Extend vs import

Question

Hi
Im getting confused about the idea of importing packages into my class and extend class. What I actually doing when I extend a class?
Thanks
D

Answer 1

is that correct using?I don't know what you mean by this question. If it does what you need it to do, then it is one correct solution. Are you asking if it's "the right way" or "the best way"? There rarely is a "right" or "best" way to do something. At minimum, it has to function according to the requirements. Beyond that, you have considerations of performance, maintainability, meeting the instructor's requirements as to which constructs to use or not to use.
One thing I would change is that I'd use a StringBuffer to build up the string from the elements, and then when you're done, call sbuf.toString() to return the String. This is more efficient than repeatedly appending to a String, alhtough unless your list is large, it's unlikely to make a significant difference.
Also, you don't have to cast before calling toString. Object obj = iter.next();
String str = obj.toString(); The correct toString method will be called without the cast.
And in fact, you don't even have to call toString str = str + obj; will end up calling toString for you. That's what the + operator does for String. It's one of the convenient little quirks of the language.
That's just general info though. If you go with the StringBuffer, you can just do sbuf.append(it.next()).append(newline); What's newline? Well, rather than using "\n" you should use System.getProperty("line.separator") to get your end of line character(s).
>
Now my problem is like that, the constructor receive
instances and I put them in the superclass,now how can
I return it to the caller?
Huh?
public class DataItemArray extends LinkedList{
/** The arr is used for cloned string storage. */
LinkedList arr = new LinkedList();
//copy the reciverd list to its superclass array
public DataItemArray(LinkedList diArray) {
for(int i=0;i<diArray.size();i++){
super.add(diArray.get(i));
}The above will work, if you're trying to construct one of your objects by adding all the elements of a LinkedList to it. I think LinkedList has a constructor that takes another List or Collection as an arg, so you can just do this: public DataItemArray(Collection col) {
    super(col);
} which will have the same effect.
By the way, never iterate over a LinkedList using a counter and list.get(i). That's O(n^2). So a list with 3 times as many items will take 9 times as long. Always use an Iterator for iterating over a Collection.
Also, you don't need to call super.add. If you're going to call an add method, just call add(). Instance methods are polymorphic, so regardless of which add() you think you're calling, it will be this.add(). In your case, this.add() is inherited from super.
public LinkedList getArray(){
return super.?????
}Why do you want a method that returns an ArrayList? Your class IS a LinkedList. If you want to return the original list from which your class was constructed (why, I can't imagine) then you probably want composition, rather than inheritance. Don't extend LL. Just implement List and take a List as an arg at construction time.
That's one solution, but it really depends on what you're trying to do. If you really want something that is a LL except that it has a different toString, then keep extending as you do, but get rid of that getArray method, as it serves no purpose.