Erste Schritte cannot be referenced from a static context

ubik

Mitglied
Hallo,

habe mir eine Klasse für eine doppelt verkettete Liste erstellt:

Code:
public class DList {
  static class Node {
    Object obj; // Element
    Node prev, next; // Zeiger auf Vorgaenger und Nachfolger

    public Node(Object o, Node p, Node n) {
      obj = o;
      prev = p; next = n;
    }

    public Node() {
      obj = null;
      prev = next = null;
    }

    public Object setElement(Object o) {
      obj = o;
    }

    public Object getElement() {
      return obj;
    }

    // Nachfolger setzen
    public void setNext(Node n) {
      next = n;
    }

    // Zugriff auf Nachfolger
    public Node getNext() {
      return next;
    }

    // Vorgaenger neu belegen
    public void setPrevious(Node p) {
      prev = p;
    }

    // Zugriff auf Vorgaenger
    public Node getPrevious() {
      return prev;
    }
  }

  private Node head = null;
  private Node tail = null; // Listenende

  public DList() {
    head = new Node();
    tail = new Node();

    // Anfang und Ende "verknuepfen"
    head.setNext(tail);
    tail.setPrevious(head);
    tail.setNext(tail);
  }

  public void addLast(Object o) {
    // Knoten zwischen tail und dessen Vorgaenger einfuegen
    Node l = tail.getPrevious();
    Node n = new Node(o, l, tail);
    l.setNext(n);
    tail.setPrevious(n);
  }

  public Object getFirst() throws ListEmptyException {
    if (isEmpty())
      throw new ListEmptyException();
    // Zugriff ueber Listenanfang
    return head.getNext().getElement();
  }

  public Object getLast() throws ListEmptyException {
    if (isEmpty())
      throw new ListEmptyException();
    // Zugriff ueber Listenende
    return tail.getPrevious().getElement();
  }

  public Object removeFirst() throws ListEmptyException {
    if (isEmpty())
      throw new ListEmptyException();
    // Zugriff ueber Listenanfang
    Object o = head.getNext().getElement();
    // Knoten zwischen head und Nachfolger entfernen
    head.setNext(head.getNext().getNext());
    head.getNext().setPrevious(head);
    return o;
  }

  public Object removeLast() throws ListEmptyException {
    if (isEmpty())
      throw new ListEmptyException();
    // Zugriff ueber Listenende
    Node n = tail.getPrevious();
    // Knoten zwischen tail und Vorgaenger entfernen
    n.getPrevious().setNext(tail);
    tail.setPrevious(n.getPrevious());
    return n.getElement();
  }

  public int size() {
    int s = 0;
    Node n = head;
    // Knoten zaehlen
    while (n.getNext() != null) {
      s++;
      n = n.getNext();
    }
  }

  public boolean isEmpty() {
    // keine Knoten zwischen head und tail
    return head.getNext() == tail;
  }
}

Und nun erstelle ich mir ein Objekt und möchte einen neuen Wert hinzufügen:

Code:
public class DListTest {
  static void main(String[] args) {
    DList meineListe = new DList();
    DList.addLast("Test");
  }
}

Aber ich bekomme beim Kompilieren die Meldung:

Code:
javac DListTest.java 
DListTest.java:4: error: non-static method addLast(Object) cannot be referenced from a static context
    DList.addLast("Test");
         ^
1 error

Was ist falsch? Die Methode ist doch public?
 

Saheeda

Top Contributor
Die Methode ist public, aber nicht statisch. Um drauf zuzugreifen, musst du meineList.addLast() eingeben.

Um die Methode mit DList.addLast() aufrufen zu können, müsste sie public static void... heißen.
 
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