Java Code Sorting List of List of Integers

Question

I am using comparator to sort my list, but I got this error. Here is my code.

// The comparators
Comparator<List<Integer>> comparator_rows = new Comparator<List<Integer>>() {

    @Override
    public int compare(List<Integer> o1, List<Integer> o2) {
        if(o1.get(0) != o2.get(0))
            return Integer.compare(o1.get(0), o2.get(0));
        else if(o1.get(1) != o2.get(1))
            return Integer.compare(o1.get(1), o2.get(1));
        else if(o1.get(2) != o2.get(2))
            return Integer.compare(o1.get(2), o2.get(2));
        else if(o1.get(3) != o2.get(3))
            return Integer.compare(o1.get(3), o2.get(3));
        else if(o1.get(4) != o2.get(4))
            return Integer.compare(o1.get(4), o2.get(4));
        else if(o1.get(5) != o2.get(5))
            return Integer.compare(o1.get(5), o2.get(5));
        else if(o1.get(6) != o2.get(6))
            return Integer.compare(o1.get(6), o2.get(6));
        else 
            return Integer.compare(o1.get(7), o2.get(7));
    }

};

Collections.sort(list, comparator_rows);

// test print
for(int i=0; i<100; i++){
    Iterator<Integer> myListIterator = list.get(i).iterator();
    System.out.print(i + " ");
    while (myListIterator.hasNext()) {
        Integer val = myListIterator.next();     
        System.out.print(val + " ");
    }
    System.out.println();
}

And I got this results:

1 3000 299 1818 3000 3000 3000 1969 15426
1 3000 1499 1808 3000 3000 1174 1995 18813
1 3000 155 1927 3000 786 1107 783 26753
1 3000 1999 681 3000 355 1314 3000 28976

Does anyone know why I got not in order result starting dimension number 2? Thank you.

Your dealing with a list of lists and not with a list of integers. — Samuel Renold, Sep 05 '16 at 10:33

score 2 · Accepted Answer · edited May 23 '17 at 11:51

2

You cannot simply compare Integers with == and !=. Use Equals instead.

if(!o1.get(0).equals(o2.get(0)))
   return Integer.compare(o1.get(0), o2.get(0));

Be carefull that this might lead to NullPointerExceptions if an Element of the List is null.

See also: Java: Integer equals vs. ==

edited May 23 '17 at 11:51

Community

1
1

answered Sep 05 '16 at 10:30

OH GOD SPIDERS

3,091
2
13
16

score 0 · Answer 2 · answered Sep 05 '16 at 10:19

0

You don't need to write a comparator for Integer, because Integer implements Comparable interface.

It's enough to sort:

Collections.sort(list);

but if you still need to create a custom comparator with some custom logic you can try this: (ofc writting your own logic inside the comparator)

Collections.sort(list, new Comparator<Integer>() {

    public int compare(Integer o1, Integer o2) {
        return Integer.compare(o1, o2);
    }
});

answered Sep 05 '16 at 10:19

Daniel Boncioaga

336
1
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I cannot directly use Collections.sort(list) because i have list of list of integers, i want to sort it for each row based on attribute 1, 2, 3, ... – hallz13 Sep 05 '16 at 10:29

Java Code Sorting List of List of Integers

2 Answers2