从链接列表中删除未指定的项目

Question

I'm trying to remove a random Node from a linked list or "bag" so to speak. Please refer to my method "T remove()". I can't get it to remove the node without it throwing a NullPointerException error.

First I have the method return a NULL if there is nothing in the list to begin with. If a node exists then I loop through the numberofNodes, find a random one and try to delete that node. I make a node that temporarily points to the data of the next node and have the previous node point to the node after the currentNode as if it doesn't exist.

Is my logic wrong?

private class Node{

        private T entry;
        private Node next;

        private Node(T entryPortion)
        {
            this(entryPortion, null);
        }

        private Node(T entryPortion, Node nextNode)
        {
            entry = entryPortion;
            next = nextNode;
        }

    }

    private Node node1;
    private Node lastNode;
    private int numItems;

    public LinkedBag() //Establishes an empty bag
    {
        node1 = null;
        numItems = 0;
    }

    @Override
    public int getCurrentSize() 
    {
        // Gets Size of Bag
        return numItems;
    }

    @Override
    public boolean isFull() 
    {
        //Checks to see if bag is full, however since it is linked list there is no specified max size. Although maximum memory can be reached
        return true;
    }

    @Override
    public boolean isEmpty() {
        // Checks to see if bag is empty
        return node1 == null;
    }

    @Override
    public boolean add(T newItem) {
        // Adds something to the bag
        Node newNode = new Node(newItem);
        newNode.next = node1;

        node1 = newNode;
        numItems++;
        return true;
    }

    @Override
    public T remove() {
        // Removes random item from bag

        if(node1.equals(null))
        {
            return null;
        }

        else
        {   
        int randItem = new Random().nextInt(numItems);
        Node currentNode = node1;
        Node previousNode = node1;
        for (int i = 0; i < randItem; i++)
            {
            previousNode = currentNode;
            currentNode = currentNode.next;
            }
            previousNode.next = currentNode.next;
            currentNode.next = null;
            numItems--;

            return null;

        }   

        /*if (numItems == 0)

            return null;
        }
        else
        {
            Node temp = node1;
            node1 = node1.next;
            numItems--;
            //if(node1 == null)
                //lastNode = null;
            return temp.entry;

        }*/

    }

    @Override
    public boolean remove(T anItem) {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public void clear() {


    }

    @Override
    public int getFrequencyOf(T anItem) {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public boolean contains(T anItem) {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public T[] toArray() {
        // Converts items in linked list to an array for easy displaying
        @SuppressWarnings("unchecked")
        T[] result = (T[])new Object [numItems];

        int i = 0;
        Node currentNode = node1;
        while((i<numItems)&&(currentNode != null))
        {
            result[i] = currentNode.entry;
            i++;
            currentNode = currentNode.next;
        }
        return result;
    }




}

This is the test program I use. "testRemove is the method that calls my 'remove()' method from my constructor class

public class LinkedBagTest {

    public static void main(String[] args) {

         System.out.println ("Creating an empty bag.");
            BagInterface < String > aBag = new LinkedBag < String > ();
            displayBag (aBag);
            testNumItems(aBag);
            testRemove(aBag);
            String [] contentsOfBag = {"A", "D", "B", "A", "C", "A", "D"};
            testAdd (aBag, contentsOfBag);
            testNumItems(aBag);
            testRemove(aBag);
            displayBag (aBag);
            testRemove(aBag);
            displayBag (aBag);
            //testIsFull(aBag, false);


    }
     private static void testAdd (BagInterface < String > aBag,
                String [] content)
        {
            System.out.print ("Adding to the bag: ");
            for (int index = 0 ; index < content.length ; index++)
            {
                aBag.add (content [index]);
                System.out.print (content [index] + " ");
            } // end for
            System.out.println ();
            displayBag (aBag);
        } // end testAdd

     private static void displayBag (BagInterface < String > aBag)
        {
            System.out.println ("The bag contains the following string(s):");
            Object [] bagArray = aBag.toArray ();
            for (int index = 0 ; index < bagArray.length ; index++)
            {
                System.out.print (bagArray [index] + " ");
            } // end for
            System.out.println ();
        } // end displayBag

     private static void testIsFull (BagInterface < String > aBag,
            boolean correctResult)
     {
        System.out.print ("\nTesting the method isFull with ");
        if (correctResult)
            System.out.println ("a full bag:");
        else
            System.out.println ("a bag that is not full:");
        System.out.print ("isFull finds the bag ");
        if (correctResult && aBag.isFull ())
            System.out.println ("full: OK.");
        else if (correctResult)
            System.out.println ("not full, but it is full: ERROR.");
        else if (!correctResult && aBag.isFull ())
            System.out.println ("full, but it is not full: ERROR.");
        else
            System.out.println ("not full: OK.");
    } // end testIsFull are here.

     private static void testNumItems (BagInterface <String> aBag)
     {
        int items = aBag.getCurrentSize();
        System.out.println("There are " + items + " items in the bag");
     }

     private static void testRemove (BagInterface <String> aBag)
     {
         aBag.remove();
         System.out.println("An Item was removed");

         testNumItems(aBag);



     }
}

Answer 1

Your null checks should be in this format: node1 == null . When you check it as node1.equals(null) , and node1 is actually null, you're trying to call a method of an object, which is null and will naturally throw a NullPointerException.

Answer 2

This is ruining things for you node1.equals(null) , initially your node1 = null , and you are asking a null node to call equals method, which is why you are getting nullpointerexception .

So, please make changes, and put node1 == null