What is the difference between and equals in Java

Question

I wanted to clarify if I understand this correctly       is a reference comparison  i e  both objects point to the same memory location  equals   evaluates to the comparison of values in the objects

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Also note that  equals   normally contains    for testing as this is the first thing you would wish to test for if you wanted to test if two objects are equal    And    actually does look at values for primitive types  for objects it checks the reference

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operator always reference is compared  But in case of      equals   method   it s depends s on implementation if we are overridden equals method than it compares object on basic of implementation given in overridden method     class A       int id     String str        public A int id String str                this id id         this str str              public static void main String arg                A obj new A 101  sam          A obj1 new A 101  sam           obj equals obj1   fasle       obj  obj1    fasle            in above code both obj and obj1 object contains same data but reference is not same so equals return false and    also  but if we overridden equals method than   class A       int id     String str        public A int id String str                this id id         this str str             public boolean equals Object obj               A a1  A obj        return this id  a1 id             public static void main String arg                A obj new A 101  sam          A obj1 new A 101  sam           obj equals obj1   true       obj  obj1    fasle            know check out it will return true and false for same case only we overridden      equals method     it compare object on basic of content id  of object     but      still compare references of object

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The    operator tests whether two variables have the same references    aka pointer to a memory address     String foo   new String  abc    String bar   new String  abc     if foo  bar     False  The objects are not the same   bar   foo   if foo  bar     True  Now the objects are the same       Whereas the equals   method tests whether two variables refer to objects   that have the same state  values     String foo   new String  abc    String bar   new String  abc     if foo equals bar      True  The objects are identical but not same    Cheers  -

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public class StringPool    public static void main String   args         String s1    Cat     will create reference in string pool of heap memory     String s2    Cat       String s3   new String  Cat     will create a object in heap memory         Using    will give us true because same reference in string pool      if  s1    s2            System out println  true          else           System out println  false                  Using    with reference and Object will give us False      if  s1    s3            System out println  true          else           System out println  false                  Using  equals method which refers to value      if  s1 equals s3             System out println  true          else           System out println  False                       ----Output-----    true    false    true

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In general  the answer to  your question is  yes   but       equals      will only compare what it is written to compare  no more  no less  If a class does not override the equals method  then it defaults to the equals Object o  method of the closest parent class that has overridden this method   If no parent classes have provided an override  then it defaults to the method from the ultimate parent class  Object  and so you re left with the Object equals Object o  method  Per the Object API this is the same as     that is  it returns true if and only if both variables refer to the same object  if their references are one and the same  Thus you will be testing  for object equality and not functional equality  Always remember to override hashCode if you override equals so as not to  break the contract   As per the API  the result returned from the hashCode   method for two objects must be the same if their equals methods show that they are equivalent  The converse is not necessarily true

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It may be worth adding that for wrapper objects for primitive types - i e  Int  Long  Double -    will return true if the two values are equal   Long a   10L  Long b   10L   if  a    b        System out println  Wrapped primitives behave like values        To contrast  putting the above two Longs into two separate ArrayLists  equals sees them as the same  but    doesn t   ArrayList lt Long gt  c   new ArrayList lt  gt     ArrayList lt Long gt  d   new ArrayList lt  gt      c add a   d add b   if  c    d  System out println  No way     if  c equals d   System out println  Yes  this is true

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Both    and  equals   refers to the same object if you don t override  equals      Its your wish  what you want to do once you override  equals    You can compare the invoking object s state with the passed in object s state or you can just call super equals

