The equals() and hashCode() methods prove to be very important, when objects implementing these two methods are added to collections. If implemented incorrectly it might screwed up your life.
equals() : This method checks if some other object passed to it as an argument is equal the object in which this method is invoked. It is easy to implement the equals() method incorrectly, if you do not understand the contract. Before overriding this method, following “properties” need to keep in mind –
- Reflexive: o1.equals(o1) – which means an Object (e.g. o1) should be equal to itself
- Symmetric: o1.equals(o2) if and only o2.equals(o1)
- Transitive: o1.equals(o2) && o2.equals(o3) implies that o1.equals(o3) as well
- Consistent: o1.equals(o2) returns the same as long as o1 and o2 are unmodified
- null comparison : !o1.equals(null) – which means that any instantiable object is not equal to null. So if you pass a null as an argument to your object o1, then it should return false.
- Hash code value: o1.equals(o2) implies o1.hashCode() == o2.hashCode() . This is very important. If you define a equals() method then you must define a hashCode() method as well. Also it means that if you have two objects that are equal then they must have the same hashCode, however the reverse is not true
From java source code
*
* @param obj the reference object with which to compare.
* @return {@code true} if this object is the same as the obj
* argument; {@code false} otherwise.
* @see #hashCode()
* @see java.util.HashMap
*/
public boolean equals(Object obj) {
return (this == obj);
}
hashCode(): This method returns a hashCode() value as an Integer and is supported for the benefit of hashing based java.util.Collection classes like Hashtable, HashMap, HashSet etc. If a class overrides the equals() method, it must implement the hashCode() method as well.Before overriding this method, you need to keep in mind
- Whenever hashCode() method is invoked on the same object more than once during an execution of a Java program, this method must consistently return the same result. The integer result need not remain consistent from one execution of the program to the next execution of the same program.
- If two objects are equal as per the equals() method, then calling the hashCode() method in each of the two objects must return the same integer result. So, If a field is not used in equals(), then it must not be used in hashCode() method.
- If two objects are unequal as per the equals() method, each of the two objects can return either two different integer results or same integer results (i.e. if 2 objects have the same hashCode() result does not mean that they are equal, but if two objects are equal then they must return the same hashCode() result).
As per java source code As much as is reasonably practical, the hashCode method defined by java.lang.Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer)