Question

The class partially shown below contains a main method. When I run the code, I see a NullPointerException (NPE) and then an error message – “Could not find the main class, program will exit”. My understanding is that if I get NPE, it means that the code is running, ie the JRE found a main method to begin execution, so why do I get the error message?

This is the console output

java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException
at com.MyWorldDemo.getValue(MyWorldDemo.java:57)
at com.MyWorldDemo.<clinit>(MyWorldDemo.java:23)
Exception in thread "main"  

In a nutshell:

• username is stored in a properties file.
• properties file is like this username=superman….etc

here is some code example

class MyClass {
    private final static String username = getData("username"); // ERROR HERE

    private static Properties prop;
    // more variables

    static {
        prop = new Properties();
        try {
            FileInputStream fis = new FileInputStream("MyDB.properties");
            prop.load(fis);
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    // this method will assign a value to my final variable username.
    public static String getData(String props) {
        String property = prop.getProperty(props);// ERROR HERE !!!
        return property;
    }
}

Answer 1

Initializing of static variables depends on its position in code (variables are initialized from top to bottom). In your code

private final static String username = getData("username"); // ERROR HERE
private static Properties prop;
// more variables

static {
    prop = new Properties();
    try {
        FileInputStream fis = new FileInputStream("MyDB.properties");
        prop.load(fis);
    } catch (IOException ex) {
        ex.printStackTrace();
    }
}

prop object will be initialized after username in static block, but since to initialize username prop is necessary and its not initialized yet you get NPE. Maybe change your code to something like:

private static Properties prop = new Properties();
private final static String username = getData("username"); 

static {

    try {
        FileInputStream fis = new FileInputStream("MyDB.properties");
        prop.load(fis);
    } catch (IOException ex) {
        ex.printStackTrace();
    }
}