Tuesday, February 15, 2011

Collections.sort returns NullPointerException

NullPointerException (NPE) is a common but annoying runtime exception in production. 

Here is JDK documentation about null object or array
Thrown when an application attempts to use null in a case where an object is required. These include:
  • Calling the instance method of a null object.
  • Accessing or modifying the field of a null object.
  • Taking the length of null as if it were an array.
  • Accessing or modifying the slots of null as if it were an array.
  • Throwing null as if it were a Throwable value.
Collections.sort() method returns NPE with below stack trace:
Exception in thread "main" java.lang.NullPointerException
    at java.util.Arrays.mergeSort(Arrays.java:1144)
    at java.util.Arrays.sort(Arrays.java:1079)
    at java.util.Collections.sort(Collections.java:117)

The reason is List, Set, HashMap etc allows null value which is ok to build the list/map/set, but will throw exception if we do sort using Collections.sort() method.

Here is one example about HashMap.

        HashMap<String, String> testMap = new HashMap<String, String>();
        testMap.put(null, "apple");
        testMap.put("key2", "orange");
        testMap.put("key3", "pear");
        List<String> keylist = new ArrayList<String>(testMap.keySet());
        List<String> valuelist = new ArrayList<String>(testMap.values());   
        Collections.sort(keylist); // this will throw NPE
        Collections.sort(valuelist); // this will not throw NPE

However, for Hashtable, putting null to the map will throw NPE, for instance
        Hashtable<String, String> testTable = new Hashtable<String, String>();
        testTable.put(null, "1"); //this will throw NPE
        testTable.put(null, "2");
        testTable.put("key3", "3");        

Exception in thread "main" java.lang.NullPointerException
    at java.util.Hashtable.put(Hashtable.java:399)

Here is the difference between Hashtable and HashMap

  HashMap Hashtable
Thread-safe N Y
Key nullable Y N
Better performance Y N

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