为什么异常类型会影响编译的内容，而不会影响编译的内容呢？

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 .

Answer 2

NullPointerException is a RunTimeException (or un-checked) and does not have to be handled at compile time. Exception , however, is considered a checked exception and must be either caught or thrown by any method which encounters it.

The second snippet can be fixed by changing the method declaration as follows:

static void Method() throws Exception

Unchecked Exceptions - The Controversy

Answer 3

Since your method "Method()" throws Exception , you have to declare it on the method signature:

static void Method() throws Exception

And it will compile. Cheers!

Answer 4

Try this as second example:

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

This should compile fine,

Answer 5

The code of Version 1 does not compile in java version 6 or lower, i guess you are using java 7.

Java 7 looks at compiletime witch exact exception type is beeing thrown. in case 1 it is a runtime exception witch musst not appear in the signatur of the method.