming the function is implemented      compares pointer locations  location of the object in memory       returns true if both objects  NOT TALKING ABOUT PRIMITIVES  point to the SAME object instance   equals   returns true if the two objects contain the same data equals   Versus    in Java  That may help you

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tests object references   equals   tests the string values     Sometimes it looks as if    compares values  because Java does some behind-the-scenes stuff to make sure identical in-line strings are actually the same object   For example     String fooString1   new String  foo    String fooString2   new String  foo        Evaluates to false fooString1    fooString2      Evaluates to true fooString1 equals fooString2       Evaluates to true  because Java uses the same object  bar      bar     But beware of nulls        handles null strings fine  but calling  equals   from a null string will cause an exception     String nullString1   null  String nullString2   null      Evaluates to true System out print nullString1    nullString2       Throws a NullPointerException System out print nullString1 equals nullString2      So if you know that fooString1 may be null  tell the reader that by writing  System out print fooString1    null  amp  amp  fooString1 equals  bar       The following are shorter  but it   s less obvious that it checks for null   System out print  bar  equals fooString1         bar  is never null System out print Objects equals fooString1   bar         Java 7 required

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equals   compares the data in a class  assuming the function is implemented      compares pointer locations  location of the object in memory       returns true if both objects  NOT TALKING ABOUT PRIMITIVES  point to the SAME object instance   equals   returns true if the two objects contain the same data equals   Versus    in Java  That may help you

User · Answer

Yes     is bad for comparing Strings  any objects really  unless you know they re canonical       just compares object references    equals   tests for equality  For Strings  often they ll be the same but as you ve discovered  that s not guaranteed always

User · Answer

Strings in Java are immutable  That means whenever you try to change modify the string you get a new instance  You cannot change the original string  This has been done so that these string instances can be cached  A typical program contains a lot of string references and caching these instances can decrease the memory footprint and increase the performance of the program   When using    operator for string comparison you are not comparing the contents of the string  but are actually comparing the memory address  If they are both equal it will return true and false otherwise  Whereas equals in string compares the string contents   So the question is if all the strings are cached in the system  how come    returns false whereas equals return true  Well  this is possible  If you make a new string like String str   new String  Testing   you end up creating a new string in the cache even if the cache already contains a string having the same content  In short  MyString     new String  MyString   will always return false   Java also talks about the function intern   that can be used on a string to make it part of the cache so  MyString     new String  MyString   intern   will return true   Note     operator is much faster than equals just because you are comparing two memory addresses  but you need to be sure that the code isn t creating new String instances in the code  Otherwise you will encounter bugs

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tests for reference equality  whether they are the same object    equals   tests for value equality  whether they are logically  quot equal quot    Objects equals   checks for null before calling  equals   so you don t have to  available as of JDK7  also available in Guava   Consequently  if you want to test whether two strings have the same value you will probably want to use Objects equals       These two have the same value new String  quot test quot   equals  quot test quot      -- gt  true          but they are not the same object new String  quot test quot       quot test quot     -- gt  false          neither are these new String  quot test quot      new String  quot test quot      -- gt  false          but these are because literals are interned by     the compiler and thus refer to the same object  quot test quot      quot test quot     -- gt  true          string literals are concatenated by the compiler    and the results are interned   quot test quot      quot te quot     quot st quot     -- gt  true         but you should really just call Objects equals   Objects equals  quot test quot   new String  quot test quot       -- gt  true Objects equals null   quot test quot      -- gt  false Objects equals null  null     -- gt  true  You almost always want to use Objects equals    In the rare situation where you know you re dealing with interned strings  you can use     From JLS 3 10 5  String Literals   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  quot interned quot  so as to share unique instances  using the method String intern   Similar examples can also be found in JLS 3 10 5-1  Other Methods To Consider String equalsIgnoreCase   value equality that ignores case  Beware  however  that this method can have unexpected results in various locale-related cases  see this question  String contentEquals   compares the content of the String with the content of any CharSequence  available since Java 1 5   Saves you from having to turn your StringBuffer  etc into a String before doing the equality comparison  but leaves the null checking to you

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The    operator checks to see if the two strings are exactly the same object   The  equals   method will check if the two strings have the same value

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equals   compares the data in a class  assuming the function is implemented      compares pointer locations  location of the object in memory       returns true if both objects  NOT TALKING ABOUT PRIMITIVES  point to the SAME object instance   equals   returns true if the two objects contain the same data equals   Versus    in Java  That may help you

