How to sort an array of objects containing null elements?

Question

In my program an array fClasses of fixed length [7] of objects is created, each object is a class FClass that contains 3 Strings , an int , and an int[] . These values are read from a .txt file and added to a specific index of the array based on the value of the int . There are less entries in the .txt file then there are indices in the array so the array ends up looking something like this:

fClasses[0] { str1, str2, str3, int1, int [] {1,2,3,4,5}}
fClasses[1] { str1, str2, str3, int1, int [] {1,2,3,4,5}}
fClasses[2] { str1, str2, str3, int1, int [] {1,2,3,4,5}}
fClasses[3] null
fClasses[4] null
fClasses[5] { str1, str2, str3, int1, int [] {1,2,3,4,5}}
fClasses[6] { str1, str2, str3, int1, int [] {1,2,3,4,5}}

Later in the program I need to sort the array based on the average of the ints in the int[] . I have a working method to return this but when I try to sort the array using compareTo and Arrays.sort I get a long list of errors starting with these:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at java.util.ComparableTimSort.countRunAndMakeAscending(Unknown Source)
    at java.util.ComparableTimSort.sort(Unknown Source)
    at java.util.Arrays.sort(Unknown Source)
    at FProg.sortClasses(FProg.java:228)

My compareTo method looks like this and it's located in a class that implements Comparable :

public int compareTo(FClass other) 
{
    if (other == null || this.avg == other.avg)
    {
        return 0;
    }
    else if (this.avg < other.avg)
    {
        return -1;
    }
    else
    {
        return 1;
    }

}

And I'm trying to call this method to do the sorting:

public void sortClasses()
{
    Arrays.sort(fClasses, 0, MAX_CLASSES);
}

I have tested it with a .txt file that contains enough entries to fill the array and the sort works correctly in that case, so I believe the problem I'm having is that my sort method can't sort an array with null elements in it. Is there any way this can be achieved?

Answer 1

Using Java 8, you can easily build the comparator you need:

Arrays.sort(fClasses, Comparator.nullsFirst(Comparator.naturalOrder()));

Use nullsLast instead if that's what you want, of course.

Answer 2

You need your own Comparator implementation and check for nulls and return 0

 Arrays.sort(fClasses, new Comparator<FClass>() {
    @Override
    public int compare(FClass o1, FClass o2) {
        if (o1 == null && o2 == null) {
            return 0;
        }
        if (o1 == null) {
            return 1;
        }
        if (o2 == null) {
            return -1;
        }
        return o1.compareTo(o2);
    }});

Answer 3

You have to create a Comparator<FClass> , rather than use a Comparable<FClass> .

public class FClassComparator implements Comparator<FClass> 
{
    public int compare(FClass left, FClass right) {
        // Swap -1 and 1 here if you want nulls to move to the front.
        if (left == null) return right == null ? 0 : 1;
        if (right == null) return -1;
        // you are now guaranteed that neither left nor right are null.

        // I'm assuming avg is int. There is also Double.compare if they aren't.
        return Integer.compare(left.avg, right.avg); 
    }
}

Then call sort via:

Arrays.sort(fClassArray, new FClassComparator());

Answer 4

使用Apache Commons Collections 4，您可以使用ComparatorUtils来做到这一点：

Collections.sort(arr, ComparatorUtils.nullLowComparator(ComparatorUtils.NATURAL_COMPARATOR));

Answer 5

By importing the org.apache.commons.collections.comparators package of the Apache 2.1.1 Release library, I'm able to sort a list, such as an ArrayList<String> , using the NullComparator as the second argument of the Collections.sort() method, as follows:

ArrayList<String> list = new ArrayList<String>();
list.add("foo");
list.add("bar");
list.add("baz");
list.add(null);

// Sort the list
Collections.sort(list, new NullComparator(true));

System.out.println(list);
// outputs:
// [bar, baz, foo, null]

The thing I like about this approach is that the NullComparator has an overload constructor which allows you to specify whether you want null to be considered a high value or a low value, which seems pretty intuitive to me.

NullComparator(boolean nullsAreHigh)

Hope this helps someone!