ArrayList Sorting Using Comparator? Exception at Runtime

[Soumya Ranjan Mohanty]

  import java.util.*;


class CompObj implements Comparator<ObjComp> {

public int compare(ObjComp one,ObjComp two) { 
return one.getName().compareTo(two.getName());
}
}
  
class ObjComp {

     String name;

ObjComp(String s) {

s=name;
}

public String getName() {

   return name;
 }
}

public class ComparatorEx {

public static void main(String args[]) {

ObjComp oc=new ObjComp("Ricky");
ObjComp oc1=new ObjComp("Michael");
ObjComp oc2=new ObjComp("Stuart");
ObjComp oc3=new ObjComp("Shaun");
ObjComp oc4=new ObjComp("Glen");
ObjComp oc5=new ObjComp("Shane");

CompObj co=new CompObj();

List<ObjComp> lis=new ArrayList<ObjComp>();
lis.add(oc);
lis.add(oc1);
lis.add(oc2);
lis.add(oc3);
lis.add(oc4);
lis.add(oc5);

Collections.sort(lis,co);
System.out.println(lis);
}
}
  

I am getting Exception at Runtime.

[Henry Wong]

Soumya Ranjan Mohanty wrote:
I am getting Exception at Runtime.

Yea. I think you're gonna need to give us a bit more detail than that.

Henry

[Himanshu Mall]

1) Please check your ObjComp constructor, the instance member "name" will contain null for all of the objects.
2) Provide a valid implementation of public String toString() if you want to print out the list elements. (Check out Arrays.toString() method as well. <It is really interesting>)

 import java.util.*;


class CompObj implements Comparator<ObjComp> {

public int compare(ObjComp one,ObjComp two) { 
return one.getName().compareTo(two.getName());
}
}
  
class ObjComp {

     String name;

ObjComp(String s) {

name = s;
}

public String getName() {

   return name;
 }

public String toString() {
	return name;
}
}

public class ComparatorEx {

public static void main(String args[]) {

ObjComp oc=new ObjComp("Ricky");
ObjComp oc1=new ObjComp("Michael");
ObjComp oc2=new ObjComp("Stuart");
ObjComp oc3=new ObjComp("Shaun");
ObjComp oc4=new ObjComp("Glen");
ObjComp oc5=new ObjComp("Shane");

CompObj co=new CompObj();

List<ObjComp> lis=new ArrayList<ObjComp>();
lis.add(oc);
lis.add(oc1);
lis.add(oc2);
lis.add(oc3);
lis.add(oc4);
lis.add(oc5);

Collections.sort(lis,co);
System.out.println(Arrays.toString(lis.toArray(new ObjComp[0])));
}
}  

What the program would print now?

[Glen, Michael, Ricky, Shane, Shaun, Stuart]


Hope it helps.

Himanshu Mall
@[e-mail]

[Abimaran Kugathasan]

Check your ObjComp class, and its field variable initializations......

[Soumya Ranjan Mohanty]

Would you please modify my code and make it working. I want to sort the elements in the ArrayList Using the Collections.sort().

[Abimaran Kugathasan]

Soumya Ranjan Mohanty wrote:Would you please modify my code and make it working. I want to sort the elements in the ArrayList Using the Collections.sort().

Himanshu Mall explained it. Have a look on it.

[Unmesh Chowdhury]

You just modify your ObjComp class constructor as follows:

    
 ObjComp(String s) {  
   
 name=s;  
 }

And override your toString() method, for instance:

    
@Override
  public String toString() {
  	return getName();
  }

Otherwise, Object class toString() will be called for your ObjComp class instances.