为什么我在Java代码中得到此输出？

Question

The student at the top of the stack is Gullion,Hailey

Student Mcglothlen,Shizue is removed from the stack


Here are all the elements of the Stack using an Iterator
--------------------------------------------------------
Stack$Node@3012db7c
Stack$Node@2607c28c
Stack$Node@477588d5
Stack$Node@756a7c99
Stack$Node@221a5d08
Stack$Node@70d1c9b5
Stack$Node@5d11c3f0
Stack$Node@3956f14c
Stack$Node@7afbd1fc
null


Here are all the elements in the Stack
--------------------------------------
Putney,John
Larkey,Ismael
Winkler,Isiah
Aceto,Liana
Hamill,Crissy
Caraway,Elmira
Gullion,Hailey
Rodrigez,Jonie
Madruga,Terrell
Williams,Diego

The first list of elements of the Stack using an Iterator apparently is not working. I do not know why. Here is my code for Iterator in my Stack class:

public Iterator<Student> iterator()  { return new ListIterator();  }

// an iterator, doesn't implement remove() since it's optional
private class ListIterator implements Iterator<Student> {
    private Node<Student> current = top;

    public boolean hasNext()  { 
        return current != null;                     
    }

    public void remove() { 
        throw new UnsupportedOperationException();  


    @SuppressWarnings("unchecked")
    public Student next() {
        if (!hasNext()) throw new NoSuchElementException();
        current = current.next; 
       return (Student)current;
    }
}

Here is the code in my Driver class that is where there seems to be a problem:

System.out.println("\nHere are all the elements of the Stack using an Iterator");
  System.out.println("--------------------------------------------------------");
  Iterator <Student> iter = myStack.iterator();
  while (iter.hasNext() )
      System.out.println(iter.next() );

HERE IS ALL OF THE CLASSES:

Stack: http://pastebin.com/2HVLVHuM

Queue class: http://pastebin.com/3q537kHW

Student class: http://pastebin.com/UnBB7kPA

Driver class: http://pastebin.com/yeA34MNd

I CAN ONLY WRITE CODE IN THE STACK CLASS. The point of this was to Implement a stack using queues. Hope this helps

Answer 1

You need to add a toString() method in your Student class. The Iterator is working correctly, but the System.out.println() doesn't know how to display the Student .

Add something to the Student class like this...

public String toString(){
    return name;
}

So that when you call System.out.println() , it can output a real value. When you call System.out.println(Object) , it always tries to output the toString() value. If this method isn't defined, it will output the java ID of the object, which is what you're seeing.

Answer 2

First of all, see nneonneo 's answer for the incorrect cast in your next() method.

Secondly, your Iterator implementation is incorrect.

The next() function in your iterator returns the element current after setting it to current.next .

After calling next() on the last element of your iteration, hasNext() should return false. But it doesn't, because current still points to the element you just returned. So you will call next() again. And in this method, current = current.next will set current to null , and then return it. Which should not happen, since hasNext was true, right?

For the same reason, the fist elemtent of your stack is missing: You set current to the top element of your stack, but before outputting anything, you already switch to current = current.next . You should do that after doing the output.

Answer 3

current in your Stack iterator is defined as Node<Student> . You return current from your next() method using a cast.

So, next() returns a Node<Student> (type-cast to Student ), instead of an actual Student . Since Node presumably doesn't have a toString method, you get the default output ( Stack$Node@<addr> ).

To fix, return something like current.item from next() instead (assuming that the item stored in the Node is called item ).

Answer 4

The first list of elements of the Stack using an Iterator apparently is not working. I do not know why.

Because your iterator is returning a Node<Student> instead of a student. The problem is at:

return (Student)current;

You probably tried to do this, but got an Incompatible Type error:

return current;

So you tried to fix by casting. The problem is that a node is not a student. A node contains a student. You need to return the student that the node contains.

Try this:

    return current.data;

No casting is required because the compiler knows that the "data" member of node is a Student, since you declared current of type Node<Student> . This will fix the problem where your student prints out incorrectly. However as pointed out by @Konstantin, your iterator is still broken. You need to save the value of current in a temporary variable, move current, then return the temporary variable. Here is one possible implementation:

    public Student next() {
        if (current == null) throw new NoSuchElementException();
        Node<Student> result = current;
        current = current.next;
        return result.data;
    }

[epilogue]

You really need to review the generics tutorial . It's not clear from the code you pasted in above but it's obvious in the paste-bin code that you are using Student as a type parameter. That is very non-standard and confusing. The convention is to use a capital letter - typically T. You should have declared stack like below. Everywhere you use Student, replace it with T.

public class Stack <T> implements Iterable<T>{   // good

instead of

public class Stack <Student> implements Iterable<Student>{ // bad

T means some type to be decided later and you can use Stack with any kind of object. Only when you actually create a stack to you use Student (or whatever)

public static void main(String [] args)
{
    Stack<Student> x = new Stack<Student>();