Sort ArrayList of objects by field in custom order

Question

How can I achieve a custom sorting to the content of field name:

first element: P followed by numbers [1-9]{2} always on first
followed by : P followed by numbers 0[0-9]
followed by : S
followed by numbers [1-9]{2}

and then the rest in normal order i1.getName().compareToIgnoreCase(i2.getName())

private static Comparator<Item> itemComperator = new Comparator<Item>() {
    @Override
    public int compare(Item i1, Item i2) {
        if (i1.getName().matches("P[1-9]{2}") && i2.getName().matches("P0[0-9]"))
            return -1;
        else if (i1.getName().matches("S[1-9]{2}"))
            return -1;
        else
            return i1.getName().compareToIgnoreCase(i2.getName());
    }
};

@Test
public void sortItem() {
    Item item01 = new Item(1, "R59");
    Item item02 = new Item(2, "S48");
    Item item03 = new Item(3, "P01");
    Item item04 = new Item(4, "P25");
    Item item05 = new Item(5, "R99");

    List<Item> items = Arrays.asList(item01, item02, item03, item04, item05);
    System.out.println("before sorting");
    long seed = System.nanoTime();
    Collections.shuffle(items, new Random(seed));
    for (Item i : items) {
        System.out.println(i.getId() + " " + i.getName());
    }

    System.out.println("after sorting");
    Collections.sort(items, itemComperator);
    for (Item i : items) {
        System.out.println(i.getId() + " " + i.getName());
    }


}

public class Item {
    private int id;
    private String name;

    public Item(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

output expected:

after sorting
4 P25
3 P01
2 S48
1 R59
5 R99

Can you include the required out-put with the question .. ??? — Madushan Perera
– Madushan Perera, Commented Apr 8, 2016 at 8:39

Andy Turner · Accepted Answer · 2016-04-08 08:50:58Z

I think that I would first map each of the inputs to a "kind" number, which is based upon the list of criteria above. For example:

int kind(String input) {
  if (input.matches("P[1-9]{2}") {
    return 0;
  } else if (input.matches("P0[0-9]")) {
    return 1;
  } else if (input.matches("S.*")) {
    return 2;
  } else if (input.matches("[1-9]{2}")) {
    return 3;
  } else {
    return 4;
  }
}

This gives you a way to see if the two strings are of the same "kind"; if not, return the ordering based on the kind. If they are the same kind, just compare them using (case insensitive) lexicographic ordering (you don't specify, but I assume you want e.g. "P11" to come before "P22"):

public int compare(Item a, Item b) {
  String nameA = a.getName();
  String nameB = b.getName();

  int kindA = kind(nameA);
  int kindB = kind(nameB);
  if (kindA != kindB) {
    return Integer.compare(kindA, kindB);
  } else {
    return nameA.compareToIgnoreCase(nameB);
  }
}

Maybe input.matches("S.*") instead of input.matches("S*")?

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