对象链表上的合并排序（升序）

Question

I am working on a homework assignment and after days of effort I cannot figure out why, after my mergeSort is implemented, my list contains only the last object in my linked list. It does not output my entire linked list, only the last object. How can I change my code to stop my list from turning null after one object.

Please note: Though I call them cubes, I know they are not since they have random lengths, widths, and heights. The assignment specifies that they be called cubes, but with these random data fields. Please ignore this.

public class SortingCubes {
    public static void main(String[] args) {
        System.out.println(" ");
        System.out.println("-----MY LINKED LIST-----");
        Cube headCubeLL = new Cube();                           //Create head cube
        int LLnum = 5;                                          //Change number of linked list items desired here
        for(int i = 0; i < LLnum; i++) {
            headCubeLL.length = Math.random() * 99 + 1;         //Create random L,W,H for each cube (between 1-100)
            headCubeLL.width = Math.random() * 99 + 1;
            headCubeLL.height = Math.random() * 99 + 1;
            headCubeLL.next = null;                             //Sets end of list to null
            if(headCubeLL.next == null) {                       //creates new cube until desired number is reached in the for loop
                Cube curr = new Cube(headCubeLL.length, headCubeLL.width, headCubeLL.height);
                headCubeLL = curr;
            }
            headCubeLL.next = null;                             //Sets last cube (next) to null to end the list.
            System.out.println(headCubeLL.toString() + " ");    //Print my Linked list until end
        }
        System.out.println(" ");
        System.out.println("-----NEW LINKED LIST AFTER MERGE SORT METHOD IS IMPLEMENTED (Asending Order by Volume)-----");
        long startTimeMergeSort = System.nanoTime();
        mergeSort(headCubeLL);
        long timeElapsedMergeSort = (System.nanoTime()- startTimeMergeSort);
        printList(headCubeLL);  //Method to print linked list
                System.out.println("Objects in list " + count(headCubeLL));
        long startTimeInsertionSort = System.nanoTime();

        System.out.println(" ");
        System.out.println("Time Record: ");
        System.out.println("Time elapsed for Merge sort on a Linked List: " + timeElapsedMergeSort + " Nanos");


    }       //End of Main

    public static void printList(Cube headCubeLL){
        while(headCubeLL != null){
            System.out.println(headCubeLL.toString() + " ");
            headCubeLL = headCubeLL.next;
        }
        System.out.println();
    }

    public static int count(Cube head){
        int count = 0;
        while(head != null){
            //System.out.println(head);
            count++;
            head = head.next;
        }
        return count;
    }

    public static Cube mergeSort(Cube headCubeLL) {

        if(headCubeLL == null || headCubeLL.next == null) {     //checking list is null
            return headCubeLL;
        }
        int count = 0;                  //To count the total number of elements
        Cube temp = headCubeLL;         //Temporary head of list 
        while(temp != null) {           //break up the list into two parts
            count++;                    //while not empty, count one and move temp to temp.next to evaluate
            temp = temp.next;
        }
        int middle = count/2;           //create an integer called middle and divide the length of your list (count) by 2
        Cube a = headCubeLL;            //another temp head cube for 1st split list
        Cube b = null;                  //another cube that is null
        Cube temp2 = headCubeLL;        //create another temp head cube for 2nd list    

        int countHalf = 0;              //start at half 0 again

        while(temp2 != null) {          //while temp I have for 2nd list is not null.... 
            countHalf++;                    //add count to get length of linked list
            Cube theNext = temp2.next;      //Create new cube that will be assigned to cube after head
            if(countHalf == middle) {       //once it reaches middle number
                temp2.next = null;          //end list (set head.next to null for list 2) 
                b = theNext;                //Take my empty null cube and assign it to end with null. 
            }
            temp2 = theNext;                //Otherwise - move along and my temp head will be the temp head.next
        }
        //Now I have 2 parts, List a and List b. 
        Cube half1 = mergeSort(a);          //Recursively call to sort each half
        Cube half2 = mergeSort(b);

        //Merge together
        Cube merged = merge(half1, half2);  //Call 2nd method to merge the 2 halves together
        //System.out.println(merged.toString());     //Print the L,W,H and Volume of each cube in my array

        return merged;      //return merged list. 
    }

    public static Cube merge(Cube a, Cube b) {      //parameters are the 2 half's of the original list
        Cube pt1 = a;                   //2 parts that I am going to merge as ascending lists according to volume
        Cube pt2 = b;

