Question

I am getting NullPointerException in the following code :

private Map<String,List<Entry>> Days;
private void intializeDays() {
    //Iterate over the DayOfWeek enum and put the keys in Map
    for(DayOfWeek dw : EnumSet.range(DayOfWeek.MONDAY,DayOfWeek.SUNDAY)){
    List<Entry> entries = null;
    Days.put(dw.toString().toLowerCase(),entries);
    }
}

I think its because of

List<Entry> entries = null;

but then how do I create an empty list and add it to a map ?

Answer 1

You must initialize your map:

private Map<String,List<Entry>> Days = new HashMap<>();

Note that you can use

List<Entry> entries = new ArrayList <Entry> ();

and add to the map, instead of adding a null.

About NullPointerException

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.
Applications should throw instances of this class to indicate other illegal uses of the null object.

Since you did not initialize you Map object when you did this:

Days.put(dw.toString().toLowerCase(),entries);

you get NullPointerException because you are “Accessing or modifying the field of a null object.”.