Queues

class Node{
    private String value = null;
    private Node next=null;
    
    public Node(String value){
        this.value= value;
    }
    
    public void setValue(String value){
        this.value = value;
    }
    
    public String getValue(){
        return this.value;
    }
    
    public void setNext(Node next){
        this.next = next;
    }
    
    public Node getNext(){
        return this.next;
    }
}

class Queue{
    private Node firstElement= null;
    private Node lastElement = null;
    private int length= 0;
    
    public Queue(){
    }
    
    public String peek(){
        if (this.length ==0){
            System.out.println("The queue is empty, not value to peek.");
            return null;
        }
        
        return this.firstElement.getValue();
    }
    
    public void enqueue(String value){
        Node newElement = new Node(value);
        
        if (firstElement == null){
            firstElement = newElement;
        }
        if (lastElement == null){
            lastElement = newElement;
        } else {
            lastElement.setNext(newElement);
            lastElement= newElement;
        }
        this.length++;
    }
    
    public String dequeue(){
        if (this.length == 0){
            System.out.println("The queue is empty, no value can be dequeued.");
            return null;
        }
        
        Node dequeuedElement = firstElement;
        firstElement = firstElement.getNext();
        this.length--;        

        return dequeuedElement.getValue();
    }
    
    public void printAll(){
        if (this.length == 0){
            System.out.println("The queue is empty, no values can be printed.");
            return;
        }
        Node currentElement = firstElement;
        while (currentElement != null){
            System.out.println(currentElement.getValue());
            currentElement = currentElement.getNext();
        }
    }
    
}

class Main {
    public static void main(String[] args) {
        Queue queue = new Queue();
        
        queue.enqueue("1");
        System.out.println("Peek: "+ queue.peek());
        queue.enqueue("2");
        queue.enqueue("3");
        queue.printAll();
        
        System.out.println("Peek: "+ queue.peek());
        queue.dequeue();
        System.out.println("Peek: "+ queue.peek());
        queue.enqueue("4");
        
        
    }
}