Accessing an instance of a variable from another class in Java

Question

Is it possible to access an instance of a variable in one class from another class in Java.

Let's say you have the following in Class A:

private BlockingQueue<byte[]> buffer = new LinkedBlockingQueue<byte[]>();

I want to make changes to the queue in this class and then be able to use to access it from another class.

How would i access the instance of buffer from another class? Is it even possible?

Answer 1

Add a getter:

public class Whatever {

    private BlockingQueue<byte[]> buffer = new LinkedBlockingQueue<byte[]>();

    public BlockingQueue<byte[]> getBuffer() {
        return buffer;
    }
}

Then if you have an instance of Whatever:

Whatever w = new Whatever();
BlockingQueue<byte[]> buffer = w.getBuffer();

Answer 2

Change the private to public. Then you can access the variable buffer like this:

A myobj = new A();
BlockingQueue<byte[]> bq = myobj.buffer;

For more details, consult this article on access control: http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

An arguably better way of doing this is using getters. That is, create a get() method in class A which simply returns buffer.

public BlockingQueue<byte[]> getBuffer() {
    return buffer
}

Then you can access it by calling getBuffer():

BlockingQueue<byte[]> bq = myobj.getBuffer();

Answer 3

Its usually best to encapsulate collections and not expose the collections itself. Instead you should expose the operations you want to perform

public class Whatever {

    private final BlockingQueue<byte[]> buffer = new LinkedBlockingQueue<byte[]>();

    public void enque(byte[] bytes) { buffer.add(bytes); }

    public byte[] takeNext() { return buffer.take(); }

}