        Cube tempHead = new Cube();         //Temp head of my New list that cubes will be merged into
        Cube ptNew = tempHead;              //Temp Head for List my new list

        while(pt1 != null || pt2 != null) {     //While either list is not empty
            if(pt1 == null) {                   //but if first is null  
                ptNew.next = new Cube(pt2.cubeVolume());    //create new cube for every cube volume
                pt2 = pt2.next;                 //loop
            }
            else if(pt2 ==null){                //but if 2nd is null
                ptNew.next = new Cube(pt1.cubeVolume());    //create new cube for every cube volume
                pt1 = pt1.next;                 //loop
                ptNew = ptNew.next;             //move on by assigning moving to next cube creating when pt2 is == null
            }
            else {
                if(pt1.cubeVolume() < pt2.cubeVolume()) {       //moving while merging - comparing volumes of the head of each list
                    ptNew.next = new Cube(pt1.cubeVolume());    //When one is greater then new cube creating in new list and cube is placed accordingly
                    pt1 = pt1.next;         //loop
                    ptNew = ptNew.next;     //loop through new merged list for next comparison
                }
                else if(pt1.cubeVolume() == pt2.cubeVolume()) { //statement if the cubes volumes are equal
                    ptNew.next = new Cube(pt1.cubeVolume());
                    ptNew.next.next = new Cube(pt1.cubeVolume());
                    ptNew = ptNew.next.next;
                    pt1 = pt1.next;             //place one next to the other
                    pt2 = pt2.next;
                }
                else {
                    ptNew.next = new Cube(pt2.cubeVolume());        //else made new cube in new list and place pt2 head
                    pt2 = pt2.next;             //loop  
                    ptNew = ptNew.next;             //loop
                }

            }
        }
        return tempHead.next;           //return new merged list
    }

}   //End of Class

My Cube Class: (all correct just for reference)

public class Cube {

    double length;
    double width;
    double height;
    Cube next = null;

    public Cube() { //Default Constructor
    }
    public Cube(double volume) {
        volume = this.cubeVolume();
    }

    public Cube(Double length, double width, double height) {
        this.length = length;
        this.width = width;
        this.height = height;
        this.next = next;
    }

    public String toString() {      //Print into a String
        return "CUBE Volume: (" + cubeVolume() + ") ----- CUBE Length: ("+ this.length +") ----- CUBE Width (" + this.width + ") ----- CUBE Height (" + this.height + ")";
    }
    //Set length
    public void setLength(double length) {
        this.length = length;
    }

    //Get length
    public double getLength() {
        return this.length;
    }

    //Set Width
    public void setWidth(double width) {
        this.width = width; 
    }

    //Get Width
    public double getWidth() {
        return this.width;
    }

    //Set Height
    public void setheight(double height) {
        this.height = height;
    }

    //Set Height
    public double setHeight() {
        return this.height;
    }

    //Set Next
    public void setNext(Cube next) {
        this.next = next;
    }

    //Get Next
    public Cube getNext() {
        return this.next;
    }

    public double cubeVolume () {
        double volume = (length*width*height);
        //System.out.println("TEST: " + volume);
        return volume;
    }

}       //End of Class

OUTPUT

-----MY LINKED LIST-----
CUBE Volume: (14550.645379921463) ----- CUBE Length: (19.526751823368887) ----- CUBE Width (54.77537177724803) ----- CUBE Height (13.604009167732666) 
CUBE Volume: (1631.5144309742377) ----- CUBE Length: (40.72878317573845) ----- CUBE Width (22.07526604887876) ----- CUBE Height (1.8146109949933575) 
CUBE Volume: (17837.576670179817) ----- CUBE Length: (4.606784797762423) ----- CUBE Width (78.5447210731351) ----- CUBE Height (49.29704940251802) 
CUBE Volume: (113668.01972101796) ----- CUBE Length: (24.366242383656253) ----- CUBE Width (54.33809524521938) ----- CUBE Height (85.85099307890556) 
CUBE Volume: (432771.5800393206) ----- CUBE Length: (83.95704403819472) ----- CUBE Width (56.0616051224998) ----- CUBE Height (91.94668276748753) 

-----NEW LINKED LIST AFTER MERGE SORT METHOD IS IMPLEMENTED (Asending Order by Volume)-----
CUBE Volume: (432771.5800393206) ----- CUBE Length: (83.95704403819472) ----- CUBE Width (56.0616051224998) ----- CUBE Height (91.94668276748753) 

Objects in list 1

Time Record: 
Time elapsed for Merge sort on a Linked List: 11831 Nanos

Answer 1

The problem is in your main method, not your sorting methods. This block

        headCubeLL.next = null;                             //Sets end of list to null
        if(headCubeLL.next == null) {                       //creates new cube until desired number is reached in the for loop
            Cube curr = new Cube(headCubeLL.length, headCubeLL.width, headCubeLL.height);
            headCubeLL = curr;
        }
        headCubeLL.next = null;   

will always go into the if section. You're not setting the next field of curr at all. So when you assign curr to headCubeLL , you lose the previous value of headCubeLL . This means that there will only ever be one object in your list. You are throwing away the object each time you create a new one.

You need to

• remove the final headCubeLL.next = null;
• add a new line curr.setNext(headCubeLL); after the object curr is created.

That way, the previous headCubeLL will be remembered, as the next element of the new object that you just created.