How can I get an element from linked list with given index in java?

Question

I'm learning about link-list in Java. and I'm want to write a method to give the value of node based on given index

I write a function, but it does not pass some of testcases, and I dont know why?. what is wrong with my logic?

//waypoint.java

public class Waypoint {
 int x ;
 int y ;
 public int getX()
 {
 return this.x;
 }
 public int getY()
 {
 return this.y;
 }
 public void setXY(int x, int y)
 {
 this.x = x;
 this.y = y;
 }

// TourElement.java

public class TourElement {
 private Waypoint points;
 private TourElement next;
 public void setWaypoint( Waypoint points)
 {
 this.points = points; 
 }
 public void setTourElement(TourElement next)
 {
 this.next = next;
 }
 Waypoint getWaypoint()
 {
 return this.points;
 }
 TourElement getNext()
 {
 return this.next;
 }
int getNoOfWaypoints()// return the number of waypoints in the list
{
 int count = 1;
 TourElement current = getNext();
 while(current.next != null)
 {
 count++;
 current = current.next;
 System.out.println(count);
 }
 return count;
} 

// here is method I'm strucking with:

Waypoint getWaypointAt(int index)
{
 int totalElement = getNoOfWaypoints();
 int count = 0;
 TourElement current = getNext();
 if(index < totalElement && index >= 0)
 {
 while (current.next != null)
 {
 if(count == index)
 {
 return getWaypoint();
 }
 count++;
 current = current.next;
 }
 }
 return null;
}

//the testcase: //case 1: pass

public void test0GetWaypointAt_First() {
 TourElement elem = createElementList(new int[][] {{0, 0}, {1, 1}, {2, 2}});
 Waypoint expected = createWaypoint(0, 0);
 assertArrayEquals(expected.toArray(), elem.getWaypointAt(0).toArray());
 }

//case 2 and case 3: failed

public void test0GetWaypointAt_Snd() {
 TourElement elem = createElementList(new int[][] {{0, 0}, {1, 1}, {2, 2}});
 Waypoint expected = createWaypoint(1, 1);
 assertArrayEquals(expected.toArray(), elem.getWaypointAt(1).toArray());
 }
 @Test
 public void test0GetWaypointAt_Last() {
 TourElement elem = createElementList(new int[][] {{0, 0}, {1, 1}, {2, 2}});
 Waypoint expected = createWaypoint(2, 2);
 assertArrayEquals(expected.toArray(), elem.getWaypointAt(2).toArray());
 }

I dont know the reason. Please help me. Thank you so much in advance

• bracco23: Please, read What is a debugger and how can it help me diagnose problems?

Answer 1

Structure you've build with list seems ok, about your method getWaypointAt I see at least two problems:

First problem

 TourElement current = getNext();

getNext() is already the next element in your list, so you're skipping the first element all the time. It should be

 TourElement current = this;

Second problem

 return getWaypoint();

It returns waypoint from the head all the time. Should be

 return current.getWaypoint();

Seems like your first test should fail as well. I don't see the way you build elements in createElementList, it could be a reason why it pass.

• Nguyen Ngoc Hai: thank you so much. I though getNext() in the first time will be the first element in my list. But I was wong. Again, thank u so much for your help!!!
• Dima Patserkovskyi: Happy to help :)