CODEWITHSUNDEEP
javapriority-queue
Curious_Bop
Curious_Bop

Reputation: 311

Java comparator Issue

I'm taking in all the information from an MySQL database table using a resultset and adding all the values into an array

public void populateQueueFromDB() {
        // create priority queue
        try {
            String url = "jdbc:mysql://localhost:3306/project";
            Connection conn = DriverManager.getConnection(url, "root", "nbuser");

            PreparedStatement stmt = conn.prepareStatement("SELECT user_id,s_date,e_date,d_date,department,projectname,projectapplication,priority,cores,disk_space,analysis FROM booking");
            ResultSet rs;
            rs = stmt.executeQuery();


            //List<JobRequest> jobList = new ArrayList<JobRequest>();

            while (rs.next()) {
                JobRequest job = new JobRequest();
                User user = new User();
                user.setUserID(rs.getString("user_id"));
                job.setUserID(user.getUserID()); // changes the /user id to the job.setuser id so can call for my queue print.
                job.setStartDate(rs.getString("s_date"));
                job.setEndDate(rs.getString("e_date"));
                job.setDeadDate(rs.getString("d_date"));
                job.setDepartment(rs.getString("department"));
                job.setProjectName(rs.getString("projectname"));
                job.setProjectApplication(rs.getString("projectapplication"));
                job.setPriority(rs.getInt("priority"));
                job.setCores(rs.getInt("cores"));
                job.setDiskSpace(rs.getInt("disk_space"));
                job.setAnalysis(rs.getString("analysis"));

                schedulerPriorityQueue.addJob( job );

            }
            schedulerPriorityQueue.printQueue();

            conn.close();

        } catch (Exception e) {
            System.err.println("Got an exception! ");
            System.err.println(e.getMessage());
        }

    }

from here I go off and call my comparator to order the data, depending on priority of being either 1,2,3 then sort the queue. some other bits of code naming etc but essentially it sends me to the comparator

public class JobQueueComparator implements Comparator<JobRequest> {

    @Override
    public int compare(JobRequest object1, JobRequest object2) {
        if(object1.getPriority() < object2.getPriority()){
            return 1;
        } else {
            return -1;
        }
    }

}

But the output I'm getting from the comparator is ordering it in priority 3, 1 then 2. I've adapted this from example online but I don't understand the returns on comparator examples I've seen.

How would I be able to change that comparator to sort my priorities, 1 being the most important, 3 being the least. I made sure I'm printing out the output after adding all the result sets into the array so I know it's working as it's changed my ordering around, just dont know how to order it how I want.

Thanks

EDIT: schedulerPriorityQueue

public class Queue {

    private Comparator<JobRequest> comparator = new JobQueueComparator(); //calls my comparator
    private PriorityQueue< JobRequest> scheduledJobs = new PriorityQueue<JobRequest>(100, comparator);

    public void addJob(JobRequest job) {
        // now add job to priority queue
        scheduledJobs.add(job); // add jobs from the resultset into queue
    }

Upvotes: 0

Views: 165

Answers (4)

Jonathan Drapeau
Jonathan Drapeau

Reputation: 2610

Make it easy on yourself and use Integer and its compareTo method.

Your comparator method would look like this

    @Override
public int compare(JobRequest object1, JobRequest object2) {
    Integer iO1 = Integer.valueOf(object1.getPriority());
    Integer iO2 = Integer.valueOf(object2.getPriority());
    return -(i01.compareTo(iO2));
}

Assuming getPriorityreturns a int or String.

Upvotes: 2

Olimpiu POP
Olimpiu POP

Reputation: 5067

You are missing the case where the priorities are equal. I would use something like this:

public class JobQueueComparator implements Comparator<JobRequest> {

@Override
public int compare(JobRequest object1, JobRequest object2) {
        return -(object1.getPriority() - object2.getPriority());
 }
}

This would work if you take into consideration the priority order like this: 1, 2, 3.

Upvotes: 0

Eric Galluzzo
Eric Galluzzo

Reputation: 3241

You should return 0 from the comparator if the two objects are equal, an integer less than 0 if object 1 < object 2, and an integer greater than 0 if object 1 > object 2. Currently the JobQueueComparator never returns 0. Try the following:

public class JobQueueComparator implements Comparator<JobRequest> {
    @Override
    public int compare(JobRequest object1, JobRequest object2) {
        return object1.getPriority() - object2.getPriority();
    }
}

See the Comparator documentation for more details.

Upvotes: 0

Carlo Pellegrini
Carlo Pellegrini

Reputation: 5686

Simply swap the signs on the compare method. This will reverse the order.

public int compare(JobRequest object1, JobRequest object2) {
    if(object1.getPriority() < object2.getPriority()){
        return -1;
    } else {
        return +1;
    }
}

Upvotes: 0

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