Object Class

#Introduction

In Java, every class directly or indirectly inherits from the Object class. It is the root (superclass) of the Java class hierarchy.

class Employee {
}

The compiler internally treats it as:

class Employee extends Object {
}

Even if you don't explicitly extend Object, Java does it automatically.

#Why Object Class?

The Object class provides common functionality that every Java object should have, such as:

• Comparing objects
• Generating hash codes
• Converting objects to strings
• Cloning objects
• Thread synchronization
• Runtime class information

Without Object, every class would need to implement these features separately.

#Object Class Hierarchy

#Methods of Object Class

The most commonly used methods are:

#1. toString()

Definition

Returns the string representation of an object. Default behaviour:

Object obj = new Object();
System.out.println(obj);

java.lang.Object@5e2de80c

Internally, System.out.println(obj); becomes System.out.println(obj.toString());

Default Implementation

getClass().getName() + '@' + Integer.toHexString(hashCode())

Example output: Employee@4a54c0de, where:

• Employee → Class Name
• 4a54c0de → Hash code (hexadecimal)

Overriding toString()

class Employee {
    int id;
    String name;

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

Employee e = new Employee();
e.id = 101;
e.name = "John";
System.out.println(e);

101 John

Real-world Usage

Useful for logging, debugging, API responses and console output.

#2. equals()

Purpose

Compares two objects. The default implementation compares memory addresses.

Employee e1 = new Employee();
Employee e2 = new Employee();
System.out.println(e1.equals(e2));

false

Because both objects are stored at different memory locations.

Overriding equals()

class Employee {
    int id;

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null || getClass() != obj.getClass())
            return false;
        Employee other = (Employee) obj;
        return id == other.id;
    }
}

Now two employees having the same id are considered equal.

#3. hashCode()

Purpose

Returns an integer hash value. Used heavily by HashMap, HashSet, Hashtable and ConcurrentHashMap.

Why Needed?

Imagine a HashMap. Instead of searching every object, Java computes hashCode() and directly jumps to the correct bucket. Search becomes much faster.

Contract

If a.equals(b) is true, then a.hashCode() == b.hashCode() must also be true.

The reverse is not guaranteed. Different objects can have the same hash code (hash collision).

Example

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

equals() vs hashCode()

HashMap Internal Flow

#4. clone()

Purpose

Creates a copy of an object.

class Employee implements Cloneable {
    @Override
    protected Object clone()
            throws CloneNotSupportedException {
        return super.clone();
    }
}

Usage:

Employee e2 = (Employee) e1.clone();

Shallow Copy

Both copied objects refer to the same nested object.

Deep Copy

Every referenced object is also copied. Preferred in enterprise applications.

#5. getClass()

Returns runtime class information.

Employee e = new Employee();
System.out.println(e.getClass().getName());

Employee

Used in reflection, frameworks, Spring and Hibernate.

#6. wait()

Makes the current thread wait until another thread notifies it. Must be called inside a synchronized block.

synchronized (obj) {
    obj.wait();
}

#7. notify()

Wakes one waiting thread.

synchronized (obj) {
    obj.notify();
}

#8. notifyAll()

Wakes every waiting thread.

synchronized (obj) {
    obj.notifyAll();
}

wait() vs sleep()

#9. finalize()

Called before garbage collection.

protected void finalize() {
}

Current Status

Use these instead:

• try-with-resources
• AutoCloseable
• Cleaner API

#Native Methods

Some Object class methods are native — implemented using platform-specific code. Examples:

• hashCode()
• wait()
• notify()
• notifyAll()