Java generic arguments

问题

Going back over my basic ADT stuff here to revise for an interview, and trying to kill two birds with one stone by learning Java while I am. Attempting to write a simple algorithm for a merge sort with a generic linked list ( which I am creating myself). It's proving to be far more difficult than I had first imagined ! Can anyone help me out please ? I will start out working on the basics and will update this post as I get further in.

My code for the generic linked list is as follows :

 public class NodeList<T extends Comparable<T> > {
  private T head;
  public NodeList<T> tail;
  public NodeList( T item, NodeList<T> list ) {
    head = item;
    tail = list;
  } 

}

I am trying to access this class in another class I have made, which is as follows :

    public class MyList<T extends Comparable<T>> {

  private NodeList<T> nodes;
  private static int size;
  public MyList( ) { 
    nodes = null; 
  }

  public MyList(T[] array ){
    for( T item : array ) {
      nodes = new NodeList<T>(item, nodes); 
    }
    size = array.length;
  }


  public void add( T item ) { 
    nodes = new NodeList<T>( item, nodes ); 
    size++;
  }


  public void addEnd( T item ) {
    NodeList<T> temp = nodes;
    while ( temp == null || temp.tail != null) {
      temp = temp.tail;
    }
    size++;
    temp.tail = new NodeList<T> ( item, null);
  }

I believe, so far, everything to be correct up until the add and addEnd methods, which should add a generic to the start of the list and end of the list respectively.

My code continues with :

 public static <S extends Comparable<S>>
    MyList<S> sort( MyList<S> list ) {

    if ( size > 1 ) {

      MyList<S> left  = leftHalf( list );
      MyList<S> right = rightHalf( list );
      list = merge( left, right );
    }

    return list;
  }

  private static <S extends Comparable<S>>
    MyList<S> merge( MyList<S> left, MyList<S> right ) {

  }

  private static <S extends Comparable<S>>
    MyList<S> leftHalf( MyList<S> list ) {
    MyList <S> leftSide = new MyList();
    int middle;
    if(size % 2 == 1) {
     middle = size +1;
    } else {
     middle = size; 
    }
    for ( int countToMiddle = 0; countToMiddle < middle ; countToMiddle++ ) {
      leftSide.addEnd(nodes);
    }


    // return elements from 0 .. list.size() / 2
  }

And I get the error:

addEnd(S) in MyList cannot be applied to (NodeList)

which occurs when I run

leftSide.addEnd(nodes);

Can anyone see a reason for this/ tell me if I am correct up to this point of my work ? Thanks so much again!

回答1:

If you want NodeList and MyList to only contain Comparable items, you can replace the generic parameter T with something like:

public class NodeList<T extends Comparable> {

Or

public class NodeList<T extends Comparable<T>> {

And replace where you use Comparable with T. This way, you know T at least implements Comparable's methods.

Oracle's tutorials for generics should be able to help you with getting the hang of them.

---

One problem you may be having is that you refer to member variables from static functions, like in leftHalf you have:

   for ( int countToMiddle = 0; countToMiddle < middle ; countToMiddle++ ) {
      leftSide.addEnd(nodes);
    }

nodes is a member variable, i.e. a non-static variable, so you can't call it from static methods. For that example, you'd have to get it from the passed MyList:

   for ( int countToMiddle = 0; countToMiddle < middle ; countToMiddle++ ) {
      leftSide.addEnd(list.nodes);
    }

And the same goes for your other static methods that try to use member variables.

---

Also, the reason you are getting an error like: addEnd(S) in MyList<S> cannot be applied to (NodeList<T>) is because S is, according to your type parameter, a Comparable. NodeList does not extend Comparable!

The two solutions you have is

1. Make NodeList extend Comparable so you can pass it to MyList.addEnd
2. Make an overload (i.e. a different method with the same name) for addEnd that takes a NodeList, and add all the items in the passed NodeList to MyList

Or come up with a different solution that better fits the need of your classes.

---

While I realize you are implementing a linked list just to sharpen your skills for an interview (I wish you good luck!), I just want to add that there is a generified LinkedList already available in Java.

回答2:

Why do you post almost the same question twice? You could extend your question, add comments etc.

We already gave you that hint. :)

回答3:

The error is happening because the class NodeList doesn't have a constructor that receives a generic T class and a NodeList. Actually, this implementation will replace the reference object that nodes is referring on every loop. You should also fix that.

What you should do is put T to be a Comparable itself, and change the attribute, something like:

public class NodeList<T extends Comparable> {
    private T head;
    private NodeList tail;
    public NodeList( T item, NodeList list ) {
        head = item;
        tail = list;
    }
}

It would be better if you tell us what exactly the code is for.

来源：https://stackoverflow.com/questions/5185577/java-generic-arguments