java using list remove it is not the same as the other one

Question

so i'm super stuck. I have a list of students with fname lname sNum etc. i'm trying to use my remove function and it doesn't work. i can't figure out where my problem is.

public void removeStudent(long sNumber) {
    //create student object
    Student student = new Student();
    //loop through the students 
    for (int i = 0; i < this.students.size(); i++) {
        //display students
        System.out.println(this.students.get(i));
        //condition if sNumber == sNumber 
        if(student.getsNumber() == sNumber){
            //remove the student from the list
            this.students.remove(student);
            System.out.println(this.students.get(i));
        }
        System.out.println("skipped the if statement");
    }
    /*for(Student student : this.students){
        if(student.getsNumber() == sNumber){
            this.students.remove(student);
        }
    }*/
}

this is where i call the method

case 3:
            // delete a student
            System.out.print("What is the Students sNumber: s");
            long sNum = input.nextLong();
            Student chkSNum = new Student();
            registry.getStudentBySNumber(sNum);
            chkSNum.setsNumber(sNum);
            if (registry.getStudentBySNumber(sNum) == chkSNum) {
                if (chkSNum.getsNumber() == sNum) {
                    registry.removeStudent(sNum);
                    System.out.println(chkSNum);
                }
            } else {
                System.out.println("Sorry no matches");
                // System.out.println(sNum);
                System.out.println(registry.getStudentBySNumber(sNum));
                System.out.println(chkSNum);
            }
            break;

my add student method works just fine.

    public void addStudent(Student student) {
    this.lastSNumber++;
    student.setsNumber(this.lastSNumber);
    this.students.add(student);
}

Possible duplicate of [Calling remove in foreach loop in Java](http://stackoverflow.com/questions/1196586/calling-remove-in-foreach-loop-in-java) — rupesh_padhye, Jan 29 '16 at 06:36
@rupesh_padhye No `foreach` here, so I don't think it is a duplicate. But to all those answering with a `foreach`... — John3136, Jan 29 '16 at 06:38
@John3136 Yes though it not direct duplicate but I can see what person trying to do removing while iterating.. — rupesh_padhye, Jan 29 '16 at 06:42
@rupesh_padhye All the people answering including a `foreach` should read your link ;-) — John3136, Jan 29 '16 at 06:43
it always skips the condition if(student.getsNumber() == sNumber){ //remove the student from the list this.students.remove(student); — tyler, Jan 29 '16 at 06:45

shmosel · Answer 1 · 2016-01-29T06:56:35.923

3

if(student.getsNumber() == sNumber)

is comparing your number to a newly constructed, blank Student object. If you want to compare it to the ones you're iterating over, you'll need something like this:

if(this.students.get(i).getsNumber() == sNumber) {
    this.students.remove(i);
}

edited Jan 29 '16 at 06:56

answered Jan 29 '16 at 06:37

shmosel

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@Optional, it won't throw a CoMod exception because there's no proper iteration. It will skip an index if the loop continues, but there should probably be a `break;` in there anyhow. – shmosel Jan 29 '16 at 07:06

osanger · Answer 2 · 2016-01-29T06:59:41.057

0

You are actually removing a new studend() object. Loop the list like this:

Student to_remove = null:
for(student s: this.students){
    if(s.getsNumber() == sNumber){
    to_remove = s;
    } 
    if(s!=null) break;
}
if(s!=null)this.students. remove(s);

This will not cause an Modification-Exception

edited Jan 29 '16 at 06:59

answered Jan 29 '16 at 06:39

osanger

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this should replace my for loop? i'm so confused on why i'm having this problem. . – tyler Jan 29 '16 at 08:41
Yes you get this exceptin because you change the list while running through it. – osanger Jan 29 '16 at 08:58
_This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible._ [source](https://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html) – osanger Jan 29 '16 at 09:01

score 0 · Answer 3 · answered Jan 29 '16 at 06:50

0

You should use the iterator to remove in the list. Something like

 Iterator<Student> it = this.students.iterator();
 while (it.hasNext()) {
 Student currentStudent = it.next();
 if(currentStudent.getsNumber() == sNumber)
  {
    it.remove();
   }
 }

answered Jan 29 '16 at 06:50

Optional

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tyler · Answer 4 · 2016-02-01T03:57:39.163

so i figured out what was wrong. my logic was dorky.

public void removeStudent(long sNumber) {
    //create student object
    //Student student = new Student();
    Student student = this.getStudentBySNumber(sNumber);
        //condition if sNumber == sNumber 
        if(student == this.getStudentBySNumber(sNumber)){
            System.out.println("removing the student from the list");
            this.students.remove(student);
            displayStudents();
        }
}

 case 3:
            // delete a student
            System.out.print("What is the Students sNumber: s");
            long sNumInput = input.nextLong();
            Student chkSNum2 = new Student();
            registry.getStudentBySNumber(sNumInput);
            chkSNum2.setsNumber(sNumInput);
            if (registry.getStudentBySNumber(sNumInput) != chkSNum2) {
                if (chkSNum2.getsNumber() == sNumInput) {
                    System.out.println("Student deleted...");
                    registry.removeStudent(sNumInput);
                }
            } else {
                System.out.println("Sorry no matches");
                System.out.println(registry.getStudentBySNumber(sNumInput));
                System.out.println(chkSNum2);
            }
            break;

score -2 · Answer 5 · answered Jan 29 '16 at 06:40

-2

It's obviously something wrong in removeStudent. First you are creating a new Student and then removing using that instance.

What you should do is this:

public void removeStudent(long sNumber) {
  for (Student s : this.students) {
    if (s.getNumber() == sNumber) { 
      this.students.remove(s);
    }
  }
}

You can ofcourse also use the for loop you have in your code. It's the same thing. Just remember to remove the right instance.

Hope that helps.

answered Jan 29 '16 at 06:40

Sten Roger Sandvik

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java.util.ConcurrentModificationException :) – Optional Jan 29 '16 at 06:46
i had it that way and had exceptions thrown – tyler Jan 29 '16 at 06:47
You will need to have a concurrent implementation of the list in your example. And it does not help íf your code is commented out :.-) – Sten Roger Sandvik Jan 29 '16 at 06:58

java using list remove it is not the same as the other one

5 Answers5