NUHMAN.com

In Java, when you override the equals() method, it is crucial to also override the hashCode() method to maintain the general contract between them. This ensures that objects that are considered equal will behave correctly when used in collections like HashSet, HashMap, or Hashtable.

Let’s walk through a proper implementation of both methods using a College class.

Overriding equals() Method

Here's an example equals() method that checks for object identity, type compatibility, and field equality based on the id field:

java

@Override
public boolean equals(Object obj) {
    if (obj == this)
        return true;
    if (!(obj instanceof College))
        return false;
    College other = (College) obj;
    return Objects.equals(this.id, other.id);
}

This method ensures:

• Reflexivity: An object is equal to itself.
• Type safety: Compares only with instances of College.
• Logical equality: Compares based on the id field.

Implementing a Consistent hashCode() Method

To maintain the contract of hashCode() (i.e., equal objects must have equal hash codes), we use the same field (id) that’s used in the equals() method:

java

@Override
public int hashCode() {
    return Objects.hash(id);
}

The Objects.hash() utility method computes a reliable hash code using the provided fields.

Full Example: College Class

java

import java.util.Objects;

public class College {
    private final String id;

    public College(String id) {
        this.id = id;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == this)
            return true;
        if (!(obj instanceof College))
            return false;
        College other = (College) obj;
        return Objects.equals(this.id, other.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }

    // Getters, setters, and other methods
}

Why This Matters

Failing to override hashCode() when equals() is overridden can result in unexpected behavior. For example:

java

Set<College> colleges = new HashSet<>();
colleges.add(new College("C101"));
System.out.println(colleges.contains(new College("C101"))); // false if hashCode isn't overridden

Without a consistent hashCode(), even logically equal objects won't be found in hash-based collections.

Summary

• Always override hashCode() when you override equals().
• Use the same fields for both methods.
• Objects.hash(...) simplifies generating robust hash codes.
• Properly implementing both methods ensures reliable behavior in collections.