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performs a reference equality check  whether the 2 objects  strings in this case  refer to the same object in the memory  The equals   method will check whether the contents or the states of 2 objects are the same  Obviously    is faster  but will  might  give false results in many cases if you just want to tell if 2 Strings hold the same text  Definitely the use of the equals   method is recommended  Don t worry about the performance  Some things to encourage using String equals     Implementation of String equals   first checks for reference equality  using      and if the 2 strings are the same by reference  no further calculation is performed  If the 2 string references are not the same  String equals   will next check the lengths of the strings  This is also a fast operation because the String class stores the length of the string  no need to count the characters or code points  If the lengths differ  no further check is performed  we know they cannot be equal  Only if we got this far will the contents of the 2 strings be actually compared  and this will be a short-hand comparison  not all the characters will be compared  if we find a mismatching character  at the same position in the 2 strings   no further characters will be checked   When all is said and done  even if we have a guarantee that the strings are interns  using the equals   method is still not that overhead that one might think  definitely the recommended way  If you want an efficient reference check  then use enums where it is guaranteed by the language specification and implementation that the same enum value will be the same object  by reference

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String a   new String  foo    String b   new String  foo    System out println a    b      prints false System out println a equals b       prints true   Make sure you understand why   It s because the    comparison only compares references  the equals   method does a character-by-character comparison of the contents   When you call new for a and b  each one gets a new reference that points to the  foo  in the string table   The references are different  but the content is the same

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If the equals   method is present in the java lang Object class  and it is expected to check for the equivalence of the state of objects  That means  the contents of the objects  Whereas the    operator is expected to check the actual object instances are same or not   Example  Consider two different reference variables  str1 and str2   str1   new String  abc    str2   new String  abc      If you use the equals    System out println  str1 equals str2    TRUE   FALSE      You will get the output as TRUE if you use        System out println  str1  str2     TRUE     FALSE      Now you will get the FALSE as output  because both str1 and str2 are pointing to two different objects even though both of them share the same string content  It is because of new String   a new object is created every time

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The    operator check if the two references point to the same object or not   equals   check 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    Case 1  String s1    Stack Overflow   String s2    Stack Overflow   s1    s2         true s1 equals s2     true   Reason  String literals created without null are stored in the String pool in the permgen area of heap  So both s1 and s2 point to same object in the pool  Case 2  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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equals   compares the data in a class  assuming the function is implemented      compares pointer locations  location of the object in memory       returns true if both objects  NOT TALKING ABOUT PRIMITIVES  point to the SAME object instance   equals   returns true if the two objects contain the same data equals   Versus    in Java  That may help you

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compares Object references    equals   compares String values   Sometimes    gives illusions of comparing String values  as in following cases   String a  Test   String b  Test   if a  b      gt  true   This is because when you create any String literal  the JVM first searches for that literal in the String pool  and if it finds a match  that same reference will be given to the new String  Because of this  we get    a  b       true                         String Pool      b ----------------- gt   test   lt -----------------a   However     fails in the following case   String a  test   String b new String  test    if  a  b      gt  false   In this case for new String  test   the statement new String will be created on the heap  and that reference will be given to b  so b will be given a reference on the heap  not in String pool   Now a is pointing to a String in the String pool while b is pointing to a String on the heap  Because of that we get   if a  b       false                   String Pool       test   lt -------------------- a                     Heap       test   lt -------------------- b   While  equals   always compares a value of String so it gives true in both cases   String a  Test   String b  Test   if a equals b       gt  true  String a  test   String b new String  test    if a equals b       gt  true   So using  equals   is always better

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You can also use the compareTo   method to compare two Strings  If the compareTo result is 0  then the two strings are equal  otherwise the strings being compared are not equal    The    compares the references and does not compare the actual strings  If you did create every string using new String somestring  intern   then you can use the    operator to compare two strings  otherwise equals   or compareTo methods can only be used

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tests object references   equals   tests the string values     Sometimes it looks as if    compares values  because Java does some behind-the-scenes stuff to make sure identical in-line strings are actually the same object   For example     String fooString1   new String  foo    String fooString2   new String  foo        Evaluates to false fooString1    fooString2      Evaluates to true fooString1 equals fooString2       Evaluates to true  because Java uses the same object  bar      bar     But beware of nulls        handles null strings fine  but calling  equals   from a null string will cause an exception     String nullString1   null  String nullString2   null      Evaluates to true System out print nullString1    nullString2       Throws a NullPointerException System out print nullString1 equals nullString2      So if you know that fooString1 may be null  tell the reader that by writing  System out print fooString1    null  amp  amp  fooString1 equals  bar       The following are shorter  but it   s less obvious that it checks for null   System out print  bar  equals fooString1         bar  is never null System out print Objects equals fooString1   bar         Java 7 required

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I think that when you define a String you define an object  So you need to use  equals    When you use primitive data types you use    but with String  and any object  you must use  equals

