Override equals() and hashCode() method in java

In Java, every object has access to the equals() method because it is inherited from the Object class.
However, this default implementation just simply compares the memory addresses of the objects.
You can override the default implementation of the equals() method defined in java.lang.Object.
If you override the equals(), you must override hashCode() otherwise a violation of the general contract for Object.hashCode will occur.
Since HashMap and Hashtable in Java relies on equals() and hashCode() method for comparing keys and values,
Java provides following rules to override equals method Java.
As per following rule equals() method in Java should be:

1) Reflexive : Object must be equal to itself.
2) Symmetric : if a.equals(b) is true then b.equals(a) must be true.
3) Transitive : if a.equals(b) is true and b.equals(c) is true then c.equals(a) must be true.
4) Consistent : multiple invocation of equals() method must result same value until any of properties are modified.
5) Null comparison : comparing any object to null must be false and should not result in NullPointerException.

Here is the contract, copied from the java.lang.Object specialization:

public int hashCode() :
Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.

The general contract of hashCode is:

1) 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.

2) 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.

3) 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.

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.

The default implementation of equals() method checks to see if the two objects have the same identity. Similarly, the default implementation of the hashCode() method returns an integer based on the object’s identity and is not based on the values of instance (and class) variables of the object. No matter how many times the values of its instance variables (data fields) change, the hash code calculated by the default hashCode implementation does not change during the life of the object.

Consider the following code, we have overridden equals() method to check if two objects are equal based on the values of their instance variables. Two objects may be stored at different memory addresses but may still be equal base on their instance variable.

 

Compile and run the above code, the output result is

 

What is wrong? The two instances of CustomerID are logically equal according to the class’s equals method. Because the hashCode() method is not overridden, these two instances’ identities are not in common to the default hashCode implementation. Therefore, the Object.hashCode returns two seemingly random numbers instead of two equal numbers. Such behavior violates “Equal objects must have equal hash codes” rule defined in the hashCode contract.

Let’s provide a simple hashCode() method to fix this problem:

 

Compile and run the above code, the output result is

 

If two or more objects are equal then they must have same hashCode() but if two or more objects have same hashCode() then they are not necessarily be equal.

If two or more objects are equal then they must have same hashCode()

hashCode() is used for only hash based collections such as HashMap and HashSet but not for ArrayList, LinkedList because these are not hash based collections. So let’s say an object is searched either in a HashMap or in a HashSet on the basis of hashCode() and if same hashCode() found in the HashSet or HashMap then only search moves to the equals() method to compare the objects whether they are equal or not.

note if two equal objects are allowed to have different hashCode(), then while searching for a particular hashCode() it will never return a correct result and hence comparison will never happen using equals(), and result will declare those two objects to be unequal though those two objects are equal.

If two or more objects have same hashCode() then they are not necessarily be equal

We all know that the generated hashCode() for each object is of type int value, so we can generate maximum of 232 unique hashCode()s in total. So if we want to store more objects in HashSet or HashMap than 232 then there must be collision amongst the objects’ hashCode(). So we don’t have any other option rather than to assign the same hashCode() to two or more different objects and thus the above statement is true.

Hence we can conclude from the above explanations that

Two or more same objects must have same hashCode().
Two or more different objects can also have same hashCode()s.

Thanks for your reading. If you have any query please leave a comment.

Soumitra

Software Professional, I am passionate to work on web/enterprise application. For more information please go to about me. You can follow on Twitter. You can be a friend on Facebook or Google Plus or Linkedin

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