Java中的ExceptionInInitializerError

Question

I was doing some tests from this page: http://scjptest.com/mock-test.xhtml?execution=e7s1 and there was a question "What is the result when this program is executed?". Here is code:

public class SuperHotel {
    static int x[];

    static {
        x[0] = 1;
    }

    public static void main(String args[]) {        
    }   
}

Answer is "ExceptionInInitializerError is thrown". Can somebody say me when "ExceptionInInitializerError" may happend and why it happend in this code?

Answer 1

An ExceptionInInitializerError is thrown to indicate that an exception occurred during evaluation of a static initializer or the initializer for a static variable.

In your code, you have not initialized the array x[] , and you are using x[0]=1 in a static block, hence the exception occurred.

Answer 2

Signals that an unexpected exception has occurred in a static initializer. An ExceptionInInitializerError is thrown to indicate that an exception occurred during evaluation of a static initializer or the initializer for a static variable.

The error means that the class failed to initialize.

In this case, you have a NullPointerException in your static initializer block.

x[0] = 1;  // NPE, because x is null

Answer 3

According to Javadoc ( https://docs.oracle.com/javase/7/docs/api/java/lang/ExceptionInInitializerError.html ) is thrown, when an Exception occurs in an static initializer Block.

when you assing x[0] = 1 your x-Array is only a reference to an array. It is not yet created and therefore a NullpointerExeption occurs in your static initializer Block, which leads to an ExceptionInInitializerError.