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tests object references   equals   tests the string values     Sometimes it looks as if    compares values  because Java does some behind-the-scenes stuff to make sure identical in-line strings are actually the same object   For example     String fooString1   new String  foo    String fooString2   new String  foo        Evaluates to false fooString1    fooString2      Evaluates to true fooString1 equals fooString2       Evaluates to true  because Java uses the same object  bar      bar     But beware of nulls        handles null strings fine  but calling  equals   from a null string will cause an exception     String nullString1   null  String nullString2   null      Evaluates to true System out print nullString1    nullString2       Throws a NullPointerException System out print nullString1 equals nullString2      So if you know that fooString1 may be null  tell the reader that by writing  System out print fooString1    null  amp  amp  fooString1 equals  bar       The following are shorter  but it   s less obvious that it checks for null   System out print  bar  equals fooString1         bar  is never null System out print Objects equals fooString1   bar         Java 7 required

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Operator    is always meant for object reference comparison  whereas the String class  equals   method is overridden for content comparison   String s1   new String  abc    String s2   new String  abc    System out println s1    s2      It prints false  reference comparison  System out println s1 equals s2       It prints true  content comparison

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compares object references in Java  and that is no exception for String objects   For comparing the actual contents of objects  including String   one must use the equals method   If a comparison of two String objects using    turns out to be true  that is because the String objects were interned  and the Java Virtual Machine is having multiple references point to the same instance of String  One should not expect that comparing one String object containing the same contents as another String object using    to evaluate as true

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Yes     is bad for comparing Strings  any objects really  unless you know they re canonical       just compares object references    equals   tests for equality  For Strings  often they ll be the same but as you ve discovered  that s not guaranteed always

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compares the reference value of objects whereas the equals   method present in the java lang String class compares the contents of the String object  to another object

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Function   public float simpleSimilarity String u  String v        String   a   u split           String   b   v split            long correct   0      int minLen   Math min a length  b length        for  int i   0  i  lt  minLen  i              String aa   a i           String bb   b i           int minWordLength   Math min aa length    bb length              for  int j   0  j  lt  minWordLength  j                  if  aa charAt j     bb charAt j                     correct                                       return  float     double  correct    Math max u length    v length          Test   String a    This is the first string     String b    this is not 1st string        for exact string comparison  use  equals  boolean exact   a equals b       For similarity check  there are libraries for this    Here I ll try a simple example I wrote  float similarity   simple similarity a b

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Function   public float simpleSimilarity String u  String v        String   a   u split           String   b   v split            long correct   0      int minLen   Math min a length  b length        for  int i   0  i  lt  minLen  i              String aa   a i           String bb   b i           int minWordLength   Math min aa length    bb length              for  int j   0  j  lt  minWordLength  j                  if  aa charAt j     bb charAt j                     correct                                       return  float     double  correct    Math max u length    v length          Test   String a    This is the first string     String b    this is not 1st string        for exact string comparison  use  equals  boolean exact   a equals b       For similarity check  there are libraries for this    Here I ll try a simple example I wrote  float similarity   simple similarity a b

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If the equals   method is present in the java lang Object class  and it is expected to check for the equivalence of the state of objects  That means  the contents of the objects  Whereas the    operator is expected to check the actual object instances are same or not   Example  Consider two different reference variables  str1 and str2   str1   new String  abc    str2   new String  abc      If you use the equals    System out println  str1 equals str2    TRUE   FALSE      You will get the output as TRUE if you use        System out println  str1  str2     TRUE     FALSE      Now you will get the FALSE as output  because both str1 and str2 are pointing to two different objects even though both of them share the same string content  It is because of new String   a new object is created every time

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Yea  it s bad        means that your two string references are exactly the same object  You may have heard that this is the case because Java keeps sort of a literal table  which it does   but that is not always the case  Some strings are loaded in different ways  constructed from other strings  etc   so you must never assume that two identical strings are stored in the same location   Equals does the real comparison for you

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performs a reference equality check  whether the 2 objects  strings in this case  refer to the same object in the memory  The equals   method will check whether the contents or the states of 2 objects are the same  Obviously    is faster  but will  might  give false results in many cases if you just want to tell if 2 Strings hold the same text  Definitely the use of the equals   method is recommended  Don t worry about the performance  Some things to encourage using String equals     Implementation of String equals   first checks for reference equality  using      and if the 2 strings are the same by reference  no further calculation is performed  If the 2 string references are not the same  String equals   will next check the lengths of the strings  This is also a fast operation because the String class stores the length of the string  no need to count the characters or code points  If the lengths differ  no further check is performed  we know they cannot be equal  Only if we got this far will the contents of the 2 strings be actually compared  and this will be a short-hand comparison  not all the characters will be compared  if we find a mismatching character  at the same position in the 2 strings   no further characters will be checked   When all is said and done  even if we have a guarantee that the strings are interns  using the equals   method is still not that overhead that one might think  definitely the recommended way  If you want an efficient reference check  then use enums where it is guaranteed by the language specification and implementation that the same enum value will be the same object  by reference

