How to create Immutable Class or Object in Java? | Benefits of immutable class

Create Immutable Class in Java | Advantages of immutable class in java

What is immutable class or object?

In simple term, once the object of the class created its fields cannot be changed or modified. Like once Student object is created, it can not be changed.

In java, all wrapper classes are immutable. like Boolean, String, Integer, Double, Long, Float.

Why we need Immutable class? or Advantages of Immutable class

• When we does not want to change its fields after created.
• It is Thread safe.
• These objects are good for use as hash key (HashMap)
• Reference of immutable objects can be cached.

How to create Immutable Class in Java?

We can create Immutable class using simple following steps :

1. Make class Final
2. Make class fields Private and Final
   
3. Do not write Setter method for fields  
4. Create all arguments constructor
   

Using above steps we can create Immutable class in Java. So lets go on code...

// Final class
final class Company {
    
    // Private fields
    final private int id;
    final private String name;

    // All arguments Constructor
    public Company(int id, String name) {
        this.id = id;
        this.name = name;
    }

    // Only getter methods
    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    @Override
    public String toString() {
        return "Company [id=" + id + ", name=" + name + "]";
    }

}

As above class follows all conditions we mentioned above, so we can call it is immutable class. But in this class if we add new field it can be mutable again lets see how we can solve that also.

Now lets create one Address class and add address field into Company class.

Address.java

public class Address {

    // Private fields
    final private int id;
    final private String location;
    
    public Address(int id, String location) {
        this.id = id;
        this.location = location;
    }
    
    public int getId() {
        return id;
    }
    
    public String getLocation() {
        return location;
    }
    
    @Override
    public String toString() {
        return "Address [id=" + id + ", location=" + location + "]";
    }
    
}

Company.java

// Final class
final class Company {
    
    // Private fields
    final private int id;
    final private String name;
    final private Address address;

    // All arguments Constructor
    public Company(int id, String name, Address address) {
        this.id = id;
        this.name = name;
        this.address = address;
    }

    // Only getter methods
    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }

    public Address getAddress() {
        return address;
    }

    @Override
    public String toString() {
        return "Company [id=" + id + ", name=" + name + ", address=" + address + "]";
    }
    
}

Demo.java

public class Demo {

    public static void main(String[] args) {
       
        Address addressObj = new Address(1, "India");
        Company companyObj = new Company(1, "Java", addressObj);

        System.out.println(companyObj);
       
        Address newAddress = companyObj.getAddress();
        newAddress.setLocation("USA");
       
        System.out.println(companyObj);
    }
}

Output :

Company [id=1, name=Java, address=Address [id=1, location=India]]
Company [id=1, name=Java, address=Address [id=1, location=USA]]

As you can see Address field changed in second output line and in immutable class we can not change any field. So how to fix that? lets see...

Step 1 : Return new Address in getAddress() method in Company class

Company.java

    public Address getAddress() {
        return new Address(address);
    }

Step 2 : Create new constructor in Address class and return address data

Address.java   

    public Address(Address address) {
        this(address.getId(), address.getLocation());
    } 

Output :

Company [id=1, name=Java, address=Address [id=1, location=India]]
Company [id=1, name=Java, address=Address [id=1, location=India]]

Now Address is not changing because every time it gives new object of Address.

Now Company class is immutable class.

Happy learning. Happy coding.

Other articles :

• Object Oriented Programming (OOP) concept in Java with Examples
• What is final in Java? | What is final class, method and variable? 
• Most asked interview questions and answers for Java developers on Core Java