Sorting two lists

Question

I am working in Java. I have two lists, let's call them A and B, which I want to sort. A is an Integer list, so I have no problem to do that. I simply use Collections.sort() to obtain a sorted list of integers. The problem comes with the list B. I want to make the same changes done before in A.. B is a list of objects, but there's no way to associate the changes in B with changes in A. I mean, there's no condition to create a comparator method.

Little example:

I have:

A -> {5,1,3,6,4}
B -> {a,b,c,d,e}

I want to sort A and apply the same changes to B to obtain:

A -> {1,3,4,5,6}
B -> {b,c,e,a,d}

Is there any way to do that using built-in Java functions? I prefer to avoid writing a sorting algorithm myself because of efficiency. Thank you!

Answer 1

I would start by creating a POJO to store A and B,

static class ABPojo implements Comparable<ABPojo> {
    public ABPojo(int a, String b) {
        this.a = a;
        this.b = b;
    }

    private int a;
    private String b;

    public int getA() {
        return a;
    }

    public String getB() {
        return b;
    }

    public int compareTo(ABPojo o) {
        if (o instanceof ABPojo) {
            ABPojo that = (ABPojo) o;
            return Integer.valueOf(a).compareTo(that.getA());
        }
        return 1;
    }
}

Then you can loop over the collection of ABPojo(s) after sorting to build your output with something like

public static void main(String[] args) {
    List<ABPojo> al = new ArrayList<ABPojo>();
    al.add(new ABPojo(5, "a"));
    al.add(new ABPojo(1, "b"));
    al.add(new ABPojo(3, "c"));
    al.add(new ABPojo(6, "d"));
    al.add(new ABPojo(4, "e"));
    Collections.sort(al);
    StringBuilder a = new StringBuilder();
    StringBuilder b = new StringBuilder();
    for (ABPojo pojo : al) {
        if (a.length() > 0) {
            a.append(",");
        } else {
            a.append("{");
        }
        if (b.length() > 0) {
            b.append(",");
        } else {
            b.append("{");
        }
        a.append(pojo.getA());
        b.append(pojo.getB());
    }
    a.append("}");
    b.append("}");
    System.out.println("A -> " + a.toString());
    System.out.println("B -> " + b.toString());
}

Output is the requested

A -> {1,3,4,5,6}
B -> {b,c,e,a,d}

Answer 2

A TreeMap will always iterate over the keys in the right order, so you can do it like this:

 Map<Integer, String> map = new TreeMap<Integer, String>();
 map.put(5, "a");
 map.put(1, "b");
 map.put(3, "c");
 map.put(6, "d");
 map.put(4, "e");
 System.out.println(map.keySet());
 System.out.println(map.values());

However if you really want to start and end with the same pair of List instances, I think you'd have to do something convoluted like this:

 List<Integer> numbers = new ArrayList<Integer>(Arrays.asList(5, 1, 3, 6, 4));
 List<String> letters = new ArrayList<String>(Arrays.asList("a", "b", "c", "d", "e"));
 Map<Integer, String> map = new HashMap<Integer, String>();
 for (int i = 0, n = numbers.size(); i < n; i++) {
     map.put(numbers.get(i), letters.get(i));
 }
 Collections.sort(numbers);
 letters.clear();
 for (int number : numbers) {
     letters.add(map.get(number));
 }
 System.out.println(numbers);
 System.out.println(letters);

Answer 3

Create a map with your elements in A as key and the elements in B as value resp. Then Collections.Sort() will automatically sort the A elements and its corresponding B elements.

Answer 4

Start with here:

How to find the permutation of a sort in Java

Then manually apply the permutation.

But it sounds like a design problem; consider one list of classes that implement Comparator<T> where the comparison function is just on the numbers.

Answer 5

I don't think there is a Java function that will do this.

However, you could use a map structure instead of a list stucture where the key to the data is your int, and the data is the unsortable list.