Custom ordering using Java Comparator – Retain the position for objects with certain value

I am trying to implement a custom comparator that sorts a list of objects in descending order, but while doing so, it ignores when the object has a specific value (-1 in my case). Thus, the objects with that value retain their original position, but all the objects around them will be sorted in descending order. For example, if the list is :

Candidate candidate1 = new Candidate("id1",  0.4);
Candidate candidate2 = new Candidate("id2", -1.0);
Candidate candidate3 = new Candidate("id3", 0.8);
List<Candidate> inputCandidates = ImmutableList.of(candidate1, candidate2, candidate3)

The resulting order should be

[
    Candidate(super=Candidate(id=id3), value=0.8), 
    Candidate(super=Candidate(id=id2), value=-1.0), 
    Candidate(super=Candidate(id=id1), value=0.4)
]

Currently, I am trying something like this:

public final Comparator<Candidate> customComparator() {
    return (Candidate one, Candidate two) -> {
        double scoreOne =  one.getValue() != null ? one.getValue() : 0d;
        double scoreTwo =  two.getValue() != null ? two.getValue() : 0d;
        if (scoreOne == scoreTwo) return 0;
        if (scoreOne == -1.0d) return 1;
        if (scoreTwo == -1.0d) return -1;
        return Double.compare(scoreTwo, scoreOne);
    };
}

However, this instead gives me:

[
    Candidate(super=Candidate(id=id3), value=0.8), 
    Candidate(super=Candidate(id=id1), value=0.4), 
    Candidate(super=Candidate(id=id2), value=-1.0)
]

Could someone please help me figure out where I am going wrong?

>Solution :

There is no solution using a single Comparator (or at least I failed to find it so). Here is a working solution requiring two steps:

1. Sort all the values in the descending (reversed()) order without the specific value and collect it as a mutable list:

   List<Candidate> sorted = inputCandidates.stream()
        .filter(candidate -> candidate.getValue() != -1.0)
        .sorted(Comparator.comparing(Candidate::getValue).reversed())
        .collect(Collectors.toCollection(ArrayList::new));
   

2. Iterate through the former collection and add the candidates with the specific value (-1.0) to its original index position in the newly sorted list:

   for (int i=0; i<inputCandidates.size(); i++) {
        Candidate candidate = inputCandidates.get(i);
        if (candidate.getValue() == -1.0) {
            sorted.add(i, candidate);
        }
    }
   

Example input:

Candidate candidate1 = new Candidate("id1",  0.4);
Candidate candidate2 = new Candidate("id2", -1.0); // fixed position
Candidate candidate3 = new Candidate("id3", -1.0); // fixed position
Candidate candidate4 = new Candidate("id4",  0.8); 
Candidate candidate5 = new Candidate("id5", -1.0); // fixed position
Candidate candidate6 = new Candidate("id6",  0.5);

Example output:

sorted.forEach(c -> System.out.println(c.getId() + " : " + c.getValue()));

id4 : 0.8
id2 : -1.0  // fixed position    
id3 : -1.0  // fixed position
id6 : 0.5 
id5 : -1.0  // fixed position
id1 : 0.4