Converting ArrayList to 2D Array - returning ArrayOutOfBounds

Question

I need to create a List of students and convert them into a Seating Chart (2d array of specific length and width). Here is my code:

public class SeatingChart {
  Student seats[][];
  public SeatingChart(List <Student> studentList, int rows, int cols) {
    seats = new Student[rows][cols];
    for (int i = 0; i < cols; i++) { //Assigns Students in Column by Row order
        for (int j = 0; j < rows; j++) {
            if (i*rows+j > studentList.size() - 1) {
                seats[i][j] = null; //Occupies remaining `seats` with null
            }
            seats[i][j] = studentList.get(i*rows+j); //Finds an error with this line
        }
    }
}

public class Tester {
  public static void main (String[] args) {
    List <Student> classroom = new ArrayList<Student>();
    classroom.add(new Student("Anna", 3));
    SeatingChart sc = new SeatingChart(classroom, 2, 2); //Finds an error with this line
  }
}

public class Student {
  //assigns Name and number of absences for each student
}

This returns:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 1, Size: 1
    at java.util.ArrayList.rangeCheck(ArrayList.java:657)
    at java.util.ArrayList.get(ArrayList.java:433)
    at stud2.SeatingChart.<init>(SeatingChart.java:14)
    at stud2.SeatingChartTester.main(SeatingChartTester.java:12)

However, the errors which are part of the ArrayList class (rangeCheck and get) are gone once I add 2 more students to classroom. Why is the error resolved, and why are the last 2 errors still present? I have double-checked the logic behind the code and replaced the 2D array with a 1D array of the same size as the ArrayList, but I'm getting the same error.

@maio290 I feel that I checked the condition where there is an out of bounds on the ArrayList with the if statement in SeatingChart. I figured out that all you need is an else statement. However, it is now causing a nullPointerException, which I figured out the cause of. Thanks! — Yaasd Palala, Mar 14 '20 at 04:36

score 1 · Answer 1 · edited Mar 14 '20 at 16:40

Here I can see the problem

  List <Student> classroom = new ArrayList<Student>();
classroom.add(new Student("Anna", 3)); //so size of class room is 1

Now you're doing this

  SeatingChart sc = new SeatingChart(classroom, 2, 2); // which says class has two rows and two colums means 4 students which is not true because your list has only one

Now notice

 for (int i = 0; i < cols; i++) { 
    for (int j = 0; j < rows; j++) { // the first iteration in this loop will go fine
        if (i*rows+j > studentList.size() - 1) {
            seats[i][j] = null;
        }
        seats[i][j] = studentList.get(i*rows+j);
 /* but when j is one and i is 0 this expression resolves to i*rows + j == 0*2 + 1 = 1
   means you're saying studentList.get(1) , but list has only one student which is at 
   index 0 , that  
   is why you're getting java.lang.IndexOutOfBoundsException: Index: 1, Size: 1 cuz 
   index 
   is 1 in the get(1) and size is also 1 means noting at index 1.*/
    }
}

Converting ArrayList to 2D Array - returning ArrayOutOfBounds

1 Answers1