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Here is a general thumb of rule for the difference between relational operator    and the method  equals    object1    object2 compares if the objects referenced by object1 and object2 refer to the same memory location in Heap  object1 equals object2  compares the values of object1 and object2 regardless of where they are located in memory  This can be demonstrated well using String Scenario 1  public class Conditionals        public static void main String   args           String str1    quot Hello quot          String str2   new String  quot Hello quot           System out println  quot is str1    str2    quot     str1    str2            System out println  quot is str1 equals str2     quot     str1 equals str2                   The result is       is str1    str2   false       is str1 equals str2    true   Scenario 2 public class Conditionals        public static void main String   args           String str1    quot Hello quot          String str2    quot Hello quot          System out println  quot is str1    str2    quot     str1    str2            System out println  quot is str1 equals str2     quot     str1 equals str2                The result is    is str1    str2   true   is str1 equals str2    true  This string comparison could be used as a basis for comparing other types of object  For instance if I have a Person class  I need to define the criteria base on which I will compare  two persons  Let s say this person class has instance variables of height and weight  So creating person objects person1 and person2 and for comparing these two using the  equals   I need to override the equals method of the person class to define based on which instance variables heigh or weight  the comparison will be  However  the    operator will still return results based on the memory location of the two objects person1 and person2   For ease of generalizing this person object comparison  I have created the following test class  Experimenting on these concepts will reveal tons of facts  package com tadtab CS5044   public class Person    private double height  private double weight   public double getHeight         return height     public void setHeight double height        this height   height     public double getWeight         return weight     public void setWeight double weight        this weight   weight       Override public int hashCode         final int prime   31      int result   1      long temp      temp   Double doubleToLongBits height       result   prime   result    int   temp    temp  gt  gt  gt  32        return result      Override        This method uses the height as a means of comparing person objects     NOTE  weight is not part of the comparison criteria     public boolean equals Object obj        if  this    obj          return true      if  obj    null          return false      if  getClass      obj getClass            return false      Person other    Person  obj      if  Double doubleToLongBits height     Double doubleToLongBits other height           return false      return true     public static void main String   args             Person person1   new Person        person1 setHeight 5 50       person1 setWeight 140 00            Person person2   new Person        person2 setHeight 5 70       person2 setWeight 160 00            Person person3   new Person        person3   person2           Person person4   new Person        person4 setHeight 5 70            Person person5   new Person        person5 setWeight 160 00            System out println  quot is person1    person2    quot     person1    person2       false      System out println  quot is person2    person3    quot     person2    person3       true        this is because perosn3 and person to refer to the one person object in memory  They are aliases      System out println  quot is person2 equals person3     quot     person2 equals person3        true           System out println  quot is person2 equals person4     quot     person2 equals person4        true              even if the person2 and person5 have the same weight  they are not equal         it is because their height is different     System out println  quot is person2 equals person4     quot     person2 equals person5        false        Result of this class execution is  is person1    person2   false is person2    person3   true is person2 equals person3    true is person2 equals person4    true is person2 equals person4    false

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Basically     compares if two objects have the same reference on the heap  so unless two references are linked to the same object  this comparison will be false   equals   is a method inherited from Object class  This method by default compares if two objects have the same referece  It means   object1 equals object2   lt    object1    object2  However  if you want to establish equality between two objects of the same class you should override this method  It is also very important to override the method hashCode   if you have overriden equals     Implement hashCode   when establishing equality is part of the Java Object Contract  If you are working with collections  and you haven t implemented hashCode    Strange Bad Things could happen   HashMap lt Cat  String gt  cats   new HashMap lt  gt     Cat cat   new Cat  molly    cats put cat   This is a cool cat    System out println cats get new Cat  molly       null will be printed after executing the previous code if you haven t implemented hashCode

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You will have to override the equals function  along with others  to use this with custom classes   The equals method compares the objects   The    binary operator compares memory addresses

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With respect to the String class   The equals   method compares the  value  inside String instances  on the heap  irrespective if the two object references refer to the same String instance or not  If any two object references of type String refer to the same String instance then great  If the two object references refer to two different String instances    it doesn t make a difference  Its the  value   that is  the contents of the character array  inside each String instance that is being compared   On the other hand  the      operator compares the value of two object references to see whether they refer to the same String instance  If the value of both object references  refer to  the same String instance then the result of the boolean expression would be  true   duh  If  on the other hand  the value of both object references  refer to  different String instances  even though both String instances have identical  values   that is  the contents of the character arrays of each String instance are the same  the result of the boolean expression would be  false    As with any explanation  let it sink in   I hope this clears things up a bit

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It is the difference between identity and equivalence   a    b means that a and b are identical  that is  they are symbols for very same object in memory   a equals  b   means that they are equivalent  that they are symbols for objects that in some sense have the same value -- although those objects may occupy different places in memory   Note that with equivalence  the question of how to evaluate and compare objects comes into play -- complex objects may be regarded as equivalent for practical purposes even though some of their contents differ   With identity  there is no such question