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In Java  when the    operator is used to compare 2 objects  it checks to see if the objects refer to the same place in memory  In other words  it checks to see if the 2 object names are basically references to the same memory location  The Java String class actually overrides the default equals   implementation in the Object class     and it overrides the method so that it checks only the values of the strings  not their locations in memory  This means that if you call the equals   method to compare 2 String objects  then as long as the actual sequence of characters is equal  both objects are considered equal   The    operator checks if the two strings are exactly the same object    The  equals   method  check if the two strings have the same value

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compares object references in Java  and that is no exception for String objects   For comparing the actual contents of objects  including String   one must use the equals method   If a comparison of two String objects using    turns out to be true  that is because the String objects were interned  and the Java Virtual Machine is having multiple references point to the same instance of String  One should not expect that comparing one String object containing the same contents as another String object using    to evaluate as true

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tests for reference equality  whether they are the same object    equals   tests for value equality  whether they are logically  quot equal quot    Objects equals   checks for null before calling  equals   so you don t have to  available as of JDK7  also available in Guava   Consequently  if you want to test whether two strings have the same value you will probably want to use Objects equals       These two have the same value new String  quot test quot   equals  quot test quot      -- gt  true          but they are not the same object new String  quot test quot       quot test quot     -- gt  false          neither are these new String  quot test quot      new String  quot test quot      -- gt  false          but these are because literals are interned by     the compiler and thus refer to the same object  quot test quot      quot test quot     -- gt  true          string literals are concatenated by the compiler    and the results are interned   quot test quot      quot te quot     quot st quot     -- gt  true         but you should really just call Objects equals   Objects equals  quot test quot   new String  quot test quot       -- gt  true Objects equals null   quot test quot      -- gt  false Objects equals null  null     -- gt  true  You almost always want to use Objects equals    In the rare situation where you know you re dealing with interned strings  you can use     From JLS 3 10 5  String Literals   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  quot interned quot  so as to share unique instances  using the method String intern   Similar examples can also be found in JLS 3 10 5-1  Other Methods To Consider String equalsIgnoreCase   value equality that ignores case  Beware  however  that this method can have unexpected results in various locale-related cases  see this question  String contentEquals   compares the content of the String with the content of any CharSequence  available since Java 1 5   Saves you from having to turn your StringBuffer  etc into a String before doing the equality comparison  but leaves the null checking to you

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Yea  it s bad        means that your two string references are exactly the same object  You may have heard that this is the case because Java keeps sort of a literal table  which it does   but that is not always the case  Some strings are loaded in different ways  constructed from other strings  etc   so you must never assume that two identical strings are stored in the same location   Equals does the real comparison for you

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compares object references in Java  and that is no exception for String objects   For comparing the actual contents of objects  including String   one must use the equals method   If a comparison of two String objects using    turns out to be true  that is because the String objects were interned  and the Java Virtual Machine is having multiple references point to the same instance of String  One should not expect that comparing one String object containing the same contents as another String object using    to evaluate as true

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The    operator checks to see if the two strings are exactly the same object   The  equals   method will check if the two strings have the same value

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All objects are guaranteed to have a  equals   method since Object contains a method   equals    that returns a boolean  It is the subclass  job to override this method if a further defining definition is required  Without it  i e  using     only memory addresses are checked between two objects for equality  String overrides this  equals   method and instead of using the memory address it returns the comparison of strings at the character level for equality    A key note is that strings are stored in one lump pool so once a string is created it is forever stored in a program at the same address  Strings do not change  they are immutable  This is why it is a bad idea to use regular string concatenation if you have a serious of amount of string processing to do  Instead you would use the StringBuilder classes provided  Remember the pointers to this string can change and if you were interested to see if two pointers were the same    would be a fine way to go  Strings themselves do not

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Operator    is always meant for object reference comparison  whereas the String class  equals   method is overridden for content comparison   String s1   new String  abc    String s2   new String  abc    System out println s1    s2      It prints false  reference comparison  System out println s1 equals s2       It prints true  content comparison

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compares the reference value of objects whereas the equals   method present in the java lang String class compares the contents of the String object  to another object

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The    operator checks to see if the two strings are exactly the same object   The  equals   method will check if the two strings have the same value

[java] How do I compare strings in Java?

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