Can I generate hashcode() and equals() with only one unique field?

Lets say, I have a class Student. Each student object has a unique field ( int ID ) plus other common fields (like String schoolName etc).

Now is it ok to generate hashcode() and equals() by considering ONLY the field ID ?

Yes, you can absolutely use ID for hashcode and equals. Based on the number of buckets in HashMap, multiple objects with different ID will share the same bucket and then the equals method will identify the right object. Since you are planning to use ID which is unique, equals method will compare the ID with each object in bucket -> LinkedList/Tree and identify the unique object.

However there is a catch. You can use ID in Hashcode if the ID is kind of an auto increment so that your objects are distributed evenly in the buckets. However if your ID field is some sort of pattern like 4, 8, 12, 16 etc., you will end up putting most of your objects in a portion of buckets. So in that case, use Hashcode to generate a prime number as prime numbers are best candidates for evenly distribution in Buckets.

It depends on the intent of your object and the intended audience.

1. Intent: There could be a case where a student gets married and changes their last name. Now if your code relies on .equals() to determine if there are changes to save then the changes won't get picked up to be saved. The same is true for any other field that may change on the Student object.

2. Audience: If you package your object into a library and give it to others. They may have an expectation that an instance of the Student object is equal if and only if all non-transient fields are equal.