This noncompliant example shows a bug in Tomcat version 4.1.24, initially discovered by Reasoning \[[Reasoning 2003|AA. Bibliography#Reasoning 03]\]. The {{cardinality}} method was designed to return the number of occurrences of object {{obj}} in collection {{col}}. One valid use of the {{cardinality}} method  is to determine how many objects in the collection are {{null}}. However, because membership in the collection is checked using the expression {{obj.equals(elt)}}, a null pointer dereference is guaranteed whenever {{obj}} is {{null}}.

public static int cardinality(Object obj, final Collection col) {
  int count = 0;
  Iterator it = col.iterator();
  while (it.hasNext()) {
    Object elt = it.next();
    if ((null == obj && null == elt) || obj.equals(elt)) {  // null pointer dereference
      count++;
    }
  }
  return count;
}

public static int cardinality(Object obj, final Collection col) {
  int count = 0;
  Iterator it = col.iterator();
  while (it.hasNext()) {
    Object elt = it.next();
    if ((null == obj && null == elt) || 
        (null != obj && obj.equals(elt))) {
      count++;
    }
  }
  return count;
}

Dereferencing a {{null}} pointer can lead to a denial of service. For example, Java Web Start applications and applets particular to JDK version 1.6, prior to update 4, were affected by a bug that had some noteworthy security consequences. In some isolated cases, the application or applet's attempt to establish an HTTPS connection with a server generated a {{NullPointerException}} \[[SDN 2008|AA. Bibliography#SDN 08]\]. The resulting failure to establish a secure HTTPS connection with the server caused a denial of service: clients were temporarily forced to use an insecure http channel for data exchange. In multithreaded programs, null pointer dereferences can violate cache coherency policies and can cause resource leaks.

Null pointer dereferences can happen in path-dependent ways. Limitations of automatic detection tools can require manual inspection of code \[[Hovemeyer 2007|AA. Bibliography#Hovemeyer 07]\] to detect instances of null pointer dereferences. Annotations for method parameters that must be non-null can reduce the need for manual inspection by assisting automated null pointer dereference detection; use of these annotations is strongly encouraged.