Collections.copy Method in java

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Hello Everyone, I am doing an assignment in java. I am making use of Collections.copy method available in Collections class. Following is a snippet of my code.

XML

public Queue(List<E> queue1) {
        queue = new ArrayList<E>(queue1.size());
        System.out.println("const" + queue1.size());
//      Collections.copy(this.queue, queue1);
        this.queue = queue1;
    }

The problem is when I'm using Collections.copy method its throwing me a NullPointerException, I don't know why? When I'm assigning directly using '=', it's working fine. Following is the snippet of code, where I am invoking this construcor.

XML

List<Integer> e = new ArrayList<Integer>(3);
        e.add(1);
        e.add(2);
        e.add(3);
        System.out.println(e.size());
        Queue<Integer> p = new Queue<Integer>(e);

Anyone knows how to solve it, please help me out. I need to use collections.copy here.
Thanks and regards,
Vinay Kumar Tiwary

Posted 16-Nov-13 3:40am

Vinay Kumar Tiwary

Updated 16-Nov-13 16:07pm

v2

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CHill60 16-Nov-13 15:49pm

how have you defined the property "queue" in the Queue class? this.queue must have at least the same capacity as queue1, and of course you must have created an instance of queue.
Note this.queue = queue1 is not actually making a copy of the ArrayList - both are pointing to the same data in memory.

Vinay Kumar Tiwary 16-Nov-13 22:09pm

Hey, thanks for reply.. I did forget to initialize it..:P Sorry.. Now u can see, I've initailized it with the same size of queue1. Now its throwing me ArrayIndexOutOfBoundsException. Please help me out.

Richard MacCutchan 17-Nov-13 6:13am

You need to provide more information; exactly where does the error occur, and what are the values of the variables.

Vinay Kumar Tiwary 17-Nov-13 7:06am

Thanks for looking into this.. I got d solution. my initialization was wrong. Actually I'm a newbie to collections classes. To initialize the new list, I just passed the old list as argument to constructor and its working fine now.
Thanks everyone. :) :) :)

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Vinay Kumar Tiwary · Answer 1 · 2013-11-17T01:09:00

Solution 1

Initialize the new list with existing list by adding as an argument to the constructor as follows. It works fine. :)

XML

public Queue(List<E> queue1) {
        queue = new ArrayList<E>(queue1);
        Collections.copy(this.queue, queue1);
    }

Posted 17-Nov-13 1:09am

Vinay Kumar Tiwary