What is wrong with my generic implementation of sort method ?

[Praveen Kumar Singh]

Hi
For Input: [abc, abb, aaa, bbc]

Collections.sort give: [aaa, abb, abc, bbc]

I tried to implement something similar to it like this

public static <T> void doBubbleShort(List<T> dataList)
	{
		for (int i = 0; i < dataList.size(); i++)
		{
			boolean oneSwap = false;
			for (int j = i; j < (dataList.size() - 1); j++)
			{
				if (((Comparable<T>) (T) dataList.get(j)).compareTo((T) dataList.get(j + 1)) > 0)
				{
					oneSwap = true;
					T temp = dataList.get(j + 1);
					dataList.set(j + 1, dataList.get(j));
					dataList.set(j, temp);

				}

			}

		}
	}

But what i get is: [abb, aaa, abc, bbc]


Why ?.....Any Help !!

[Paul Clapham]

It's probably just a bad implementation of bubble-sort. Nothing to do with generics, I wouldn't think.

(It would be better if the method's name was "doBubbleSort" rather than "doBubbleShort" but that isn't the source of your problem.)

[Matthew Brown]

Think about the scope of your inner loop.

Because you're looping from the start to finish, at the end of one iteration you've got the last element in the right place. So the next iteration you need to move across all elements except the last one. But instead, you're moving across all but the first.

[Wouter Oet]

If you change your method definition to:

public static <T extends Comparable<T>> void doBubbleShort(List<T> dataList)  

Then you don't need to cast. Also oneSwap is never used.

[Rob Spoor]

Wouter Oet wrote:If you change your method definition to:

public static <T extends Comparable<T>> void doBubbleShort(List<T> dataList)  

Then you don't need to cast. Also oneSwap is never used.

Make that

public static <T extends Comparable<? super T>> void doBubbleSort(List<T> dataList)

Without the ? super part you couldn't sort a List<java.sql.Timestamp>, as Timestamp extends java.util.Date and therefore implements Comparable<Date> - not Comparable<Timestamp>.

[Praveen Kumar Singh]

Thanks Paul and Matthew
Its end up in wrong implementation only !
I almost presume that error is in 'generic' logic only
Final code

public static <T extends Comparable<T>> void doBubbleSort(List<T> dataList)
	{
		for (int i = dataList.size() - 1; i > 0; i--)
		{
			for (int j = 0; j < i; j++)
			{
				if (( dataList.get(j)).compareTo( dataList.get(j + 1)) > 0)
				{
					T temp = dataList.get(j + 1);
					dataList.set(j + 1, dataList.get(j));
					dataList.set(j, temp);

				}

			}
		}
	}

Thanks Wouter
Its works very well
I definitely need more study on Generics !

Thanks Rob
Look like you are an expert on generics

[Rob Spoor]

Expert, no. I am somewhat experienced though, even though I didn't switch from Java 1.4 to Java 5.0 for the first year or so.