Sorting Array: Bubble sort

Question

public static void sortByNumber(Course[] list) {
    Course temp = new Course();

    boolean fixed = false;

    while(fixed == false) {
        fixed = true;
    for (int i = 0; i<list.length-1; i++) {

        if (list[i].getNum() > list[i+1].getNum()) {
            temp.setNum(list[i].getNum());
            temp.setDept(list[i].getDept());
            temp.setTitle(list[i].getTitle());

            list[i] = list[i+1];
            list[i+1] = temp;
            fixed = false;
        }
    }
    }}

This is a method for sorting courses offered by university.

For example, each course has its department (i.e. MATH), number (i.e. 263) and title (i.e. Ordinary Differential Equations for Engineers) - MATH 263 Ordinary Differential Equations for Engineers.

In my another class, I have created an object Course, which has its own accessors and mutators (i.e. getNum(), setNum(), getDept(), so on).

Given a long list of courses, I wanted to arrange them according to their course numbers, but the above method does not seem to work.

Can someone hint reason for this?

Hint: Look at your setters. You are not thinking in terms of object oriented programming. What was temp before you called its setters? What is it after? — Benjamin Bau
– Benjamin Bau, Commented Dec 2, 2015 at 1:52

Ramón Gil Moreno · Accepted Answer · 2015-12-02 01:53:27Z

3

The temp variable is a reference to a Course object.

Actually the array list is an array of references to Course objects.

You only need to change references and not copy the object's values into temp. Just do temp = list[i]; to keep a reference to the i-th element of the array.

answered Dec 2, 2015 at 1:53

Ramón Gil Moreno

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3 Comments

Corp. and Ltd. Over a year ago

I see that my redundancy was inefficient, but why does it produce incorrect result?

Ramón Gil Moreno Over a year ago

You were pointing the i-th reference to the next object (the i+1-th one, that's OK), but made that i+1-th reference always point the temp object instead of pointing to the i-th object.

Oss Over a year ago

You can initialize the temp inside your loop which will be correct but THIS will be reduntant and inefficient. Pointing your temp to i+1 is the best solution. Check out the bubble sort algorithm on RosettaCode and some other sorting alternatives to learn. rosettacode.org/wiki/Sorting_algorithms/Bubble_sort#Java

Oss · Accepted Answer · 2015-12-02 02:01:07Z

0

I would rather implement the Comparable interface.

class Course implements Comparable<Course>{
    @Override
    public int compareTo(Course other){
        return this.getNum().compareTo(other.gerNum());
    }
}

And then you can use Array.sort(Course[] list) to sort your courses array.

answered Dec 2, 2015 at 2:01

Oss

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