Question

I have two versions of code. In the first version type of exception which is throwed in Method() – NullPointerException, in the second version – Exception. However first version will compile but second won’t compile. Why is this happen?

public class Demo
{
    static void Method()
    {
        try
        {
            throw new NullPointerException("error");
        }
        catch(Exception ex)
        {
            throw ex;
        }
    }
    public static void main(String argv[])
    { 
        try
        {
            Method();
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
}

This is the second version.

public class Demo
{
    static void Method()
    {
        try
        {
            throw new Exception("error");
        }
        catch(Exception ex)
        {
            throw ex;
        }
    }
    public static void main(String argv[])
    { 
        try
        {
            Method();
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
}

stack trace for first version:

java.lang.NullPointerException: error
    at Demo.Method(Demo.java:7)
    at Demo.main(Demo.java:18)

Answer 1

This is because NullPointerException is a so-called “unchecked” exception. You don’t need to declare them in the throws clause.

However, a casual Exception is not unchecked, and you do need to declare in a throws declaration. You need to make Method() throw Exception in your second code snippet.

Unchecked exceptions are RuntimeException, Error and derivate classes. NullPointerException derivates from RuntimeException.