Sorting Array of objects using bubble sort algorithm

[Jordi Sedo]

Hello to everybody,

I'm working on my project about managing Authors and OnlineCourses, this time I'm trying to sort alphabetically the list of authors. Here's my code for the sorting method and for the listing one:

    private static void sortStringBubble(Author b[]) {
    	int count;
    	boolean sortFlag = true;
    	Author temp;
      	while(sortFlag) {
    		sortFlag = false;
    		for (count = 0; count < b.length - 1; count ++) {
    			if (b[count].getLastName().compareToIgnoreCase(b[count+1].getLastName()) > 0) {
    				temp = b[count]; 
    				b[count] = b[count+1];
    				b[count+1] = temp;
    				sortFlag = true;
    			}
    			   			
    		}
    	}
    }
    
    /**
     * Show Authors by alphabetical order.
     */
     public void getAuthors(){
     	System.out.println(getTitle());
    	//Should show a list of the authors sorted by lastname but if a Online Course don't have any authors added
    	//will show No added authors for this OnlineCourse
    	if(nbrAuthors==0) {
    		 System.out.println("No added authors for this OnlineCourse");
    	 }else {
    	    sortStringBubble(authors);
    		 for(int cont = 0; cont<nbrAuthors; cont++) {
    			 System.out.println(authors[cont]);
    		 }
    	 }
     }

When I execute my code I get this output:

--- Showing info ---
Object Oriented Programming. Java practices
Exception in thread "main" Author [firstName=Swarnalatha, lastName=Ashok]
Author [firstName=Derek, lastName=Kiong]
Author [firstName=Josh, lastName=MacQueen]
Author [firstName=Danny, lastName=Poo]
UML design
java.lang.NullPointerException
	at exercise2.OnlineCourse.sortStringBubble(OnlineCourse.java:81)
	at exercise2.OnlineCourse.getAuthors(OnlineCourse.java:102)
	at exercise2.Main.main(Main.java:32)

By the exception and the return of the program it seems that the Bubble Sort is working but I'm probably doing something wrong with indices. Can someone lend me a hand?

Thank you

[Piet Souris]

hi Jordi,

I have not checked your code yet, but could there be some nulls in your Author[] array?

[Jordi Sedo]

Hello Piet,
That’s correct, it’s not mandatory to fill the array

[Jordi Sedo]

Hey again,
I think I solved the problem, I changed the method from static to normal, and I used the variable I was using to counting the number of authors for each OnlineCourse, now I get my output as expected:

    private void sortStringBubble(Author b[]) {
      	boolean sortFlag = true;
      	Author temp;
      	while(sortFlag) {
    		sortFlag = false;
    		for (int count = 0; count < nbrAuthors-1; count ++) {
    			if (b[count].getLastName().compareToIgnoreCase(b[count+1].getLastName()) > 0) {
    				temp = b[count]; 
    				b[count] = b[count+1];
    				b[count+1] = temp;
    				sortFlag = true;
    			}
    		}
    	}
    }

Output:

--- Showing info ---
Object Oriented Programming. Java practices
Author [firstName=Swarnalatha, lastName=Ashok]
Author [firstName=Derek, lastName=Kiong]
Author [firstName=Josh, lastName=MacQueen]
Author [firstName=Danny, lastName=Poo]
UML design
Author [firstName=Danny, lastName=Poo]
Introduction to Programming
Author [firstName=John, lastName=Crisp]
Digital Systems Theory
Author [firstName=John, lastName=Crisp]
Graph Theory
No added authors for this OnlineCourse

[Piet Souris]

Well done!

But if you are working with an array that may or may not be fully filled, why not use a List (an ArrayList or so)? That would be much simpler.

[Jordi Sedo]

For this project I was forced to use a simple array for authors.

[Campbell Ritchie]

I am not convinced you have found the best solution. I think those methods should all be static because they need no information other than their parameters, and they emit no information other than their returned values.
I think yoiu should have a utility class with a method that swaps two elements in an array.

public static <T> void swapTwoElements(T[] array, int i, int j)
{
   // Details not providedJJJ
}

I also think yoiu should have a bubbleSort method in the same utility class:-

public static <T extends Comparable<? super T>> void bubbleSort(T[] array)
{
   // Details not provided
}

public static <T> void bubbeSort(T[] array, Comparator<T> comp)
{
   // Details not provided
}

Use the sort method to do the sorting, and pass on swapping to the swap method.

Read about <T> here, and about Comparable and Comparator here, both in the Java™ Tutorials. There are several ways you can create a Comparator<Author> including alphabetical order of name. I think the tutorials link will explain everything you need to know.

[Piet Souris]

This is an assignment, concerning possibly partly filled arrays. This is not common, so putting it in a utility class seems not obvious. But why isn't the variable 'nbrAuthors' a parameter in this case?

[Jordi Sedo]

Sorry Piet, the nbrAuthors it’s a variable I have defined in the OnlineCourse, and initialized it at the constructor. So I had to stop using the static modifier for my sorting method so I could use nbrAuthors as the limit of my for

[Jordi Sedo]

Campbell Ritchie wrote:I am not convinced you have found the best solution. . . . .
Read about <T> here, and about Comparable and Comparator here, both in the Java™ Tutorials. There are several ways you can create a Comparator<Author> including alphabetical order of name. I think the tutorials link will explain everything you need to know.

I’ll give a look to these tutorials today! Thank you!

[Campbell Ritchie]

Piet Souris wrote:This is an assignment, concerning possibly partly filled arrays. This is not common . . .

That means there are going to be nulls in the array somewhere, and that is impossible to sort using conventional methods as taught atColleges.

putting it in a utility class seems not obvious. But why isn't the variable 'nbrAuthors' a parameter in this case?

You can still use utility methods to sort arrays containing nulls. You can't use Comparable obects, however, because you cannot compare null references like that. You can however create a Comparator<Author> that copes well with nulls. I can't remember whether the tutorials link I gave tells you anything about nulls, nor whether the static methods in Comparable<T> that cope with nulls are permissible, but you can write a Comparator like this:-

Comparator<Author> byNameNullLast =
   new Comparator<>()
   {
      @Override
      public int compare(Author a1, Author a2)
      {
         return a1 == null ? -1 :
                a2 == null ?  1 :
                             somethingElse;
      }
   };

I am presuming that OP doesn't know how to turn that anonymous class into a λ. If both  arguments are null, that Comparator will cause a redundant swap of the two nulls. You can sort that out by inserting a1 == a2 ? 0 : after return.

[Piet Souris]

Jordi Sedo wrote:Sorry Piet, the nbrAuthors it’s a variable I have defined in the OnlineCourse, and initialized it at the constructor. So I had to stop using the static modifier for my sorting method so I could use nbrAuthors as the limit of my for

I understand, but the Author array is a parameter to the method ('private void sortStringBubble(Author b[]) {...'), so can be any Author array (that is why Campbell said this method could be static), and in the body you are using a variable that is specific to a certain array. That is incorrect. So, either make this nbrAuthors a parameter too, or in the body of the method, check from what index the nulls start.