CODEWITHSUNDEEP
java
Rootesh
Rootesh

Reputation: 11

How to use only one instantiated object to store several values?

I just got hooked up with queues in JAVA but I can't figure out how to use objects. The below is a piece of code in which I'm trying to figure out how to use only one instantiated object instead of several objects to store multiple data.

public class value {

    private int A;

    public int getA() {
        return A;
    }

    public void setA(int a) {
        this.A = a;
    }

    public String toString()
    {
        return Integer.toString(A);
    }
}

__

Queue<value> q=new PriorityQueue<value>();

    value v1 = new value();
    value v2 = new value();
    value v3 = new value();
    value v4 = new value();
    value v5 = new value();

    v1.setA(1);
    v2.setA(7);
    v3.setA(5);
    v4.setA(6);
    v5.setA(2);

    q.add(v1);
    q.add(v2);
    q.add(v3);
    q.add(v4);
    q.add(v5);


    while(!q.isEmpty())
    {
        System.out.println(q.poll());
    }

The above returns: 1 2 5 6 7

__

Then I tried a second piece of code

    Queue<value> q=new PriorityQueue<value>(a);

    value v1 = new value();
    v1.setA(0);
    q.add(v1);
    v1.setA(3);
    q.add(v1);
    v1.setA(5);
    q.add(v1);

    while(!q.isEmpty())
    {
        System.out.println(q.poll().getA());
    }

The second one returns me: 5, 5, 5

Do i need to instantiate the class for every new data i'm adding? Seems quite resourceful if I got to do it like that

Upvotes: 1

Views: 82

Answers (3)

Stephen C
Stephen C

Reputation: 719739

Do i need to instantiate the class for every new data i'm adding?

Basically, yes.

In your second example, you are adding one object to the queue three times, and changing it between the times you add it. No copies are being made, so you end up with the same object in the queue three times.

In addition to not doing what you expect, this is actually a really bad idea. When you modify an object while it is in the queue in a way that alters the queue's priority ordering you are liable to break the data structure, causing elements to be added and removed in an unexpected order.

(Interestingly, the javadoc does not mention this explicitly. However, it it is clear that it would happen from reading the source code.)

Upvotes: 1

user2241537
user2241537

Reputation:

First of all you have to know answers of following questions:

  • What is an object?
  • What is being added in the queue?

So, here is an answer to the first one: Your class is a mold and your object is a bowl made from that mold.

Here is an answer to the second one: Queue is storing only references to the object.

You are storing a reference to same one bowl(object) three times and when you "see soup in your bowl"(poll your queue) you see the same soup(value). This soup(value) will be the same as you poured in your bowl last time. So, you have to make different bowls(objects) for storing different soups(values).

Upvotes: 0

Jos
Jos

Reputation: 2013

q.add(v1) will not make a copy and store the copy in the Queue, Instead it will add a reference to v1 to the queue. Since what we add to the queue is reference, all the elements in the queue (for 2nd example) are the same. Since references point to same object, in effect there is only one object. here the final state of the object has value 5. Hence the output 5.

If you need to store separate values, do need to create new objects. There might be a slight overhead compared to primitives but that's how Object oriented langues work. In order to use collections, you need objects any way.. In that case your value class is equivalent to an Integer class.

Upvotes: 0

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