verkettete lineare Liste

[Guest]

bin gerade dabei, eine klasse zu schreiben, die eine verkettete lineare liste von objekten darstellt. das interface habe ich vorgegeben:

public interface List {
 
   public void addFirst(Object o);
 
   public void add(Object o);
 
   
   public void clear();
 
   public boolean contains(Object o);
 
   
   public Object getFirst();
 
   
   public boolean isEmpty();
 
   public int size();
 
   
   public Object[] toArray();
 
   
   public boolean remove(Object o);
 
}

jetzt hab ich nur das problem, dass mir beim testen:

333
---------
Exception in thread "main" java.lang.NullPointerException
at LinkedList.add(LinkedList.java:31)
at Test.TestAdd(test.java:11)
at Test.main(Test.java:83)

ausgegeben wird...

public class LinkedList implements List {
 

   protected class ListNode {
 
      
      protected Object value = null;
 
      
      protected ListNode next = null;
    }
 
      protected ListNode head = null;
 
   // Methoden ...
   
   public void addFirst(Object o){
	   ListNode newNode = new ListNode();
	      newNode.value = o;
	      newNode.next  = head;
	      head = newNode;
   }
   
   public void add(Object o){
	    ListNode newNode = new ListNode();
		newNode.value = o;
		newNode.next = null;
		ListNode node = head;
		while (node.next != null) {
			node = node.next;
		}
		node.next = newNode;
   }
   
   public void clear(){
	   head = null;
   }
   
   public boolean contains(Object o){
	     ListNode node = head;
	      while (node != null) {
	         if (node.value.equals(o)) return true;
	         node = node.next;
	      }
	      return false;    // Ende der Liste erreicht, nicht gefunden
   }
   
   public Object getFirst(){
	   return head;
   }
   
   public boolean isEmpty(){
	   return head == null;
   }
   
   public int size(){
	   int length = 0;
	   ListNode node = head;
		while (node != null) {
			node = node.next;
			length = length + 1;
		}
	   return length;
   }
   
   public Object[] toArray(){
	   Object[] feld = new Object[size()];
		ListNode node = head;
		for (int i = 0; i < feld.length; i++) {
			feld[i] = node.value;
			node=node.next;
		}
		return feld;
   }
 
   public boolean remove(Object o){
	   ListNode node = head;
	   
	   if (node.value.equals(o)){
	     head = node.next;   
	   }
	   else{
	      while (node != null) {
	         if (node.next.value.equals(o)){
	        	 node.next = node.next.next; 
	        	 return true;
	         }
	         node = node.next;
	      }
	      return false;
	   }
	   return false;
   }
 
   public void Print(){
	    ListNode node = head;
		while (node != null) {
			System.out.println(node.value);
			node = node.next;
		}  
   }
   
   public void PrintElement(Object o){
	    ListNode node = head;
		System.out.println(node.value);
  }
 public String toString() {
      StringBuilder sb = new StringBuilder("Liste: < ");
      for (ListNode node = head; node != null; node = node.next) {
         sb.append(node.value).append(" ");
      }
      sb.append(">");
      return sb.toString();
   }
}

public class Test {
	
      public static void TestaddFirst(){
           LinkedList test = new LinkedList();
      	   test.addFirst("333");
	   test.Print();
        }

	public static void TestAdd(){
	   LinkedList test = new LinkedList();
	   test.add("777");
	   test.add("222");
	   test.add("111");
	   test.Print();
	}
	public static void Testclear(){
           LinkedList test = new LinkedList();
           test.addFirst("777");
           test.add("222");
           test.add("111");
           test.clear();
           test.Print();
        }	
	public static void Testcontains(){
           LinkedList test = new LinkedList();
           test.addFirst("777");
           test.add("222");
           test.add("111");
           test.add("888");
           test.add("666");
           test.add("444");
              	
           if (test.contains("222"))
        	   System.out.println("enthaelt '222'");
           else
        	   System.out.println("enthaelt '222' nicht");
           
           if (test.contains("555"))
        	   System.out.println("enthaelt '555'");
           else
        	   System.out.println("enthaelt '555' nicht");
        }
        public static void TestgetFirst(){
            LinkedList test = new LinkedList();
            test.addFirst("777");
            test.add("222");
            test.add("111");
            test.add("888");
            test.PrintElement(test.getFirst());
        }
        public static void TestisEmpty(){
             LinkedList test = new LinkedList();
             test.addFirst("777");
             test.add("222");
             test.add("111");
             test.add("888");
             if (test.isEmpty())
		    System.out.println("leer");
	     else
		    System.out.println("nicht leer");
        }
        public static void Testsize(){
            LinkedList test = new LinkedList();     
            test.addFirst("777");
    	    test.add("222");
    	    test.add("111");
            System.out.println(test.size());

        }
	public static void TestRemove(){
		LinkedList test = new LinkedList();
		test.addFirst("777");
		test.add("222");
		test.add("111");
		test.Print();
		test.remove("111");
		System.out.println("after remove");
		test.Print();
	}
        public static void main(String[] args) {
             TestaddFirst();
             System.out.println("--------");
             TestAdd();
             System.out.println("--------");
             Testclear();
             System.out.println("--------");
             Testcontains();
             System.out.println("--------");
             TestgetFirst();
             System.out.println("--------");
             TestisEmpty();
             System.out.println("--------");
             Testsize();
             System.out.println("--------");
        
	}

}

 

[SlaterB]

head ist wahrscheinlich null, einfach zu testen mit
System.out.println("head ist: "+head);
am Anfang der Operation add()

 

[Beni]

   public void add(Object o){
       ListNode newNode = new ListNode();
      newNode.value = o;
      newNode.next = null;
      ListNode node = head;
      while (node.next != null) {
         node = node.next;
      }
      node.next = newNode;
   }

"head" ist null -> node ist null -> node.next wirft eine Exception. Du musst den Spezialfall prüfen, dass die Liste leer ist.

[Edit: zu spät, SlaterB die Rennschnecke gewinnt]