How to return a list?

Question

Hi m assignment is to create a linked list implementation..
so far i have this..
public class Set<Value> {
Value Element;
Set<Value> next;
Set(Value data)
            Element=data;
            next=null;
        Set(Value data, Set n)
            Element = data;
            next    = n;
public interface MySet<Value> {
boolean isIn(Value v);
void add(Value v);
void remove(Value v);
MySet union(MySet s);
MySet intersect(MySet s);
MySet difference(MySet s);
int size();
void printSet();
public class MyLLSet<Value> implements MySet<Value>{
    private Set<Value> head;
public MyLLSet(){
    head=new Set(null);  
public boolean isIn(Value v){
    Set<Value> p;
    p=head;
    boolean flag=false;
     while(p.next!=null){
        p=p.next;
        if(p.Element==v){
            flag=true;
return flag;
public void add(Value v){
    Set<Value> p;
    Set<Value> q;
    p=head;
    q=head.next;
    while(p.next!=null){
        p=p.next;
        q=q.next;
    p.next=new Set(v,q);
public void remove(Value v){
Set<Value> p;
Set<Value> q;
p=head;
q=head.next;
while(p.next!=null){
     p=p.next;
     q=q.next;
     if(p.Element==v){
         p.next=p.next.next;
         q.next=null;
         break;
public MySet union(MyLLSet s){
    Set<Value> p;
    Set<Value> q;
    p=s.head;
    q=this.head;
    while(p.next!=null){
        p=p.next;
    p=q.next;
    s.head.next=null;
RETURN ???
}However in my union method i cant figure out what to return.. i am sure that the method creates a union of two lists.. but how do i return the final unionized list.... ( i am new to programming so sorry if i dont make sense)
Edited by: haramino1 on Mar 22, 2009 1:31 PM

Answer 1

Have some patience! You can't expect an answer in 10 minutes.
Your original post looked like you wanted to make an implementation of a linked list, using a set as the underlying code (which doesn't make a lot of sense). However, your real assignment, as you stated later, is to make an implementation of a set, with a linked list as the underlying code.
In your original post, your Set<Value> doesn't make sense to be your Set class. It does make sense to have that code as your linked list's "node" class. So, rename it from Set<Value> to Node<Value>. Everywhere in that class that says "Set" should say "Node". It will only confuse you more if you call it "Set" when it is not a "Set" (it surely confused me!). Also, "Element" should be lowercase "element" (to follow standard naming conventions).
You want to make a linked list class that implements everything in the MySet<Value> interface. So, your MyLLSet<Value> should have all of those methods. Your MyLLSet should have one variable (which you have, but you call it Set<Value>, it should be Node<Value>).
private Node<Value> head;Your MyLLSet constructor probably shouldn't set head to a new Node with a null value (otherwise, your Set will always include a null member). The MyLLSet constructor probably doesn't need to do anything, so you can just remove that whole constructor and let the compiler generate its standard no-argument constructor.
When comparing values for equality in isIn, you probably want to use .equals, not ==:
if (p.element == v) // WRONGshould be:
if (p.element.equals(v)) // CORRECTOtherwise, you are only comparing reference values, not the data they point to. So, for instance, if Value was a String, you wouldn't be able to write code that asked the user for a String value to test if the String was in the Set--I don't think you'd ever be able to get "true" for the answer.
In isIn, your test should not be p.next != null --it should be "p != null". Otherwise, if p is null, you will get a NullPointerException trying to get "next". You want:
while (p != null) {
   // Test p.element for equality to v.
   //        If equal, you can break out of the 'for' loop.
   // Set p to p.next.
}Similarly in other methods, you need to check p != null . Your 'add' method is wrong--you didn't check duplicates, for one thing. Your loop isn't right, either.