Getting Error "Exception in thread "main" java.lang.ExceptionInInitializerError"

Question

I am trying to initialize HashMap with Enum Month. However, getting an error like: "Exception in thread "main" java.lang.ExceptionInInitializerError". Please let me know why I am getting this error. I am getting this in static main class method and for line ChatGroups.initializeMap();

package static_initialization;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class ChatGroups {
    public static final Map<String, Month> months = new HashMap<String, Month>() {{
        months.put("Jan", Month.January);
        months.put("Feb", Month.February);
        months.put("Mar", Month.March);
        months.put("Apr", Month.April);
        months.put("Jun", Month.June);
        months.put("Jul", Month.July);
        months.put("Aug", Month.August);
        months.put("Sep", Month.September);
        months.put("Oct", Month.October);
        months.put("Nov", Month.November);
        months.put("Dec", Month.December);
    }};

    public static void initializeMap() {
        System.out.println();
    }
}


package static_initialization;

public enum Month {
    January(0),
    February(1),
    March(2),
    April(3),
    May(4),
    June(5),
    July(6),
    August(7),
    September(8),
    October(9),
    November(10),
    December(11);

    private int month;

    Month(int i) {
        month = i;
    }
}


package static_initialization;

public class Main {
    public static void main(String[] args) {
        initializeMaps();
    }
    public static void initializeMaps() {

        try {
            ChatGroups.initializeMap();
        } catch (Exception ex) {
            System.out.println("Neelabh exception = " + ex);
        }
    }
}

Please find the stack trace

Exception in thread "main" java.lang.ExceptionInInitializerError
    at static_initialization.Main.initializeMaps(Main.java:10)
    at static_initialization.Main.main(Main.java:5)
Caused by: java.lang.NullPointerException
    at static_initialization.ChatGroups$1.<init>(ChatGroups.java:9)
    at static_initialization.ChatGroups.<clinit>(ChatGroups.java:8)

please share full stacktrace, the error message is just a part of it. The full stack is usually giving a lot more information — Wisthler
– Wisthler, Commented May 27, 2019 at 5:47
Use just put instead of months.put in the static initializer — rdas
– rdas, Commented May 27, 2019 at 5:51
@rdas can you explain why I need this.put instead of months.put. — Neelabh Singh
– Neelabh Singh, Commented May 27, 2019 at 5:53
During static initialization, the months pointer is not bound to the map object you are defining yet. So it's still null when that code runs. — rdas
– rdas, Commented May 27, 2019 at 5:54

Eran · Accepted Answer · 2019-05-27 05:52:47Z

It should be:

public static final Map<String, Month> months = new HashMap<String, Month>() {{
    put("Jan", Month.January);
    put("Feb", Month.February);
    put("Mar", Month.March);
    put("Apr", Month.April);
    put("Jun", Month.June);
    put("Jul", Month.July);
    put("Aug", Month.August);
    put("Sep", Month.September);
    put("Oct", Month.October);
    put("Nov", Month.November);
    put("Dec", Month.December);
}};

months.put() leads to NullPointerException, since the static variable months is still null when the put statements are executed.

As an alternative, avoid creating an anonymous class instance, and instead write:

public static final Map<String, Month> months = new HashMap<String, Month>();
static {
    months.put("Jan", Month.January);
    months.put("Feb", Month.February);
    months.put("Mar", Month.March);
    months.put("Apr", Month.April);
    months.put("Jun", Month.June);
    months.put("Jul", Month.July);
    months.put("Aug", Month.August);
    months.put("Sep", Month.September);
    months.put("Oct", Month.October);
    months.put("Nov", Month.November);
    months.put("Dec", Month.December);
}

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