Java TreeSet比较器

Question

I have a Vehicle class with fields: baseVehicleId, subModelId, notes and partNo

If is the same baseVehicleId, subModelId and partNo , i want the object with the longest notes to remain( and delete the others)

I have this:

Comparator<Vehicle> comparator = new Comparator<Vehicle>() {
            @Override
            public int compare(Vehicle a, Vehicle b) { 
                if(a.baseVehicleId().equals(b.baseVehicleId()) && String.valueOf(a.subModelId ()).equals(String.valueOf(b.subModelId ()))
                        && String.valueOf(a.partNo ()).equals(String.valueOf(b.partNo ())) 
                ))
                    return a.getOurnotes().compareTo(b.getOurnotes());
                // not good I know
                 return 1;
            }
        };

        TreeSet<Vehicle> treeSet = new TreeSet<Vehicle>(comparator);

How can i modify this code :/? So for example if i have all the fields equal and the length of notes are greater then delete the other objects.

Answer 1

The comparator's job is simply to say which of two objects comes first - there's no mechanism to manipulate the underlying collection during it's comparisons. I think you would want to think about this in a slightly different way: perform your filtering logic prior to adding an object to your collection.

I'd recommend just using a standard list, say ArrayList for sake of simplicity. Also you'd need to override the equals() method in your Vehicle class which returns true if two Vehicles share the same IDs and part numbers.

When it comes to adding new vehicles you can do something like this:

int vehicleIndex = myvehicles.indexOf(vehicle) // -1 if missing, otherwise 0 or greater to represent the index position

if (vehicleIndex == -1) {
    // No item in the list shares the ids/model/part numbers so can add
    myvehicles.add(vehicle)
} else {
    // There's another similar vehicle, better compare their notes:
    Vehicle existingVehicle = myvehicles.get(vehicleIndex); // retrieve the object from list so that we can compare it

    // check if the new vehicle has longer notes
    if (vehicle.getOurNotes().length > existingVehicle.getOurNotes().length) {
        // if it does, remove the old one from the list
        myvehicles.remove(vehicleIndex);
        // and add the new one. Effectively we've done a replacement
        myvehicles.add(vehicle)
    }
}

Obviously it's not automatically sorted but you can do that after adding a batch of items by running the Collections.sort() method which also accepts a Comparator, but this time your comparator can focus on the specific task of detailing the ordering now that you know the list is pre-filtered so you are not concerned with trying to filter items out.

Answer 2

Just loop through the collection and remove everything that doesn't meet your requirements using it.remove() .

You may also want to look at the Java 8 collections functionality that will allow you to do things like filter a collection as that will also do what you are looking for.

Answer 3

I think you should use the method

maximum = Collections.max(collection, comparator);

for searching the maximum of the elements and then just delete the others with

collection.remove();

Inside the comparator you can't delete from the collection.