LinkedList.java

package data.angelic;

public class LinkedList {
    private Entry header;
    private int size;
    
    public LinkedList() {
        super();
        header = new Entry(null, null, null);
        header.next = (header.previous = header);
        size = 0;
    }
    
    public Object getFirst() { return header.next.element; }
    
    public Object getLast() { return header.previous.element; }
    
    public Object removeFirst() {
        Object first = header.next.element;
        this.remove(header.next);
        return first;
    }
    
    public Object removeLast() {
        Object last = header.previous.element;
        this.remove(header.previous);
        return last;
    }
    
    public void addFirst(Object o) { this.addBefore(o, header.next); }
    
    public void addLast(Object o) { this.addBefore(o, header); }
    
    public int size() { return size; }
    
    public void add(Object o) { this.addBefore(o, header); }
    
    private Entry addBefore(Object o, Entry e) {
        Entry newEntry = new Entry(o, e, e.previous);
        newEntry.previous.next = newEntry;
        newEntry.next.previous = newEntry;
        size++;
        return newEntry;
    }
    
    private void remove(Entry e) {
        e.previous.next = e.next;
        e.next.previous = e.previous;
        size--;
    }
    
    private static class Entry {
        Object element;
        Entry next;
        Entry previous;
        
        Entry(Object element, Entry next, Entry previous) {
            super();
            this.element = element;
            this.next = next;
            this.previous = previous;
        }
    }
    
}