Sort arraylist by double and String

Question

We were given a task to:

sort an array list by a dog's tail length (double)
if two dogs or more have the same tail length, sort them by name

I was able to create a code BUT the correct results do not show up during the first print-out. I have to try again for the sorting to actually happen.

I'm very new to programming and this confuses me. Any ideas? Thanks!

private void listDogs() {
    boolean length = false;
    for (int i = 0; i < dogs.size(); i++) {
        length = true;
    }
    if (dogs.isEmpty()) {
        System.out.println("Error: no dogs in register");

    }else {
        System.out.println("Please enter the tail lenght minimum: ");
        double tailLength = scan.nextDouble();
        scan.nextLine();

        Collections.sort(dogs);
        for (int i = 0; i < dogs.size(); i++) {
            if (dogs.get(i).getTailLength() >= tailLength) {
                System.out.println(dogs.get(i));
                length = true;
            }
        }
        if (length == false) {
                System.out.println("Error: No dog is registered with this tail length.");   
        }

@Override
public int compareTo(Dog o) {
    // TODO Auto-generated method stub
    int compare = Double.compare(tailLength, o.tailLength);   
    if (compare == 0) {
        compare = Double.compare(tailLength, o.tailLength);    
    }
    if (compare == 0) {
        compare = name.compareTo(o.name);
    }
    return compare;
}

The compareTo method looks ok (except for the first 3 lines, remove them). You can write some tests to verify that it is working. The problem might be somewhere else, e.g. the if (dogs.get(i).getTailLength() >= tailLength) { looks potentially suspicious. — Moritz Petersen
– Moritz Petersen, Commented Feb 12, 2019 at 14:39

Malte Kölle · Accepted Answer · 2019-02-12 14:39:13Z

2

I am not sure if this solves your Problem, but you have a

scan.nextLine();

in your Code and I do not understand if this has to be there, because as far as I know it only waits for you to enter something and then it doesn't even save it in a variable. So you might delete this line (Line 12).

answered Feb 12, 2019 at 14:39

Malte Kölle

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2 Comments

Zabuzard Over a year ago

It is to consume the newline left in the buffer after entering the number for nextDouble(). It is a poor design, Double.parseDouble(scan.nextLine()); should have been used instead, but it is correct.

Zabuzard Over a year ago

While your comment is not bad, it is definitely not an answer to OPs question, more a comment. I suggest to remove it, see How to Answer, thanks.

alexflex25 · Accepted Answer · 2019-02-12 15:13:32Z

Do you try this?

import java.util.ArrayList;
import java.util.Collections;

public class Dog implements Comparable<Dog> {

    private double tailLength;
    private String name;

    public Dog(final double _tailLength, final String _name) {
        tailLength = _tailLength;
        name = _name;
    }

    @Override
    public String toString() {
        return "Dog [tailLength=" + tailLength + ", name=" + name + "]";
    }

    @Override
    public int compareTo(final Dog o) {
        int res = Double.compare(tailLength, o.getTailLength());
        if (res == 0) {
            res = name.compareTo(o.getName());
        }
        return res;
    }

    public double getTailLength() {
        return tailLength;
    }

    public void setTailLength(final double tailLength) {
        this.tailLength = tailLength;
    }

    public String getName() {
        return name;
    }

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

    public static void main(final String[] args) {
        final ArrayList<Dog> dogs = new ArrayList<Dog>();
        dogs.add(new Dog(2, "Dog D"));
        dogs.add(new Dog(2, "Dog A"));
        dogs.add(new Dog(5, "Dog C"));
        dogs.add(new Dog(4, "Dog A"));
        dogs.add(new Dog(3, "Dog A"));
        dogs.add(new Dog(3, "Dog B"));
        dogs.add(new Dog(1, "Dog A"));

        // Sort Dog by tailLength and name
        Collections.sort(dogs);

        final Scanner sc = new Scanner(System.in);
        while (true) {
            System.out.println("tailLength: ");
            final double inputTaillength = sc.nextDouble();
            for (final Dog dog : dogs) {
                if (dog.tailLength == inputTaillength) {
                    System.out.println(dog);
                }
            }

        }
    }
}

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