How do I send email with the gmail API?

Question

I want to send email using the code in this tutorial. https://developers.google.com/gmail/api/quickstart/java

I get an exception:

Aug 01, 2016 1:34:08 PM com.google.api.client.util.store.FileDataStoreFactory setPermissionsToOwnerOnly
WARNING: unable to change permissions for everybody: C:\Users\johndoe\.credentials\gmail-java-quickstart.json
Aug 01, 2016 1:34:08 PM com.google.api.client.util.store.FileDataStoreFactory setPermissionsToOwnerOnly
WARNING: unable to change permissions for owner: C:\Users\johndoe\.credentials\gmail-java-quickstart.json
Exception in thread "main" java.lang.NullPointerException
    at java.io.Reader.<init>(Reader.java:78)
    at java.io.InputStreamReader.<init>(InputStreamReader.java:72)
    at GmailQuickstart.authorize(GmailQuickstart.java:68)
    at GmailQuickstart.getGmailService(GmailQuickstart.java:91)
    at GmailQuickstart.main(GmailQuickstart.java:99)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

This is the code snippet where I got the exception:

public static Credential authorize() throws IOException {
    // Load client secrets.
    InputStream in =
        GmailQuickstart.class.getResourceAsStream("/client_secret.json");

I changed the folder permissions to give full access to everyone, but that did not help. I also, changed the code to write to a folder outside of user directory and that also failed with the same exception.

I also used the full path for "in" as suggested here and that did not work.

I don't understand how to use the "grant" code snippet here & how it works.

How do I fix my problem ?