Immutable code to mutable

[Tonyuk2]

Hello,

I would like to get any suggestion or help to convert a immutable code to mutable.

The immutable code is a implementation of Poly data type.

Any links or suggestions are welcome

[KevinWorkman]

Huh? Do you have access to the code? Can you change it? Then just add a setter.

[Tonyuk2]

Hi,

Thanks for the quick reply

I am given a Immutable implementation of poly data type. I would like to change the same to immutable. I am using Netbeans IDE

What do you mean by a setter?

[Fubarable]

A setter is a method that sets a property. For instance if the class has an int field called "value", then a setter would be:

public void setValue(int value) {
   this.value = value;
}

and would allow users of the class to set the value's value.

And rather than try to make it mutable, why not just create a wrapper class?

[KevinWorkman]

And rather than try to make it mutable, why not just create a wrapper class?

I came back to suggest exactly this. Beat me to it!

[Tonyuk2]

Suppose the immutable code is like this, how would the mutable one look like?

// Effects: If q is null throw NullPointerException
    // else return the Poly this * q
    public Poly mul(Poly q) throws NullPointerException {

       if ((q.deg == 0 && q.trms[0] == 0) || (deg == 0 && trms[0] == 0)) {
          return new Poly();
       }

       Poly r = new Poly(deg + q.deg);
       r.trms[deg + q.deg] = 0;
       for (int i = 0; i <= deg; i++) {
          for (int j = 0; j <= q.deg; j++) {
             r.trms[i+j] += trms[i]*q.trms[j];
          }
       }
       return r;
    }

[KevinWorkman]

String is immutable. What if I want to make it mutable? I add a wrapper:

public class StringWrapper{
   private String string;


   public StringWrapper(String s){
      string = s;
   }

   public String getString(){
      return string;
   }

   public void setString(String s){
      string = s;
   }

}

Now wherever I want to use a mutable String, I simply use this wrapper class instead.

[Fubarable]

Suppose the immutable code is like this, how would the mutable one look like?

// Effects: If q is null throw NullPointerException
    // else return the Poly this * q
    public Poly mul(Poly q) throws NullPointerException {

       if ((q.deg == 0 && q.trms[0] == 0) || (deg == 0 && trms[0] == 0)) {
          return new Poly();
       }

       Poly r = new Poly(deg + q.deg);
       r.trms[deg + q.deg] = 0;
       for (int i = 0; i <= deg; i++) {
          for (int j = 0; j <= q.deg; j++) {
             r.trms[i+j] += trms[i]*q.trms[j];
          }
       }
       return r;
    }

Nothing in that code makes the code or class immutable. I suggest that you read a tutorial on Java immutable classes. For e.g., Java Practices -> Immutable objects

[pbrockway2]

Nothing in that code makes the code or class immutable.

Yes.

But I think the thing is that this method creates a new Poly instance to represent the product and returns that. This doesn't, in itself, make the class immutable, but it is what you would do if you wanted an immutable Poly class. Another common way of making a product would be for the method to alter the state of the Poly of which it is a method so that the multiplication is "in place" ie:

  /** Changes this polynomial so that it represents the product of its initial value with some other given polynomial. */
public void mul2(Poly q) {
    // your code here
}

This has to be done method by method to create this (quite different) behaviour for the other class. It will involve the Poly instance variables like deg and trm[] which you haven't described.