Single linked lists - help

[Srcee]

Java can't compile the next program segment because it should return Object. What can I do to make it compile? May I write return null after if?

public Object deleteFirst(){
if (first != null){
SLLNode tmp = first;
first = first.succ;
return tmp.element;
}
}

[Fubarable]

What do you want it to return as a default if first == null?

[Srcee]

What do you want it to return as a default if first == null?

The code is for deleting a first node in a single linked list. If first==null it should throw Exception.

[Fubarable]

The code is for deleting a first node in a single linked list. If first==null it should throw Exception.

Then do that.

if (first == null) {
  throw new // my exception...
}
// ... rest of code of method goes here.

[r035198x]

You are not throwing that exception. Think about the error message you are getting.
Java is smart enough to realize that your if test might return false meaning your if block is never entered. In that case there is nothing to return. But the method says that it returns an Object.

[Srcee]

Then do that.

if (first == null) {
  throw new // my exception...
}
// ... rest of code of method goes here.

Thank you a lot ;)

[Srcee]

I do not want to open a new topic, so I will ask here. Can you send mi a link to a site with solved exercises about Single linked lists, Double linked lists, stacks, queues, decks, hash tables and maps?

Tnx

[PhHein]

Trail: Collections (The Java™ Tutorials)
Not really solved exercises though.

[Srcee]

Not really solved exercises though.

Oh but I need solved exercises, for example:"write a class named Playlist as Double linked list that should allow i don't know some functions". Or "employers in a company as a tree". Help pls

[PhHein]

I don't know such a site, I've never needed that.

[Srcee]

I need help again. The next program segment is for adding a first node in a Double linked list:

public void insertFirst(Object o){
DLLNode ins = new DLLNode(o, first, null);//1
if (first == null)
last = ins;
else
first.succ.prev = first;//2
first = ins;
}

Can someone explain me:
what does the line marked with //1 and //2 mean? Possibly with pictures :) I mean, I know what they mean but I can't see how that adds node in the list.