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Introduction

In Java, the equals() and hashCode() methods are essential when comparing objects and working with collections like HashMap, HashSet, and Hashtable. Understanding how to correctly override and use these methods is crucial for ensuring predictable behavior in your applications.

The equals() Method

The equals() method determines logical equality between two objects. By default, it compares object references (just like ==), but it can be overridden to compare values or fields.

Method Signature:

java

public boolean equals(Object obj)

Best Practices for Overriding:

• Check for reference equality: if (this == obj) return true;
• Check for null and class type: if (obj == null || getClass() != obj.getClass()) return false;
• Cast and compare relevant fields.

Example:

java

@Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null || getClass() != obj.getClass()) return false;
    Person person = (Person) obj;
    return age == person.age && Objects.equals(name, person.name);
}

The hashCode() Method

The hashCode() method returns an integer hash code representing the object. It’s used in hash-based collections to quickly find buckets where an object might reside.

Method Signature:

java

public int hashCode()

Best Practices for Overriding:

• Always override hashCode() when you override equals().
• Use Objects.hash() or a consistent algorithm based on significant fields.

Example:

java

@Override
public int hashCode() {
    return Objects.hash(name, age);
}

The equals() and hashCode() Contract

These methods are interdependent. To ensure correct behavior in collections, they must follow this contract:

Equals Contract:

1. Reflexive: x.equals(x) is true.
2. Symmetric: If x.equals(y), then y.equals(x).
3. Transitive: If x.equals(y) and y.equals(z), then x.equals(z).
4. Consistent: Multiple invocations should return the same result (if state doesn’t change).
5. Non-null: x.equals(null) is always false.

HashCode Contract:

1. If x.equals(y) is true, then x.hashCode() == y.hashCode().
2. If x.equals(y) is false, x.hashCode() can still be equal or different.
3. Repeated calls must return the same value (if the object is unchanged).

Why They Matter

When objects are used as keys in a HashMap or elements in a HashSet, correct implementation ensures:

• No unexpected duplicates.
• Fast lookup and retrieval.
• Logical consistency.

Conclusion

Mastering equals() and hashCode() is vital for any Java developer. By following the contract and best practices, your objects will behave reliably in collections, supporting cleaner and more predictable code.