Move part of array to the front in Java

Question

I have an array similar to that:

[null, null, 3, 4, null]

I want to change it to array similar to that:

[3, 4, null, null, null]

I know how to do it manually, but it looks ugly. Is there any built-in(with minimum custom code) way to do that? (maybe something with System.arraycopy)

I finished with something like this(thanks @Braj for idea):

@Test
public void test() {
  Integer[] k = new Integer[]{null, null, 3, 2, 1};
  k[1] = null;
  Arrays.sort(k, new Comparator<Object>() {
    public int compare(Object o1, Object o2) {
      System.out.println(o1);
      if (o1 == null) return 1;
      else if(o2 == null) return -1;
      else return 0;
    }
  });
  assertArrayEquals(
    new Integer[]{3, 2, 1, null, null},
    k
  );
}

If somebody interested there is a bigger problem which tried to solve with this code

Do you care about the ordering of non-null elements? Should they be sorted in any specific way? Should they retain their initial order? — toniedzwiedz, Jul 05 '14 at 11:41
1) It looks ugly --> put that ugly code in a method, and you no longer have to see that ugly code (abstraction). As for using the API, there is a `java.util.Arrays`. If it is not there, you will waste more time searching for a solution than solving it yourself — SJuan76, Jul 05 '14 at 11:43
You can take a look at Guava and its [`nullsLast`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html) ordering. I'm not sure you can use it without sorting the collection in any way at the same time so it may not be that concise a solution. — toniedzwiedz, Jul 05 '14 at 11:48
If somebody interested there is a [bigger problem](http://codereview.stackexchange.com/questions/56193/queue-over-resizable-array-implementation) which I tried to solve with this code — kharandziuk, Jul 05 '14 at 15:05

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

6

Use Arrays.sort(T[],Comparator) and pass Comparator to sort based on your own logic.

Read more here and find sample code as well

Here is the sample code that sorts the array as well while pushing null values in the end.

Note: (change it as per your need, not fully tested)

    final Integer[] data = new Integer[] { null, null, 3, 4, null };
    Arrays.sort(data, new Comparator<Integer>() {
        public int compare(Integer o1, Integer o2) {
            if(o1 != null && o2!=null){
                return o1.compareTo(o2);
            }else if(o2 != null){
                return o2;
            }else{                  
                return Integer.MIN_VALUE; // null values in the end
            }
        }
    });

    System.out.println(Arrays.toString(data));

output:

   [3, 4, null, null, null]

EDIT

If you are interested only in moving the null values in the end then find sample code in the below post that is answered by @Dmitry Ginzburg

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 05 '14 at 11:43

Braj

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on `[null, null, 4, 3, null]` this code would return `[3, 4, null, null, null]`, which is incorrect. – Dmitry Ginzburg Jul 05 '14 at 12:20
@DmitryGinzburg what is expected? – Braj Jul 05 '14 at 12:21
@Braj `[4, 3, null, null, null]` – Dmitry Ginzburg Jul 05 '14 at 12:21
@Braj I adopted your solution a little bit. Can you check it(now it's a part of question)? – kharandziuk Jul 05 '14 at 12:23
@kharandziuk use `Comparator` instead of `Comparator – Braj Jul 05 '14 at 12:27
@kharandziuk I tested your code and it result into incorrect output. input `[null, null, 3, 4, 2, null]` and output `[3, 4, 2, null, null, null]` – Braj Jul 05 '14 at 12:30
@Braj, What is the reason to do that? I chose Integer type just for example – kharandziuk Jul 05 '14 at 12:31
@kharandziuk I am using`Integer#compareTo()` method inside `Comparator` and there is no meaning of comparing Dog with Cat or String with Integer. That's why make it more generic. – Braj Jul 05 '14 at 12:32
@DmitryGinzburg What OP says in the post at line 3 and 4. *I want to change it to array similar to that:*`[3, 4, null, null, null]` – Braj Jul 05 '14 at 12:34
@Braj about >I tested your code. It's a correct behavior) – kharandziuk Jul 05 '14 at 12:34
@kharandziuk Time to leave. Change the code as per your need. – Braj Jul 05 '14 at 12:35
@Braj, I've changed the input array a little bit, so you should look at it more careful. – Dmitry Ginzburg Jul 05 '14 at 12:44

score 2 · Answer 2 · answered Jul 05 '14 at 12:24

2

The sort algo from Java standard library is stable, so, if we're making the elements, which are not null the same (in the context of ordering), it would keep the order:

Arrays.sort(data, new Comparator<Integer>() {
    public int compare (Integer x1, Integer x2) {
        if (x1 == null && x2 != null)
            return 1;
        else if (x1 != null && x2 == null)
            return -1;
        else
            return 0;
    }
});

answered Jul 05 '14 at 12:24

Dmitry Ginzburg

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Incorrect result. I tested your code for input `[null, null, 3, 4, 2, null]` and output `[3, 4, 2, null, null, null]` – Braj Jul 05 '14 at 12:29
Your code is correct, but Braj provided an idea earlier. Thanks! – kharandziuk Jul 05 '14 at 12:38
@DmitryGinzburg Sorry I have taken it in wrong way. I was thinking from sorting perspective as well. As per sorting logic it should be []2, 3, 4, null, null, null] but I was wrong. I think OP just want to move the items and not interested in sorting. – Braj Jul 05 '14 at 19:31
@Braj, yep, it's really what OP requesting. – Dmitry Ginzburg Jul 06 '14 at 08:12

Move part of array to the front in Java

2 Answers2

EDIT