What Is the Purpose of hashCode in Java?

What is the purpose of the hashCode method in Java?

In Java, the hashCode method returns an integer hash value that represents the object. It plays a crucial role in efficient data retrieval, particularly in collections such as HashMap and HashSet.

@Override
public int hashCode() {
    int result = 17; // Non-zero constant
    result = 31 * result + (field1 != null ? field1.hashCode() : 0);
    result = 31 * result + field2;
    return result;
}

Causes

• Optimizes performance by enabling constant-time complexity for insertions and lookups in hash tables.
• Facilitates quick comparisons between objects.
• Supports proper functioning of hash-based collections.

Solutions

• Override the hashCode() method in user-defined classes to provide a meaningful hash code that reflects the object’s state. For example, if two objects are equal according to the equals() method, they must return the same hash code.
• Ensure that your hashCode() implementation is consistent with equals() method; that is, if two objects are equal based on their equals() method, their hashCode()s must be the same.
• Use prime numbers in hash calculations to spread the hash values uniformly. This aids in reducing collisions.

Mistake: Not overriding hashCode when equals is overridden.

Solution: Always override hashCode whenever equals is implemented, to maintain the contract between these methods.

Mistake: Using mutable fields to compute hash code.

Solution: Avoid using mutable fields in the hashCode method to prevent inconsistencies after object mutation.

Mistake: Returning a constant value from hashCode.

Solution: Ensure that the hashCode method returns different values for different objects to avoid collisions.