排序的双链表中的添加方法-Java

Question

I am really struggling to create an add method for my sorted doubly linked list. I am trying to add an object to a list in order based on a comparable. My code at the moment:

       public void add(DistanceEvent obj){

       DistanceEvent newNode =  obj;

       if (firstLink == null)
       {
           firstLink = newNode;
           return;
       }

       else if (obj.compareTo(firstLink) < 0) {
           newNode.next = firstLink;
           firstLink = newNode;
       }

       else
       {DistanceEvent after = firstLink.next;
       DistanceEvent before = firstLink;
       while (after != null){
           if (obj.compareTo(after) < 0)
               break;
           before = after;
           after = after.next;

       }

       newNode.next =before.next;
       before.next = newNode;

       }
   }
}

I am trying to add 10 objects to this list, however i am only returning two when i display the contents of the list. I have been struggling with this for a while and I'm very new to java so any help would be appreciated. Here is the rest of my linkedlist class:

public class DoubleEndedLinkedList {
    DistanceEvent firstLink;
    DistanceEvent lastLink;

public void insertInFirstPosition(String name, String country, int turn){

    DistanceEvent theNewLink = new DistanceEvent(name, country, turn);

    if(isEmpty()){
        lastLink = theNewLink;
    } else {
        firstLink.previous = theNewLink;
    }

    theNewLink.next = firstLink;
    firstLink = theNewLink;

}

public void insertInLastPosition(String name, String country, int turn){

    DistanceEvent theNewLink = new DistanceEvent(name, country, turn);

    if(isEmpty()){

        firstLink = theNewLink;

    } else {


    lastLink.next = theNewLink;
    theNewLink.previous = lastLink;

    }

    lastLink = theNewLink;


}

public boolean isEmpty(){
    return(firstLink == null); }

public void display() {
    DistanceEvent theLink = firstLink;
    while(theLink != null){
        theLink.display();
        theLink = theLink.next;
        System.out.println();
    }
}

public boolean insertAfterKey(String name, String country, int turn,  int key){
    DistanceEvent theNewLink = new DistanceEvent(name, country, turn);
    DistanceEvent currentDistanceEvent = firstLink;
    while (currentDistanceEvent.turn != key){
        currentDistanceEvent =  currentDistanceEvent.next;
        if(currentDistanceEvent == null){
            return false;
        }

    }
    if(currentDistanceEvent == lastLink){
        theNewLink.next = null;
        lastLink = theNewLink;
    } else {
        theNewLink.next = currentDistanceEvent.next;
        currentDistanceEvent.next.previous = theNewLink;


    }

    theNewLink.previous = currentDistanceEvent;
    currentDistanceEvent.next = theNewLink;
    return true;
}
       public void add(DistanceEvent obj){

       DistanceEvent newNode =  obj;

       if (firstLink == null)
       {
           firstLink = newNode;
           return;
       }

       else if (obj.compareTo(firstLink) < 0) {
           newNode.next = firstLink;
           firstLink = newNode;
       }

       else
       {DistanceEvent after = firstLink.next;
       DistanceEvent before = firstLink;
       while (after != null){
           if (obj.compareTo(after) < 0)
               break;
           before = after;
           after = after.next;

       }

       newNode.next =before.next;
       before.next = newNode;

       }
   }
  }

I apologise for the readability of my code. Thanks for your time!

Answer 1

I have been struggling with this for a while and I'm very new to java so any help would be appreciated.

Well, I'm sure that someone could find the bug or bugs in your code for you. But I think it would be more helpful to you (in the long run) to encourage you to improve your debugging skills.

Here are a couple of suggestions:

• Read your IDE's tutorial material on how to use its debugger. Learn how to set breakpoints, single step, look at the values of variables, look inside objects, and so on.

• Try adding some "trace print" statements to your code at carefully chosen places to get a handle on what it is doing.

• Learn to read your own code and visualize what it is doing. One technique that is useful for beginners is to "hand execute" the code. Get a piece of paper a pencil and an eraser, write out all of the variables, and draw the objects as boxes with cells containing literal values and references to other objects drawn as arrows. Then "execute" the program by stepping through code one statement at a time, updating the variables and fields.

  After some practice, you will be able to do this all in your head ...

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I've only scratched the surface of this topic, but if you Google for "how to debug java", there are lots of good resources: tutorials, videos, and so on.

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Finally, a couple of tips:

• Be methodical. Understand the what the code actually says. Look at the evidence that you have.

• Don't rely on "hunches". It is easy to waste time looking for phantoms if you make assumptions about what is happening that are not evidence-based.

• Assume that 99% of the time, the bug will be in your code. And make that 99.99% if you are using just the standard Java class libraries.

• Debugging multi-threaded programs is hard .....