Question

I Have made a prority blocking queue.Inserting is properly done based on priority.when i delete the head of the queue,last element of the queue becomes the head of the queue ,procedure repeats.
Suppose i inserted 123456789 in the queue.

First deleted : 1;  queue elements :92345678  
Second deleted:  9; queue elements :8234567  

and so on.

I need to achieve queue property, which is not being maintained.

i.e
First delete : 1 queue elements :23456789
Second delete :2 queue elements :3456789

package com.block.ui;

import java.util.Iterator;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class PriorityBlockQueSamplActivity extends Activity {
/** Called when the activity is first created. */

EditText et;
EditText et2;
Button b1;
Button b2;

Controller c ;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    et=(EditText)findViewById(R.id.editText1);
    et2=(EditText)findViewById(R.id.editText2);
    b1 =(Button)findViewById(R.id.button1);
    b2 =(Button)findViewById(R.id.button2);
    c= new Controller();


    Message m = new Message();
    m.mPriority=1;
    m.Subject="1";
    c.insert(m);

    Message n = new Message();
    n.mPriority=1;
    n.Subject="2";
    c.insert(n);
    Message p = new Message();
    p.mPriority=1;
    p.Subject="3";
    c.insert(p);

    Message q = new Message();
    q.mPriority=1;
    q.Subject="4";
    c.insert(q);


    b1.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

            Iterator<Message> iter = c.getQueue().iterator();
            while(iter.hasNext()){
                Message m =iter.next();
                Log.i("MESSAGE", "Element "+m.Subject+" priority :"+m.mPriority);
            }

        }
    });
    b2.setOnClickListener(new  OnClickListener() {

        @Override
        public void onClick(View v) {
            Message m=c.getMessage();
            c.delete(m);

        }
    });
}

}

and one more class

 `public class Controller {

private int queueSize=300;
Comparator<Message> comparator = new PriorityComaparator();
PriorityBlockingQueue<Message> qu = new PriorityBlockingQueue<Message>(queueSize,comparator);

            public Controller() {

}

public void insert(Message msg) {

    boolean status = qu.add(msg);
    if (status)
        log("Added successfully " + msg.Subject + " priority "
                + msg.mPriority + " size :" + qu.size());
    else
        log("Failed to add " + msg.Subject + " with priority "
                + msg.mPriority);

}

public void delete(Message msg) {
    if (qu.contains(msg)) {
        boolean status = qu.remove(msg);
        if (status)
            log("deleted successfully " + msg.Subject + " priority " + msg.mPriority
                    + " size :" + qu.size());
        else
            log("Failed to delete " + msg.Subject + " with priority "+ msg.mPriority);
    }
}

public Message getMessage() {
    return qu.peek(); 
}

public PriorityBlockingQueue<Message> getQueue(){
    return qu;
}
void log(String msg) {
    Log.i("QUEUE", msg);
}

class PriorityComaparator implements Comparator<Message> {

    @Override
    public int compare(Message m, Message n) {
        int y = ((Message) m).mPriority; // 1 -highest 3-least
        int z = ((Message) n).mPriority;


        if (y < z) {
            return -1;
        }
        if (y > z) {
            return 1;
            }
            return 0;
            }

        }

}`

Answer 1

If the priority queue you are using is your personal implementation, then try to an heap data structure.

As the wikipage says:

The heap is one maximally efficient implementation of an abstract data type called a priority queue.

Here is the link to the page.

Remember that in Java you don’t need to implement it, you can use the standard PriorityQueue class or PriorityBlockingQueue if you need that it works in a multithread program.