Comparator

[Duran Harris]

Hi...

I'm getting the error :"int cannot be dereferenced" for the following code:

class firstNameSort implements Comparator<customer>
{
	public int compare(customer a,customer b)
        {
        return (a.getFirst()).compareTo(b.getFirst());
        }
}

What is the problem??

[James Tharakan]

i think a.getFirst() method is returning int.
using int you can't call a method ...
you are doing something like 5.compareTo()

[Duran Harris]

Nope...

getFirst() returns a String..first name

[Ankit Garg]

Just use a cast if a.getFirst actually returns an int

((Integer)a.getFirst()).compareTo(b.getFirst())

[Duran Harris]

Look:

import java.io.*;
import java.util.*;
public class customer implements Serializable
{
private String first;
private String last;
private int age;
static customer[] list;
static int count;

public customer(String first,String last,int age)
{
  this.first=first;
  this.last=last;
  this.age=age;
  
}

public customer()
{
}

public String getFirst()
{
return first;

}

public String getLast()
{
return last;

}

public int getAge()
{
return age;

}


 class ageSort implements Comparator<customer>
{
	public int compare(customer a,customer b)
        {
         return (a.getAge()).compareTo(b.getAge());      


        }
}

 class firstNameSort implements Comparator<customer>
{
	public int compare(customer a,customer b)
        {
        return (a.getFirst()).compareTo(b.getFirst());
        }
}

 class lastNameSort implements Comparator<customer>
{
	public int compare(customer a,customer b)
        {
        return (a.getLast()).compareTo(b.getLast());
        }
}

private void readObject(ObjectInputStream ois)
{
try{ois.defaultReadObject();



   }
catch(IOException ioe){System.out.println("IO Error");}


}

private void writeObject(ObjectOutputStream oos)
{

try{oos.defaultWriteObject();

    }
catch(IOException ioe){System.out.println("IO Error");}


}

public String toString()
{

 return ""+this.first+""+this.last+""+this.age;

}

public static void createCustomer(String first,String last,int age)
{ count++;
  customer a = new customer(first,last,age);
 try{FileOutputStream fos = new FileOutputStream("customers.txt");
     ObjectOutputStream oos = new ObjectOutputStream(fos);
     oos.writeObject(a);   
     oos.close();
    }
catch(Exception e){e.printStackTrace();}

}

public static void populateArray()
{
   customer c= null;

 try{
     FileInputStream fis = new FileInputStream("customers.txt");
     ObjectInputStream ois = new ObjectInputStream(fis);
     c = (customer)ois.readObject();   
     list[count]=(customer)c;
     ois.close();
 
   }


catch(Exception e){e.printStackTrace();}

}

public static void main(String[] args)
{
 createCustomer("duran","harris",22);
 createCustomer("neil","van tonder",20);
 createCustomer("jackie","chan",35);
 createCustomer("melissa","lourens",17);
 
 populateArray();
 


}













}

[James Tharakan]

we have given the solution , now go through the code again

[Ankit Garg]

I think this code will give the error

46. return (a.getAge()).compareTo(b.getAge());

and not this one

56. return (a.getFirst()).compareTo(b.getFirst());

[Duran Harris]

Hmm doesn't int implement Comparable?

[Duran Harris]

Maybe I should change to Integer

[Ankit Garg]

Duran Harris wrote:Hmm doesn't int implement Comparable?

int is a primitive datatype. It cannot implement anything

[James Tharakan]

Ankit Garg wrote:Just use a cast if a.getFirst actually returns an int

((Integer)a.getFirst()).compareTo(b.getFirst())

this is better option

[Duran Harris]

Ok that's fixed...

but now I have a null pointer exception in populateArray() ops:

[Ankit Garg]

Did you call createCustomer before calling populateArray??

[Duran Harris]

yes,and I can see the customers.txt file that has been created...

[James Tharakan]

Nothing

[Duran Harris]

well I changed the customer[] to an arraylist and it works..