What is the importance of hashCode() and equals() methods?

The hashCode() and equals() methods in Java are fundamental to the operation of collections such as HashMap, HashSet, and Hashtable, which store elements in a hash table. These methods are crucial for correctly storing and retrieving objects from these collections, ensuring that each object's uniqueness and equivalence are accurately maintained.

Importance of hashCode() Method

The hashCode() method provides a way of distributing objects into buckets in hash table implementations. According to the general contract of the hashCode() method:

1. Consistency: 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[7].
2. Equality: 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[7].
3. Unequal Objects and Hash Codes: It is not required that if two objects are unequal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce distinct integer results. However, programmers should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables[7].

By providing a high-quality implementation of hashCode() that distributes objects evenly across the hash buckets, you can improve the performance of hash table-based collections by reducing the number of potential collisions.

Importance of equals() Method

The equals() method is used to determine logical equality of two objects. The general contract of the equals() method states that it should be:

1. Reflexive: For any non-null reference value x, x.equals(x) should return true[7].
2. Symmetric: For any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true[7].
3. Transitive: For any non-null refe...