Java APIs

I still have no idea what I'm doing with Generics. How do I get rid of the warning?

The latest warning message is: unchecked call to compareTo(T) as a member of the raw type java.lang.Comparable

and the offending line of code:

return ((Comparable)c1).compareTo(c2);

The entire program:

import java.util.Comparator;
import java.util.List;
/**
 * A comparator to sort Lists of data based on a column within each List
 */
public class ListComparator implements Comparator
{
	private int column;
	private boolean isAscending;
	private boolean isNullsLast;
	private boolean isIgnoreCase = false;

	ListComparator(int column)
	{
		this(column, true);
	}

	ListComparator(int column, boolean isAscending)
	{
		this(column, true, true);
	}

	ListComparator(int column, boolean isAscending, boolean isNullsLast)
	{
		setColumn( column );
		setAscending( isAscending );
		setNullsLast( isNullsLast );
	}

	public void setColumn(int column)
	{
		this.column = column;
	}

	public void setAscending(boolean isAscending)
	{
		this.isAscending = isAscending;
	}

	public void setNullsLast(boolean isNullsLast)
	{
		this.isNullsLast = isNullsLast;
	}

	public void setIgnoreCase(boolean isIgnoreCase)
	{
		this.isIgnoreCase = isIgnoreCase;
	}

	public int compare(Object a, Object b)
	{
		List list1 = (List)a;
		List list2 = (List)b;

		Object o1 = list1.get(column);
		Object o2 = list2.get(column);

		// Treat empty strings like nulls

		if (o1 instanceof String && ((String)o1).length() == 0)
		{
			o1 = null;
		}

		if (o2 instanceof String && ((String)o2).length() == 0)
		{
			o2 = null;
		}

		// Handle sorting of null values

		if (o1 == null && o2 == null) return 0;

		if (o1 == null) return isNullsLast ? 1 : -1;

		if (o2 == null) return isNullsLast ? -1 : 1;

		//  Compare objects

		Object c1;
		Object c2;

		if (isAscending)
		{
			c1 = o1;
			c2 = o2;
		}
		else
		{
			c1 = o2;
			c2 = o1;
		}

		if (c1 instanceof Comparable)
		{
			if (c1 instanceof String
			&&  isIgnoreCase)
				return ((String)c1).compareToIgnoreCase((String)c2);
			else
				return ((Comparable)c1).compareTo(c2);
		}
		else  // Compare as a String
		{
			return c1.toString().compareTo(c2.toString());
		}
	}
}