如何编写自定义比较器以使用 SimpleEntry 对树集进行排序？

Question

I'm currently working on an assignement which requires me to use a Treeset to sort pairs of value. I'm asked to use SimpleEntry. I'm storing them on a Treeset:

    TreeSet treeSet = new TreeSet(new PairComparator());
    SimpleEntry pair = new SimpleEntry(weight,source);
    treeSet.add(pair);

Here is my custom Comparator:

static class PairComparator implements Comparator<AbstractMap.SimpleEntry<Integer, Integer>> {

        @Override
        public int compare(AbstractMap.SimpleEntry<Integer,Integer> o1, AbstractMap.SimpleEntry<Integer,Integer> o2) {
            int key1 = o1.getKey() ;
            int key2 = o2.getKey();
            return key1 - key2;
        }
    }

I get an error I don't understand, which is:

class java.util.HashMap cannot be cast to class java.lang.Integer

Could you explain me what is happening? I'm not totally familiar with defining a custom comparator.

Thanks for your attention !

EDIT: I was adding a Hashmap as a key on my SimpleEntry and not an integer, which is now quite obvious...

Answer 1

Seems like somehow you have added a HashMap as a key to one of the SimpleEntries in your TreeSet.

Show a complete runnable example please.

This code works for me:

import java.util.AbstractMap;
import java.util.AbstractMap.SimpleEntry;
import java.util.Comparator;
import java.util.TreeSet;

public class Main {

    static class PairComparator implements Comparator<SimpleEntry<Integer, Integer>> {

        @Override
        public int compare(SimpleEntry<Integer, Integer> o1, SimpleEntry<Integer, Integer> o2) {
            int key1 = o1.getKey();
            int key2 = o2.getKey();
            return key1 - key2;
        }
    }

    public static void main(String[] args) {
        TreeSet treeSet = new TreeSet(new PairComparator());
        treeSet.add(new SimpleEntry(42, 69));
        treeSet.add(new SimpleEntry(37, 65));
        treeSet.add(new SimpleEntry(23, 19));
        treeSet.add(new SimpleEntry(54, 12));
        System.out.println(treeSet);
    }
}

Answer 2

You didn't specify the types for the TreeSet and some other data structures but I did it here and it works just fine. I did change your comparator. Subtracting integer values to do the comparisons is not a good idea as it can cause problems for large numbers.

        
TreeSet<SimpleEntry<Integer, Integer>> treeSet =
        new TreeSet<>(new PairComparator());

Random r = new Random(23);
for (int i = 0; i < 10; i++) {
    int weight = r.nextInt(100);
    int source = r.nextInt(100);
    
    SimpleEntry<Integer, Integer> pair =
            new SimpleEntry<>(weight, source);
    treeSet.add(pair);
}

treeSet.forEach(System.out::println);

Prints the following (duplicate keys ignored)

18=43
22=17
24=30
27=48
81=95
83=47
89=70
90=10
94=87

The comparator

static class PairComparator implements
        Comparator<AbstractMap.SimpleEntry<Integer, Integer>> {
    public int compare(
            AbstractMap.SimpleEntry<Integer, Integer> o1,
            AbstractMap.SimpleEntry<Integer, Integer> o2) {
        int key1 = o1.getKey();
        int key2 = o2.getKey();
        return key1 < key2 ? -1 : key1 > key2 ? 1 : 0;
    }
}

You can also specify the following as a comparator.

Comparator<AbstractMap.SimpleEntry<Integer,Integer>> pairComparator =
     (se1,se2)-> Integer.compare(se1.getKey(),se2.getKey());