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In short  the answer is  Yes    In Java  the    operator compares the two objects to see if they point to the same memory location  while the  equals   method actually compares the two objects to see if they have the same object value

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is an operator and equals   is a method   Operators are generally used for primitive type comparisons and thus    is used for memory address comparison and equals   method is used for comparing objects

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can be used in many object types but you can use Object equals for any type   especially Strings and Google Map Markers

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String w1   Sarat    String w2   Sarat    String w3   new String  Sarat      System out println w1 hashCode         3254818  System out println w2 hashCode         3254818  System out println w3 hashCode         3254818   System out println System identityHashCode w1      prints 705927765  System out println System identityHashCode w2      prints 705927765  System out println System identityHashCode w3      prints 366712642    if w1  w2         705927765  705927765        System out println  true        else       System out println  false          prints true   if w2  w3         705927765  366712642        System out println  true        else       System out println  false          prints false    if w2 equals w3          Content of 705927765   Content of 366712642        System out println  true        else       System out println  false          prints true

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The major difference between    and equals   is  1     is used to compare primitives   For example             String string1    Ravi           String string2    Ravi           String string3   new String  Ravi            String string4   new String  Prakash             System out println string1    string2      true because same reference in string pool         System out println string1    string3      false   2  equals   is used to compare objects  For example            System out println string1 equals string2       true equals   comparison of values in the objects         System out println string1 equals string3       true         System out println string1 equals string4       false

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Just remember that  equals      has to be implemented by the class you are trying to compare  Otherwise  there isn t much of a point  the version of the method for the Object class does the same thing as the comparison operation  Object equals   The only time you really want to use the comparison operator for objects is wen you are comparing Enums  This is because there is only one instance of an Enum value at a time  For instance  given the enum  enum FooEnum  A  B  C    You will never have more than one instance of A at a time  and the same for B and C  This means that you can actually write a method like so   public boolean compareFoos FooEnum x  FooEnum y        return  x    y       And you will have no problems whatsoever

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The String pool  aka interning  and Integer pool blur the difference further  and may allow you to use    for objects in some cases instead of  equals  This can give you greater performance      at the cost of greater complexity   E g    assert  ab      a     b    Integer i   1  Integer j   i  assert i    j    Complexity tradeoff  the following may surprise you   assert new String  a      new String  a     Integer i   128  Integer j   128  assert i    j    I advise you to stay away from such micro-optimization  and always use  equals for objects  and    for primitives   assert  new String  a    equals new String  a      Integer i   128  Integer j   128  assert i equals j

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Since Java doesn   t support operator overloading     behaves identical for every object but equals   is method  which can be overridden in Java and logic to compare objects can be changed based upon business rules  Main difference between    and equals in Java is that  quot    quot  is used to compare primitives while equals   method is recommended to check equality of objects  String comparison is a common scenario of using both    and equals   method  Since java lang String class override equals method  It return true if two String object contains same content but    will only return true if two references are pointing to same object  Here is an example of comparing two Strings in Java for equality using    and equals   method which will clear some doubts   public class TEstT           public static void main String   args                     String text1   new String  quot apple quot        String text2   new String  quot apple quot                     since two strings are different object result should be false     boolean result   text1    text2      System out println  quot Comparing two strings with    operator   quot    result                    since strings contains same content   equals   should return true     result   text1 equals text2       System out println  quot Comparing two Strings with same content using equals method   quot    result                  text2   text1        since both text2 and text1d reference variable are pointing to same object        quot    quot  should return true     result    text1    text2       System out println  quot Comparing two reference pointing to same String with    operator   quot    result

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The difference between    and equals confused me for sometime until I decided to have a closer look at it  Many of them say that for comparing string you should use equals and not     Hope in this answer I will be able to say the difference   The best way to answer this question will be by asking a few questions to yourself  so let s start   What is the output for the below program   String mango    mango   String mango2    mango   System out println mango    mango2   System out println mango    mango2     if you say    false true   I will say you are right but why did you say that  and If you say the output is   true false   I will say you are wrong but I will still ask you  why you think that is right   Ok  Let s try to answer this one   What is the output for the below program   String mango    mango   String mango3   new String  mango    System out println mango    mango3   System out println mango    mango3     Now If you say   false true   I will say you are wrong but why is it wrong now  the correct output for this program is  true false   Please compare the above program and try to think about it   Ok  Now this might help  please read this   print the address of object - not possible but still we can use it    String mango    mango   String mango2    mango   String mango3   new String  mango    System out println mango    mango2   System out println mango    mango2   System out println mango3    mango2   System out println mango3    mango2      mango2    mang   System out println mango      mango2   System out println mango    mango2   System out println mango    mango2    System out println System identityHashCode mango    System out println System identityHashCode mango2    System out println System identityHashCode mango3      can you just try to think about the output of the last three lines in the code above  for me ideone printed this out  you can check the code here    false true true false mango mango false true 17225372 17225372 5433634   Oh  Now you see the identityHashCode mango  is equal to identityHashCode mango2  But it is not equal to identityHashCode mango3   Even though all the string variables - mango  mango2 and mango3 - have the same value  which is  mango   identityHashCode   is still not the same for all   Now try to uncomment this line    mango2    mang   and run it again this time you will see all three identityHashCode   are different  Hmm that is a helpful hint  we know that if hashcode x  N and hashcode y  N    x is equal to y  I am not sure how java works internally but I assume this is what happened when I said   mango    mango     java created a string  mango  which was pointed referenced  by the variable mango something like this  mango ---- gt   mango    Now in the next line when I said   mango2    mango     It actually reused the same string  mango  which looks something like this  mango ---- gt   mango   lt ---- mango2   Both mango and mango2 pointing to the same reference Now when I said   mango3   new String  mango     It actually created a completely new reference string  for  mango   which looks something like this   mango ----- gt   mango   lt ------ mango2  mango3 ------ gt   mango    and that s why when I put out the values for mango    mango2  it put out true  and when I put out the value for mango3    mango2  it put out false  even when the values were the same    and when you uncommented the line    mango2    mang   It actually created a string  mang  which turned our graph like this   mango ---- gt  mango  mango2 ---- gt   mang  mango3 ----- gt   mango    This is why the identityHashCode is not the same for all   Hope this helps you guys  Actually  I wanted to generate a test case where    fails and equals   pass  Please feel free to comment and let me know If I am wrong

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When you evaluate the code  it is very clear that      compares according to memory address  while equals Object o   compares hashCode   of the instances   That s why it is said do not break the contract between equals   and  hashCode   if you do not face surprises later        String s1   new String  Ali        String s2   new String  Veli        String s3   new String  Ali         System out println s1 hashCode         System out println s2 hashCode         System out println s3 hashCode           System out println   s1  s2       s1    s2        System out println   s1  s3       s1    s3          System out println  s1 equals s2       s1 equals s2         System out println  s1 equal s3       s1 equals s3             Output      96670          3615852     96670      s1  s2  false      s1  s3  false     s1 equals s2  false     s1 equal s3  true

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There are some small differences depending whether you are talking about  primitives  or  Object Types   the same can be said if you are talking about  static  or  non-static  members  you can also mix all the above     Here is an example  you can run it    public final class MyEqualityTest       public static void main  String args                   String s1   new String   Test             String s2   new String   Test              System out println    n1 - PRIMITIVES             System out println  s1    s2       false         System out println  s1 equals  s2        true          A a1   new A            A a2   new A             System out println    n2 - OBJECT TYPES   STATIC VARIABLE             System out println  a1    a2       false         System out println  a1 s    a2 s       true         System out println  a1 s equals  a2 s         true          B b1   new B            B b2   new B             System out println    n3 - OBJECT TYPES   NON-STATIC VARIABLE             System out println  b1    b2       false         System out println  b1 getS      b2 getS         false         System out println  b1 getS   equals  b2 getS           true          final class A          static     public static String s      A                 this s   new String   aTest              final class B       private String s      B                 this s   new String   aTest                public String getS                 return s             You can compare the explanations for       Equality Operator  and   equals        method in the java lang Object class  through these links        http   docs oracle com javase tutorial java nutsandbolts op2 html  equals       http   docs oracle com javase 7 docs api java lang Object html equals java lang Object

[java] What is the difference between == and equals() in Java?

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