How to make a priority int list that regards the insertion order in case of duplicates in Java?

Question

I want to make a priority list entering int values. I know that the list would normally have an ascending natural order (please correct me if I got that wrong). But what I as a newbie want to do is to sort same values.

Say, I entered the values 3, 6, 4, 1, 2, 4, 7, 4 and I'm expecting a list of ascending int values. but I want in the above list the second 4 to have lower priority, hence I want it coming right after the first 4 value and the third 4 value after the second etc.

Are there any classes /methods in Java for that? Where should I look for tips?

If you're storing just integers there's not really any priority; they're both 4s. The way to do it is allow duplicates; simple as that. — ChiefTwoPencils, Dec 20 '15 at 04:05
Yes, any implementation of SortedSet, like [TreeSet](https://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html), can do it. But, obviously, the value 4 is identical to all other values of 4, so you'll need to create your own custom class that wraps the number along with whatever other information determines their order. You will also want to have your class implement [Comparable](https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html) so it can define that ordering. — VGR, Dec 20 '15 at 04:13
thx VGR! that sounds like the answer i was looking for. if you post it as an answer i'll accept :-) — melar, Dec 20 '15 at 04:23

score 2 · Accepted Answer · edited May 23 '17 at 12:02

1 if you want to distinguish between 4 and 4, you have to add something => make a class

2 not to reinvent the wheel, use Collections (which accept duplicates) and sort

3 then you have to implement some comparison (implements Comparable)

see this: How to sort an ArrayList in Java

it gives this:

// It is like Integer + !

public class IntegerPlus implements Comparable<IntegerPlus>
{
int value=0;
String id="";

public IntegerPlus(int _val) {value=_val;}
public IntegerPlus(int _val, String _id) {value=_val; id=_id;}

@Override
public String toString() {return value+"("+id+")";}

@Override
public int compareTo(IntegerPlus _other)
    {
    if (value>_other.value) return 1;
    if (value<_other.value) return -1;
    return 0;
    }

}

4 good luck: you can use a regular sort: it preserves initial order on duplicates:

see this post: Does Collections.sort keep order on equal elements?

it gives:

IntegerPlus ip1=new IntegerPlus(4,"first");
System.out.println(ip1);

List<IntegerPlus> lipl=new ArrayList<IntegerPlus>();
lipl.add(ip1);
lipl.add(new IntegerPlus(2,"b"));
lipl.add(new IntegerPlus(4,"second"));
lipl.add(new IntegerPlus(1));
lipl.add(new IntegerPlus(3));
lipl.add(new IntegerPlus(5));
lipl.add(new IntegerPlus(6));
lipl.add(new IntegerPlus(4,"third"));
lipl.add(new IntegerPlus(2,"c"));


System.out.println("BEFORE SORT:"+lipl);

=> BEFORE SORT:[4(first), 2(b), 4(second), 1(), 3(), 5(), 6(), 4(third), 2(c)]

Collections.sort(lipl);

System.out.println("AFTER SORT:"+lipl);

=> AFTER SORT:[1(), 2(b), 2(c), 3(), 4(first), 4(second), 4(third), 5(), 6()]

hope it helps !

oh thats great. from what i understood in your answer i dont neccessarily need to use a prioritylist since the collections class is also doing the work? is that correct? — melar, Dec 20 '15 at 11:49
your are right. standard arraylist does the job . I think your are thinking about priority queue. but you dont use priority. — guillaume girod-vitouchkina, Dec 20 '15 at 12:19
yea sorry, i was referring to the priority queue. ok but now i got my answer. neat. thx a lot! — melar, Dec 20 '15 at 12:35

How to make a priority int list that regards the insertion order in case of duplicates in Java?

1 Answers1