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 with 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;
}

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.

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. A {{NullPointerException}} was generated in some isolated cases when the application or applet attempted to establish an HTTPS connection with a server \[[SDN 2008|AA. Bibliography#SDN 08]\]. The 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.