Why we must declare immutable class as Final? Why is an immutable class supposed to be final?

Why immutable class declared as final in Java?

Immutable class means, once object is created it can not be changed. 

Learn more about Immutable class in details :

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

Now lets see why we must declare immutable class as final in Java.

If immutable class does not defined as final then any other class can extends that class and change properties of that class and broke the objective of immutability.

So final class is important in immutability. Lets see using example.

Programming.java

public class Programming {
    
    private final String name;
    
    public Programming(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Programming [name=" + name + "]";
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Programming other = (Programming) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }

}

MutableClass.java

public class MutableClass extends Programming {

    private String mutableName;
    
    public MutableClass(String name) {
        super(name);
       
        // Setting same name as programming name field
        this.setMutableName(name);
    }

    public String getMutableName() {
        return mutableName;
    }

    public void setMutableName(String mutableName) {
        this.mutableName = mutableName;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = super.hashCode();
        result = prime * result + ((mutableName == null) ? 0 : mutableName.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (!super.equals(obj))
            return false;
        if (getClass() != obj.getClass())
            return false;
        MutableClass other = (MutableClass) obj;
        if (mutableName == null) {
            if (other.mutableName != null)
                return false;
        } else if (!mutableName.equals(other.mutableName))
            return false;
        return true;
    }
    
}

Demo.java

import java.util.HashMap;

public class Demo {

    public static void main(String[] args) {
       
        HashMap<Programming, Integer> mapOfProgramming = new HashMap<>();
       
        MutableClass mutableObj = new MutableClass("Java");
       
        // Assigning obj reference of mutable to immutable class
        Programming programmingObj = mutableObj;
       
        mapOfProgramming.put(programmingObj, 1);
       
        // Changing immutable class property "name" using mutable class
        mutableObj.setMutableName("Python");
       
        System.out.println(mapOfProgramming.get(programmingObj));
       
    }
    
}

Output :

null

As you can see, we are getting "null" answer rather than "1". because MutableClass extends Programming class and change the property of "name" field and broke the immutability of Programming class.

Map only knows key holding as Object "Java", but using mutable class we are set name property to "Python" and if we get from map using "Python" name property it will return null because there is no key as name "Python". 

If we comment "mutableObj.setMutableName("Python");" line of code we are getting answer as 1.

So for create immutable class, one must declare class as Final otherwise some other class can extends it and change the object of immutable class.

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• Guide to Collection framework in Java with Examples | Guide for choose which collection to use