How is this code snippet swapping Queue contents (Java)?

Here's the code I wrote:

public static void main(String[] args) {

    // Why is this snippet actually swapping? 
    Queue<Integer> q1 = new LinkedList<>();
    Queue<Integer> q2 = new LinkedList<>();
    q1.add(10); q1.add(20); q1.add(30);
    q2.add(11); q2.add(22); q2.add(33);
    System.out.println("q1 before name swap --> " + q1);
    System.out.println("q2 before name swap --> " + q2);
    Queue<Integer> temp = q1;
    q1 = q2;
    q2 = temp;
    System.out.println("q1 after name swap --> " + q1);
    System.out.println("q2 after name swap --> " + q2);
}

Here's the output I got:

q1 before name swap --> [10, 20, 30]   
q2 before name swap --> [11, 22, 33]   
q1 after name swap --> [11, 22, 33]  
q2 after name swap --> [10, 20, 30]

Even after pondering over this for quite some time, I can't seem to understand why this code actually swaps the Queues. Shouldn't temp also store q2 contents, as I've written temp = q1 and then q1 = q2? That is, shouldn't changes to q1 reflect on temp as well, since Queue is non-primitive/reference data type in Java?

Solution

Queues are indeed Non-primitive or Reference Data Type.

There are 3 Queues (q1, q2 & temp) here in the sample.

You do the following

You assign q1 to [10, 20, 30] and q2 to [11, 22, 33]. The value of q1 and q2 is stored in memory.
You then assign temp to q1 and temp value is now [10, 20, 30]. At this point of time if you modify temp or q1 then both of them will change.
You then assign q1 to q2 and q1 starts pointing to reference of q2 and now q1 value is [11, 22, 33]
You then assign q2 to temp and q2 starts pointing to reference of temp and now q2 value is [10, 20, 30]

You are basically changing references using q1=q2 and q2=temp and not the value.

Hopefully this helps.