Why Were equals() and hashCode() Defined in the Object Class?

What is the significance of the equals() and hashCode() methods defined in Java's Object class?

The methods equals() and hashCode() in Java's Object class are critical for establishing object equality and ensuring proper functioning in collections such as HashMap and HashSet. Understanding their definition and usage is key to effective Java programming.

@Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null || getClass() != obj.getClass()) return false;
    MyObject myObject = (MyObject) obj;
    return Objects.equals(attribute1, myObject.attribute1);
}

@Override
public int hashCode() {
    return Objects.hash(attribute1);
}

Causes

• Object equality checks are necessary for comparing instances of objects based on their logical content rather than their memory addresses.
• Hash-based collections require consistent and clear definitions of equality to function correctly.

Solutions

• Override the equals() method to define custom equality logic for your objects, focusing on meaningful attributes that represent the object's state.
• Override the hashCode() method alongside equals() to ensure that equal objects produce the same hash code, which is vital for hash-based collections.

Mistake: Not overriding hashCode() when equals() is overridden.

Solution: Always override hashCode() whenever equals() is overridden to maintain consistency in hash-based collections.

Mistake: Using default implementation of equals() for complex objects.

Solution: Provide a meaningful implementation of equals() for complex objects that compares their significant fields.