Java Object Class Tutorial

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Java Object Class Tutorial

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  • Post last modified:July 22, 2024

Introduction

In Java, the Object class is the root class for all classes. Every class in Java is implicitly or explicitly a subclass of Object. It provides some common methods that can be used by all objects in Java.

Common Methods in Object Class

  1. toString() Method:
  • The toString() method returns a string representation of the object.
  • The default implementation in the Object class returns a string consisting of the class name followed by the “@” character and the object’s hashcode.
 public class ExampleObject {
       public static void main(String[] args) {
           ExampleObject obj = new ExampleObject();
           System.out.println(obj.toString()); // Prints something like "ExampleObject@<hashcode>"
       }
   }
  1. equals(Object obj) Method:
  • The equals() method is used to compare the equality of two objects.
  • By default, it compares the memory addresses of the two objects. However, it is often overridden in user-defined classes to compare the contents.
public class ExampleObject {
       public static void main(String[] args) {
           ExampleObject obj1 = new ExampleObject();
           ExampleObject obj2 = new ExampleObject();
           System.out.println(obj1.equals(obj2)); // Prints false
       }
   }
  1. hashCode() Method:
  • The hashCode() method returns a hash code value for the object.
  • This method is often overridden in user-defined classes for better hash code computation.
public class ExampleObject {
       public static void main(String[] args) {
           ExampleObject obj = new ExampleObject();
           System.out.println(obj.hashCode());
       }
   }
  1. getClass() Method:
  • The getClass() method returns the runtime class of an object.
public class ExampleObject {
       public static void main(String[] args) {
           ExampleObject obj = new ExampleObject();
           Class<? extends ExampleObject> clazz = obj.getClass();
           System.out.println(clazz.getName()); // Prints "com.example.ExampleObject"
       }
   }

Overriding toString(), equals(), and hashCode()

It’s common to override these methods in your custom classes to provide meaningful implementations for your objects. Here’s an example:


public class Person {
    private String name;
    private int age;

    // Constructor, getters, and setters...

    @Override
    public String toString() {
        return "Person{name='" + name + "', age=" + age + '}';
    }

    @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);
    }

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

    public static void main(String[] args) {
        Person person1 = new Person("John", 30);
        Person person2 = new Person("John", 30);

        System.out.println(person1.equals(person2)); // Prints true
        System.out.println(person1.hashCode() == person2.hashCode()); // Prints true
    }
}

Conclusion

Understanding the Object class and its methods is essential when working with Java, as it provides a foundation for object-oriented programming in the language. By overriding methods like toString(), equals(), and hashCode(), you can customize the behavior of your classes to suit your specific needs.

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