Using The CompareTo Method

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#1 soltan

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Post icon Posted 01 May 2009 - 01:15 AM

Hi, I�m having a problem using the compareTo method. The compareTo method is used to store strings in alphabetical order of last name then first name if last name is the same.

That is the interface

public interface Comparable<T> 
{
	
	int compareTo(T rhs);
}

The implementation

public int compareTo(Object rhs)
	{

		int compare = theLastName().compareTo(rhs.theLastName());
		return (compare !=0 ? compare : theFirstName().compareTo(rhs.theFirstName()));

	}

So as you can see I�m using objects here and I�m using an arrayList to store them , How do i employ my compareTo method and using it to sort my collection with Collection.sort(theArrayList) in my main class ?

Thanks

This post has been edited by soltan: 01 May 2009 - 01:17 AM

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#2 pbl

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Posted 01 May 2009 - 03:48 AM

soltan, on 1 May, 2009 - 01:15 AM, said:

public interface Comparable<T> 
{
	
	int compareTo(T rhs);
}

The implementation

public int compareTo(Object rhs)
	{

		int compare = theLastName().compareTo(rhs.theLastName());
		return (compare !=0 ? compare : theFirstName().compareTo(rhs.theFirstName()));

	}

What is T ?
If T is an Object : class Rhs implements Comparable<Object>
then you cannot do:
rhs.theLastName()); because rhs is an Object and Object do not have a method called "theLastName" so you will have to cast

int compareTo(Object o) {
Rsh rsh = (Rhs) o;
... continue as you did

Now if T is and Rhs: class Rhs implements Comparable<Rhs>
then you'll have to change the signature of your method from

int compareTo(Object rhs)
to
int compareTo(Rhs rhs)

hope this hekps

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#3 Mikeyp926

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Posted 01 May 2009 - 11:14 AM

Also, to actually use this method with Collections.sort, I'm thinking you might just call that sort method like you do normally, and it should automatically use the compareTo method you wrote to sort them....at least I think that's how you would do it, I don't think you need to use a comparator or anything, but I could be mistaken.

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