Search linkedlist for object variable

Question

I need to search a LinkedList and compare the objects in the list so I don't add duplicates. I can't figure out how to pull a node so I can compare it to the StudentInfo object I'm trying to add. This is my attempt to use an iterator:

private LinkedList<CourseInfo> classes = new LinkedList<>();

  public void addCourse(String cid, String name, String prof, String days, 
  String time, String room)
  {

     CourseInfo course = new CourseInfo(cid, name, prof, days, time, room);
     Iterator i = classes.iterator();
     while(i.hasNext())
     {
         if(i.equals(course))
         {
             System.out.println("Caught");
             break;
         }

     }

 }

I specifically need to compare the cid variables

Don't want duplicates: use HashSet. Find an object: list.contains(...). — markspace
– markspace, Commented Jun 18, 2018 at 23:26
Did you override equals() and hashcode() in your CourseInfo class? — Code-Apprentice
– Code-Apprentice, Commented Jun 18, 2018 at 23:38

StaticBeagle · Accepted Answer · 2018-06-18 23:50:37Z

2

Minor note, traversing a LinkedList to check if an element exists is expensive. You might want to consider HashSet. That being said, in your code you are comparing the iterator with the CourseInfo, you need to compare the element pointed by the iterator instead by using next(). Also, next() is required in order to advance the iterator (thanks to Code-Apprentice for the suggestion) otherwise you could end up in an infinite loop in i.hasNext().

private LinkedList<CourseInfo> classes = new LinkedList<>();

  public void addCourse(String cid, String name, String prof, String days, 
  String time, String room)
  {

     CourseInfo course = new CourseInfo(cid, name, prof, days, time, room);
     // Don't use raw types e.g. Iterator it
     // Specify the type of element you are iterating on
     // Iterator<T> it
     Iterator<CourseInfo> i = classes.iterator();
     while(i.hasNext())
     {
         CourseInfo cInfo = i.next(); // get the element pointed by the iterator
         if(cInfo.equals(course))
         {
             System.out.println("Caught");
             break;
         }
     }
 }

Or using enhanced for-loops:

for(CourseInfo ci : classes) {
    if(ci.equals(course)) {
        System.out.println("Caught");
        break;
     }
}

edited Jun 18, 2018 at 23:50

answered Jun 18, 2018 at 23:19

StaticBeagle

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10 Comments

I. McFee Over a year ago

it says incompatible types: Object cannot be converted to CourseInfo

David Conrad Over a year ago

tmp is probably the worst imaginable variable name.

Code-Apprentice Over a year ago

Note that the call to next() is required in order to advance the iterator. The original while loop would continue indefinitely without this.

Code-Apprentice Over a year ago

@I.McFee You should edit your question with an update so that you can format the code correctly. This answer assumes that you already overrode the equals() and hashcode() method correctly.

StaticBeagle Over a year ago

@I.McFee are you overriding equals & hashcode for CourseInfo?

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