How to Use Collection in Java

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Java

sockets

datalist

Hi,
I am newer to Java just started it a while back. I am trying to make chat messenger (Client-Server) and I need a data structure in which I can store online users (One socket and one String containing Username). Now I did a lot of research and thought AbstractList could help me decently, but I faced a problem rather quickly

public class Collections <E> extends AbstractList<E> {
    
    private int size;
    private E[] list;
    
    public Collections() {
        size = 0;
        list = new E[50];
    }
    
    @Override
    public E get(int index) {
        return list[index];
    }
    @Override
    public boolean add(E element) {
        
        
        return false;
    }
    
    @Override
    public int size() {
        return list.length;
    }
    public static void main(String[] args) {
        Collections <String>obj = new Collections();
        obj.add(new String("1"));
        obj.add(new String("5"));
        
        System.out.println("Size = " + obj.size());
    }
}

This is my code and frankly I couldn't get a way around the add function...
I tried using

list = new E[50];

but that gives generic initialization error. In short I have no idea how to initialize the list object.
So now I have Three questions:
1. Is my approach to creating a list of all the clients on Server good?
2. What should I do with the add() function?
3. Why are there '<e>' twice in the first line? Obviously it is a template but wouldn't you need it only once? Obviously you need it both places so what is the first one for and what is the second one for? (The Tutorial I learnt from didn't quiet explain it)