How to transform this class to immutable?

Question

I am in process of learning immutability but I am not able to exactly digest how this works. So in order for me to understand immutability, I created a test program.

The funtion getArray(Box b) will make an ArrayList of Box objects.

    Expected output:    Actual output:

    Output              Output
    a is 5              a is 5
    b is 10             b is 10

    Output              Output
    a is 0              a is 4
    b is 0              b is 40

    Output              Output
    a is 1              a is 4
    b is 10             b is 40

    Output              Output
    a is 2              a is 4
    b is 20             b is 40

    Output              Output
    a is 3              a is 4
    b is 30             b is 40

    Output              Output
    a is 4              a is 4
    b is 40             b is 40

Logic:

public class Box {

    static int a;
    static int b;

    public Box() {
        a = 5;
        b = 10;
    }

    public int getA() {
        return a;
    }

    public void setA(int x) {
        a = x;
    }

    public int getB() {

        return b;
    }

    public void setB(int x) {
        b = x;
    }

    public void display() {
        System.out.println("Output");
        System.out.println("a is " + a);
        System.out.println("b is " + b);
        System.out.println();

    }
}

Main Class

import java.util.ArrayList;

public class Check {

    public static void main(String[] args) {

        Box b = new Box();
        b.display();

        ArrayList<Box> arr2 = new ArrayList<Box>();

        arr2 = getArray(b);

        for (int i = 0; i < arr2.size(); i++) {
            arr2.get(i).display();
        }

    }

    public static ArrayList<Box> getArray(Box b) {

        ArrayList<Box> arr = new ArrayList<Box>();
        for (int i = 0; i < 5; i++) {
            b.setA(i);
            b.setB(i * 10);
            arr.add(b);
        }
        return arr;

    }

}

How do I change the logic in such a way that I get the desired output? How do we decide how and where to edit the code to ensure immutability?

Answer 1

This would be an immutable:

public final class Box {
   final int a;
   final int b;

   public Box(int a, int b) {
      this.a = a;
      this.b = b;
   }
}

And then your array method would be:

public static ArrayList<Box> getArray(Box b) {

   ArrayList<Box> arr = new ArrayList<Box> ();
   for(int i =0 ;i <5; i++) {
      arr.add(new Box(i, i*10));
   }
   return arr;

}

The data members are declared final because they're immutable, and so getters are pointless and setters just make no sense.

The class is declared final so you cannot subclass it.

Answer 2

In short, an immutable object is an object whose state cannot be modified after it's created. Your immutable Box object would look like this:

public final class Box {

    private final int a;
    private final int b;

    public Box(int a, int b) {
        this.a = a;
        this.b = b;
    }

    public int getA() {
        return a;
    }

    public int getB() {
        return b;
    }

    public void display() {
        System.out.println("Output");
        System.out.println("a is " + a);
        System.out.println("b is " + b);
        System.out.println();
    }

}

Notice that the variables a and b are assigned exactly once, during the construction of the Box instance. There are no setters, because Box's immutability means that its state (including variables a and b) will not change over its lifetime.

The final keyword in front of a and b means that you must assign them exactly once. It's actually considered good practice to make all your variables final unless you specifically need them not to be; but for an immutable object it's essential.

You were using the static keyword. Static has nothing to do with immutability. It means the variable is shared among all instances of the Box class. In my example, each Box instance has its own copies of a and b, because I didn't make them static.

To wrap this up, I'll give an example of your main class which has the desired output:

public class Check {

    public static void main(String[] args) {
        final List<Box> arr2 = getArray();
        for (int i = 0; i < arr2.size(); i++) {
            arr2.get(i).display();
        }
    }

    public static ArrayList<Box> getArray() {
        final ArrayList<Box> arr = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            final Box box = new Box(i, i * 10);
            arr.add(box);
        }
        return arr;
    }

}

Note that a new instance of box is created at every iteration of the loop.