equals hashcode contract in java | why we need to override equals and hashcode in java

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#Java #Equals #Hashcode #Contract

Java Interview Questions

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equals() vs == operator
In Java, both equals() method & == operator are used to check equality of objects.
But, some of the differences between these two:
● one is method & other is operator (==).
● We can use == operator for reference comparison (address comparison) &
equals() method for content comparison.
● In simple words, == operator checks, if both objects are pointing to the same
memory location, whereas equals() method evaluates to the comparison of
values ( fields ) in the objects.
● If a class does not override the equals() method, then by default it uses equals(Object o) method of the closest parent class that has overridden this method.

Background of equals() & hashCode()
equals() & hashCode() methods have been defined in Object class,
which is parent class for all java classes. For this reason, all java objects inherit a
default implementation of these methods.
equals() method
The equality can be compared in two ways:
Shallow Comparison: The default implementation of equals method is defined in
Object class which simply checks if two objects are references (say a &
b) to the same memory location. i.e, It checks if a == b.

Deep Comparison: Suppose a class provides its own implementation of equals()
method in order to compare the objects of that class w.r.t state of the objects. That
means, fields (data members) of objects are to be compared one with another.
Some principles of equals() method of Object class :
If some other object is equal to a given object, then it follows these rules:
Reflexive : for any reference value a, a.equals(a) should return true.

Symmetric : for any reference values a & b, if a.equals(b) should return true then
b.equals(a) must return true.
Transitive : for any reference values a, b, & c, if a.equals(b) returns true &
b.equals(c) returns true, then a.equals(c) should return true.
Consistent : for any reference values a & b, multiple invocations of a.equals(b)
consistently return true or consistently return false, provided no information used in
equals comparisons on the object is modified.
Note: For any non-null reference value a, a.equals(null) should return false.

hashCode() method
● A hashCode is an integer value that represents the state of the current object.
● If you write a custom type then you are responsible for creating a hashCode
implementation that will represent the state of the current instance.
● Hashcode value is mostly used in hashing based collections like HashMap... etc
● hashCode() method must be overridden in every class which overrides equals()
method.

● Objects which are equal by equals() MUST have the same hashCode.
● Two different objects are NOT equal by equals() method may return similar
hashCode
● HashCode() method should return the same integer value for the same object for each calling of this method unless the value stored in the object is modified.
● If you only override the hashCode() method, it returns a proper hashCode but the
equality fails while comparing two similar objects with same content.
● If you only override the equals method, if a.equals(b) is true, it means the
hashCode of a & b must be the same, but that does not happen since you did
not override the hashCode method.
● Note : hashCode() method of Object class always returns a new hashCode for each object.

● If you don't override hashcode() then the default implementation in Object class
will be used by collections. This implementation gives different values for different objects, even if they are equal according to the equals() method.
● Some collections, like HashMap... etc use the hash code to store its data & to retrieve it.
If you don't implement hashcode() & equals() in a
consistent manner, then they will not function properly.
● Consider eclipse generated source code for equals, hashCode & other methods

Use of equals() & hashCode()
In HashMap,
● hashCode() is used to calculate the bucket & therefore calculate the index.
● equals() method is used to check that 2 objects are equal or not.
● HashMap uses equals() to compare the keys, whether the keys are equal
or not. If equals() method return true then the keys are equal otherwise not
equal.
● equals() & hashCode() methods are executed on Key in the HashMap