How to create an immutable class which has mutable instance object?

Hi everyone;

Sometimes you need an immutable class in your app. Maybe you want to convert your mutable class to immutable class.

Firstly;

Why we need a immutable class or what is advantages of immutable class?

• Thread-safe object
• Cacheable object

How to do an immutable class?

• immutable class must be final
• all properties must be private (access control)
• all properties must be final (can not change)
• using any setter method for properties

For example an immutable class

public final class Employee {         

     private final String name;
     private final Integer age;

     public Employee(String name, Integer age) {
         this.name = name;
         this.age = age;
     }

     public String getName() {
         return name;
     }

     public Integer getAge() {
         return age;
     }

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

• final class
• private final properties
• no setter method

Now we add a mutable class instance to immutable class

For example a mutable class (Address)

public class Address {

     private String city;
     private String state;

     public Address(String city, String state) {
         this.city = city;
         this.state = state;
     }

     public String getCity() {
         return city;
     }

     public void setCity(String city) {
         this.city = city;
     }

     public String getState() {
         return state;
     }

     public void setState(String state) {
         this.state = state;
     }

     @Override
     public String toString() {
         return "Address{" +
                 "city='" + city + '\'' +
                 ", state='" + state + '\'' +
                 '}';
     }
 }

And we add this instance object (Address) to immutable class (Employee)

public final class Employee {

     private final String name;
     private final Integer age;
     private final Address address;  //mutable object

     public Employee(String name, Integer age, Address address) {
         this.name = name;
         this.age = age;
         Address cloneAddress = new Address(address.getCity(),address.getState()); // deep copy
         this.address = cloneAddress;
     }

     public String getName() {
         return name;
     }

     public Integer getAge() {
         return age;
     }

     public Address getAddress() {
         return new Address(address.getCity(),address.getState());
     }

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

Did you notice that how define mutable class inside constructor method and getter method of mutable class?

Firstly test this immutable class

public class TestClass{

    public static void main(String[] args) {
        Address address = new Address("city1","state1");
        Employee employee = new Employee("employee1",20,address);

        System.out.println(employee);

        //after change
        address.setCity("city2");
        address.setState("state2");

        System.out.println(employee);

        Address address2 = employee.getAddress();
        //after change other way
        address2.setCity("city3");
        address2.setState("state3");

        System.out.println(employee);
    }
}

Output is :

Employee{name='employee1', age=20, address=Address{city='city1', state='state1'}}
Employee{name='employee1', age=20, address=Address{city='city1', state='state1'}}
Employee{name='employee1', age=20, address=Address{city='city1', state='state1'}}

You can see mutable class in immutable class can not change values.

But if I change our class structure like below

public final class Employee {

    private final String name;
    private final Integer age;

    private final Address address;  //mutable object

    public Employee(String name, Integer age, Address address) {
        this.name = name;
        this.age = age;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public Integer getAge() {
        return age;
    }

    public Address getAddress() {
        return address;
    }

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

when run code example, output is :

Employee{name='employee1', age=20, address=Address{city='city1', state='state1'}}
Employee{name='employee1', age=20, address=Address{city='city2', state='state2'}}
Employee{name='employee1', age=20, address=Address{city='city3', state='state3'}}

In actually, because of changing of mutable class our immutable class changed.

What I did?

1. In consturctor of immutable class, I did deep copy :

Address cloneAddress = new Address(address.getCity(),address.getState()); // deep copy
this.address = cloneAddress;

2. In getter method of mutable class inside immutable class :

return new Address(address.getCity(),address.getState());

I hope, it was useful for you.

Thanx to reading.