Is it mandatory to override hashCode If you override equals method?

If you override the equals(), you MUST also override hashCode(). Otherwise, a violation of the general contract for Object. hashCode() will occur, which results unexpected behavior when your class is in conjunction with all hash-based collections.

What happens if you override equals but not hashCode?

Overriding only equals() method without overriding hashCode() causes the two equal instances to have unequal hash codes, which violates the hashCode contract (mentioned in Javadoc) that clearly says, if two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two …

What happens if we do not override hashCode () and equals () in HashSet?

now if you have not override the hashcode and equals then after putting all the objects till line 5 if you put obj5 in the map as By Default HashCode you get different hashCode so the row(Bucket will be different). So in runtime memory it will be stored like this.

Why override Gethashcode when Equals method is overridden?

It is because the framework requires that two objects that are the same must have the same hashcode. If you override the equals method to do a special comparison of two objects and the two objects are considered the same by the method, then the hash code of the two objects must also be the same.

Why do we need to override equals and hashCode?

31 Answers. You must override hashCode() in every class that overrides equals(). Failure to do so will result in a violation of the general contract for Object. hashCode(), which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.

Why do we need to override equals method in Java?

Why we override equals() method? It needs to be overridden if we want to check the objects based on the property. For example, we want to check the equality of employee object by the id. Then, we need to override the equals() method.

What happens when you don’t override equals?

If you don’t override equals, it will compare the internal address of the two references, which matches the logic behind hashCode.

When should we override GetHashCode?

If you’re implementing a reference type, you should consider overriding the Equals method if your type looks like a base type, such as Point, String, BigNumber, and so on. Override the GetHashCode method to allow a type to work correctly in a hash table. Read more guidance on equality operators.

What happens if we override only equals?

If we only override equals(Object) method, when we call map. put(g1, “CSE”); it will hash to some bucket location and when we call map. put(g2, “IT”); it will hash to some other bucket location because of different hashcode value as hashCode() method has not been overridden.

What happens if we don’t override equals?

You must override hashCode() in every class that overrides equals(). Failure to do so will result in a violation of the general contract for Object. hashCode(), which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.

What is method overriding Why would you override a method?

Declaring a method in sub class which is already present in parent class is known as method overriding. Overriding is done so that a child class can give its own implementation to a method which is already provided by the parent class.

Why GetHashCode () method should be overridden along with equals () method in each DTO class?

What happens if you don’t override hashCode() in Java?

Joshua Bloch says on Effective Java You must override hashCode () in every class that overrides equals (). Failure to do so will result in a violation of the general contract for Object.hashCode (), which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.

What happens if you only override the equals method in Java?

If you only override the hash-code method nothing happens, because it always returns a new hashCode for each object as an Object class. If you only override the equals method, if a.equals (b) is true it means the hashCode of a and b must be the same but that does not happen since you did not override the hashCode method.

What is equals-hashCode contract in Java?

These collection classes rely on rules of Java programming around equals and hashcode to work according to their specification, popularly known as an equals-hashcode contract. According to which, you must override hashcode if you are overriding equals and vice-versa.

Why do I need to @override HashMap/hashtable?

Because HashMap/Hashtable will lookup object by hashCode () first. If they are not the same, hashmap will assert object are not the same and return not exists in the map. The reason why you need to @Override neither or both, is because of the way they interrelate with the rest of the API.