Relationship between hashCode and equals method in Java

Question

I read in many places saying while override equals method in Java  should override hashCode method too  otherwise it is  violating the contract    But so far I haven t faced any problem if I override only equals method  but not hashCode method   What is the contract  And why am I not facing any problem when I am violating the contract  In which case will I face a problem if I haven t overridden the hashCode method

User · Answer

The contract is that if obj1 equals obj2  then obj1 hashCode      obj2 hashCode     it is mainly for performance reasons  as  maps are mainly using hashCode method to compare entries keys

User · Answer

Yes  it should be overridden  If you think you need to override equals    then you need to override hashCode   and vice versa  The general contract of hashCode   is         Whenever it is invoked on the same object more than once during an execution of a Java application  the hashCode method must consistently return the same integer  provided no information used in equals comparisons on the object is modified  This integer need not remain consistent from one execution of an application to another execution of the same application    If two objects are equal according to the equals Object  method  then calling the hashCode method on each of the two objects must produce the same integer result    It is not required that if two objects are unequal according to the equals java lang Object  method  then calling the hashCode method on each of the two objects must produce distinct integer results  However  the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables

User · Answer

See JavaDoc of java lang Object  In hashCode   it says      If two objects are equal according to the equals Object  method    then calling the hashCode method on each of the two objects must   produce the same integer result     Emphasis by me    If you only override equals   and not hashCode   your class violates this contract   This is also said in the JavaDoc of the equals   method      Note that it is generally necessary to override the hashCode method   whenever this method is overridden  so as to maintain the general   contract for the hashCode method  which states that equal objects must   have equal hash codes

User · Answer

The problem you will have is with collections where unicity of elements is calculated according to both  equals   and  hashCode    for instance keys in a HashMap   As its name implies  it relies on hash tables  and hash buckets are a function of the object s  hashCode     If you have two objects which are  equals    but have different hash codes  you lose   The part of the contract here which is important is  objects which are  equals   MUST have the same  hashCode     This is all documented in the javadoc for Object  And Joshua Bloch says you must do it in Effective Java  Enough said

User · Answer

Have a look at Hashtables  Hashmaps  HashSets and so forth  They all store the hashed key as their keys  When invoking get Object key  the hash of the parameter is generated and lookup in the given hashes    When not overwriting hashCode   and the instance of the key has been changed  for example a simple string that doesn t matter at all   the hashCode   could result in 2 different hashcodes for the same object  resulting in not finding your given key in map get

User · Answer

According to the doc  the default implementation of hashCode will return some integer that differ for every object     As much as is reasonably practical  the hashCode method defined by class Object does   return distinct integers for distinct objects   This is typically implemented by   converting the internal address of the object into an integer  but this implementation   technique is not required by the JavaTM programming language     However some time you want the hash code to be the same for different object that have the same meaning  For example  Student s1   new Student  John   18   Student s2   new Student  John   18   s1 hashCode      s2 hashCode       With the default implementation of hashCode   This kind of problem will be occur if you use a hash data structure in the collection framework such as HashTable  HashSet  Especially with collection such as HashSet you will end up having duplicate element and violate the Set contract

User · Answer

A contract is  If two objects are equal then they should have the same hashcode and if two objects are not equal then they may or may not have same hash code   Try using your object as key in HashMap  edited after comment from joachim-sauer   and you will start facing trouble  A contract is a guideline  not something forced upon you

[java] Relationship between hashCode and equals method in Java

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