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());   
       
        System.out.println(keylist);
        System.out.println(valuelist);
       
        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