Object Class Methods · Classes and Objects in Depth in Java | Java

Object Class Methods

Every Class Extends Object

Every Java class automatically extends java.lang.Object. So every object has these methods:

Method	Default behaviour	Usually override?
`toString()`	class name + @ + hash, e.g. `Product@1b6d3586`	yes, for readable logs
`equals(Object o)`	true only for the same object (like `==`)	yes, to compare by value
`hashCode()`	number based on identity	yes, whenever you override equals
`getClass()`	runtime class of the object	no (final)

Overriding toString and equals

Example

Two products with the same id are equal by value after we override equals. The == operator still compares references.

import java.util.Objects;

public class Main {
    public static void main(String[] args) {
        Product a = new Product(101, "Phone");
        Product b = new Product(101, "Phone");
        System.out.println(a);
        System.out.println("a == b      : " + (a == b));
        System.out.println("a.equals(b) : " + a.equals(b));
        System.out.println("same hash   : " + (a.hashCode() == b.hashCode()));
        System.out.println("class       : " + a.getClass().getSimpleName());
    }
}

class Product {
    int id;
    String name;

    Product(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        return "Product{id=" + id + ", name=" + name + "}";
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Product)) return false;
        Product other = (Product) o;
        return id == other.id && name.equals(other.name);
    }

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

Output

Product{id=101, name=Phone}
a == b      : false
a.equals(b) : true
same hash   : true
class       : Product

Tip: The full equals/hashCode contract and immutable classes are covered in the Advanced level. For now: if you override equals, always override hashCode too.