为什么要进入循环？

Question

It works fine and gives me the correct output until it enters the last object to add to nonEmptyList in the sample() method. I have managed to find out where it is looping, which is in the while loop in the add(item) method. I can't change the methods or returns, so if anyone could suggest a way I could prevent this infinite loop, that would be appreciated.

public class SampleableListImpl implements SampleableList {

    public int size;
    public Object firstLink = null; //Made a link class to manage each object, this class is the linkedlist (manager) of the object private 
    public ReturnObjectImpl ro;
    int count;
    SampleableListImpl emptyList;
    SampleableListImpl nonEmptyList;

    public ReturnObject add(Object item) { 
        if (firstLink == null){
            firstLink = item;
            size++;
            firstLink.setIndex(0);
        System.out.println("Added linkedlink at 0");
        } else if (firstLink != null){
            Object current = firstLink; 
            while (current.getNextNode() != null){ //LOOPS HERE AFTER sample() sends the last object to be added to nonEmptyList
                current = current.getNextNode();
            } 
            current.setNextNode(item);
            size++;
            current.getNextNode().setIndex(size - 1);
            System.out.println("Added a new link to the existing linkedlist at " + current.getNextNode().getIndex());   
        }
        return null;
    }

    public SampleableList sample() { 
        if (firstLink == null){
            System.out.println("List is empty, so returning an empty sampableList");
            return emptyList = new SampleableListImpl();
        }
        Object current = firstLink;
        if (firstLink.getNextNode().getNextNode() != null){
            nonEmptyList = new SampleableListImpl();
            System.out.println("Adding to firstNode in nonEmptyList");
            nonEmptyList.add(firstLink);
            while (current.getNextNode().getNextNode() != null){
                current = current.getNextNode().getNextNode();
                System.out.println("Adding " + current.getIndex() + " to nonEmptyList");
                nonEmptyList.add(current);
            }
        } else {
                nonEmptyList.firstLink = current;
                System.out.println("There is only a head - no other objects to sample");
            }
        System.out.println("returning nonEmptyList");
        return nonEmptyList;
    }

}

And I am running this

SampleableListImpl sampList = new SampleableListImpl();
        Object ob = new Object();
        Object ob1 = new Object();
        Object ob2 = new Object();
        Object ob3 = new Object();
        Object ob4 = new Object();
        sampList.add(ob);
        sampList.add(ob1);
        sampList.add(ob2);
        sampList.add(ob3);
        sampList.add(ob4);
        sampList.sample(); 

Answer 1

When all 5 objects are added just before call sample() :

• ob.getNextNode() will return ob1 .
• ob1.getNextNode() will return ob2 .
• ob2.getNextNode() will return ob3 .
• ob3.getNextNode() will return ob4 .
• ob4.getNextNode() will return null .

sample() will in the first loop add ob again:

• ob.getNextNode() will return ob1 .
• ob1.getNextNode() will return ob2 .
• ob2.getNextNode() will return ob3 .
• ob3.getNextNode() will return ob4 .
• ob4.getNextNode() will return ob .

In the second loop within sample() it will try to add ob2 , but it can no longer reach the end of your list.

What you could do to get around this, is create a copy of each object you want to add from sample() (and set their next node to null before adding).