How to manually implement a Linked List in Java without using a Constructor?

I realize that there are a few good threads already existing which discuss how to implement a linked list in Java, however I can't seem to find one which does not use a constructor like this,

public LinkList() {
    Link = null;
}

or something along these lines. However, I have not done this in my code, and therefore I am having trouble with writing a few of the methods.

I have a class called Link which contains the following code, note without any constructor like the one displayed above:

public class Link {
private Link next = null;
private String value = null;

public Link getNext(){
    return next;
}

public void setNext(Link nextLink){
    next = nextLink;
}

public String getValue(){
    return value;
}

public void setValue(String aValue){
    value = aValue;
}
}

and then I have my LinkedList class, which contains the following code:

public class LinkedList {
private Link head = null;
private Link tail = null;

public Link getHead(){
    return head;
}

public void setHead(Link aLink){
    head = aLink;
}

public Link getTail(){
    return tail;
}

public void setTail(Link aLink){
    tail = aLink;
}

public boolean isEmpty(){
    return(head == null && tail == null);
}

public void addLast(String aString){
    Link link = new Link();
    link.setValue(aString);
    //tail.setNext(link);
    if(isEmpty()){
        head = link;
    }else{
        tail.setNext(link);
    }
    tail = link;

    }

public void addFirst(String aString) {
    Link link = new Link();
    link.setValue(aString);

    if(isEmpty()){  
        tail = link;
    }else{
        head.setNext(link);
    }
    head = link;        
}

public Link removeFirst(){
    Link temp = head;
    head = temp.getNext();
    temp.setNext(null);

    return temp;
}

}

My addFirst Method, and my isEmpty Method both seem to be working but my addLast and removeFirst Methods are not. I have drawn pictures, looked at pseudo code, and searched around the internet, but I just cannot seem to figure this out.

I need to know how to get my addLast and removeFirst methods working without having to add in the constructor at the beginning like all the rest of the example code I keep seeing.

Here are the Tests they need to pass:

    @Test
public void testRemoveFirst(){
    list.addFirst("three");
    list.addFirst("two");
    list.addFirst("one");
    assertTrue("one".equals(list.removeFirst()));
    assertTrue("two".equals(list.removeFirst()));
    assertTrue("three".equals(list.removeFirst()));
}

@Test
public void testAddLast(){
    list.addFirst("three");
    list.addFirst("two");
    list.addFirst("one");
    assertTrue( "three".equals(list.removeLast()));
    assertTrue( "two".equals(list.removeLast()));
    assertTrue( "one".equals(list.removeLast()));
    assertNull(list.removeLast());
}