Thread: Pass-by-Value scheme, puzzled with passing/assigning a reference to another object

i just encountered a strange thing with object reference as i was creating my own linked list,

The main class

public class WordLinkedListSampleProgram {
 
    public static void main(String[] args) {
 
        WordLinkedList nL = new WordLinkedList();
 
        nL.addValue("A");
        nL.addValue("B");
 
        nL.reversedLink();
    }
}

The WordLinkedList class

public class WordLinkedList {
 
    private Node start;
    private Node next;
    private Node last;
    private int listSize;
 
    public void addValue(String val) {
 
        if (listSize == 0) {
 
            start = new Node(val);
            next = start;
            start.setNext(next);
        }
        else {
 
            next.setNext(new Node(val));
            last = next.getNext();
            next = last;
            last.setNext(null);
        }
 
        listSize++;
    }
 
    public void reversedLink() {
 
        Node tempStart, tempNext;
 
        tempStart = last;
        tempNext = tempStart;
        // tempStart.setNext(tempNext);
 
        // i was expecting this part should throw a null pointer exception,
        // but if the above statement tempStart.setNext(tempNext); is UNCOMMENTED
        // this statement doesnt throw any exception even with more .getValue() consecutive calls
        System.out.println(start.getNext().getNext().getValue());
    }
}

The Node class

public class Node {
 
    private Node nextNode;
    private String value;
 
    public Node(String val) {
 
        value = val;
    }
 
    public Node getNext() {
 
        return nextNode;
    }
 
    public void setNext(Node n) {
 
        nextNode = n;
    }
 
    public String getValue() {
 
        return value;
    }
}

base on what i know so far is, the local 'tempStart' affects the reference of the data member 'last'?
what i know is, this kind of reference control is only possible with java arrays
how come it happens with data fields and objects?

the data member 'last''s references should not be affected when i pass it in another variable.. how come in my case it does?
am i missing something?

please correct and enlighten me, thank you in advance. any help would be greatly appreciated

that is what confuses me, please let me do my best to explain it further.
in the whole program, i got a linked-list with 2 nodes

start - as the first one, set to next(another node)
last - is the last node/another node(i.e the next node the start is pointing at)
last does not point to any node anymore, so it has a null 'nextNode' inside of it
so if there is a call like last.getNext(); it should throw a null, or a start.getNext().getNext(); should throw a null or a last.getNext().getNext() should throw a null exception as well , that is basically what i am expecting to happen because i only have a 2 nodes linked to each other

im so puzzle that why this following statements doesnt throw a null
exception, any of these should throw a null

start.getNext().getNext()
// or 
last.getNext()
// or
last.getNext().getNext()

with printing the values of the nodes,
printing values.. like these calls

last.getNext().getValue(); // prints "B"
last.getNext().getNext(); // prints "B"
last.getNext().getNext().getNext(); // still prints "B"

all of which should throw a null exception when the statement..

tempStart.setNext(tempNext);

is commented or removed


if not, every node starting from 'start' to 'last' is being affected, last.getNext()/start.getNext().getNext() should never have any more nodes to call(a null exception should be thrown), but with those local variable NODES(temp nodes), Im totally mystified why the data member nodes are affected.. and just like what i've said, my assumption was the reference of 'last' data member is being affected when that code above is uncommented

my expectation is even that code is there or not, last.getNext() or start.getNext().getNext() should always throw a null, but why doesnt it?

my goal was to reverse the links, i was able to do so, by making a doubly-linked list(i.e with a set previous and a set next nodes), but i wanna get ahead of this one, as this is one of the crucial things that i need to deal with java references, i even got another one like this, but i might be able to understand it by just getting ahead of this one. please ask me and im willing to explain it as possible as i can.. thanks again norm.

yes i put a toString() in the Node class but it throws a StackOverFlow error, pointing at the toString() method itself,
and i made a call on the toString() inside the reverselink method .print(last.toString) or .print(tempStart.toString);

Exception in thread "main" java.lang.StackOverflowError
	at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:391)
	at java.lang.StringBuilder.append(StringBuilder.java:119)
	at xUnfinished_Errorsx.WordNode_UC.toString(WordNode_UC.java:38)
	at java.lang.String.valueOf(String.java:2826)
	at java.lang.StringBuilder.append(StringBuilder.java:115)
	at xUnfinished_Errorsx.WordNode_UC.toString(WordNode_UC.java:38)

yes i manage to track down what this statement does with the local node

tempStart.setNext(tempNext);

because of these first 2

tempStart = last;
tempNext = tempStart;

the tempStart is just pointing to itself, somehow it becomes a dog chasing its own tail...


thats why this statement

System.out.println(tempStart.getNext().getNext().getValue());

clearly shows that tempStart.getNext() or more call to .getNext() is just calling its ownself thats why it never throws a null pointer exception

but this one still confusing me, i cant track it down. considering its a data member just passing its reference to a local one

System.out.println(last.getNext().getNext().getValue());

how it is being affected with the statement tempStart.setNext(next);
how come the 'last' points it self to itself.

@KevinWorkMan thats what i was looking for on the web.. i dont know what keyword should i type or if i would, i got bunch of hard to understand one than to get an ABC explanatory kind of article for this kind of problem, and thank you norm. yes i see that.. even the 'last' points to itself. Please hang on me for a while, this is what i understand base on the articles kevin gave me, the "Cup with remote controls",..

when the statement

tempStart = last;
tempNext = tempStart;
tempStart.setNext(tempNext);

i passed not a copy to a reference but instead i passed the thing that the 'last' controls into the variable tempStart?
so whenever i do something USING the tempStart.. it will affect what the 'last' is referring to?

is my understanding correct?