Add method for inserting nodes

app/src/main/java/ch/nuth/zhaw/exbox/MyList.java

@@ -188,11 +188,24 @@ public class MyList extends AbstractList<Object> {
         return anchor;
     }
 
+    /**
+     * Inserts the specified `item` after the specified `node`.
+     *
+     * @param node
+     * The node to insert the `item` after.
+     *
+     * @param item
+     * The item to insert.
+     */
+    protected void insertAfter(IListNode node, Object item) {
+        IListNode newNode = new ListNode(item, node, node.getNextNode());
+        node.setNextNode(newNode);
+        newNode.getNextNode().setPreviousNode(newNode);
+    }
+
     @Override
     public boolean add(Object e) {
-        IListNode newNode = new ListNode(e, getAnchor(), getAnchor().getPreviousNode());
+        insertAfter(getAnchor().getPreviousNode(), e);
-        getAnchor().getPreviousNode().setNextNode(newNode);
-        getAnchor().setPreviousNode(newNode);
         return true;
     }