抛出异常时编译错误？

Question

What is wrong with this code?

public class Mocker<T extends Exception> {
    private void pleaseThrow(final Exception t) throws T{
        throw (T)t;
    }
    public static void main(String[] args) {
        try{
            new Mocker<RuntimeException>().pleaseThrow(new SQLException());
        }
        catch (final SQLException e) {
            // TODO: handle exception
            e.printStackTrace();
        }
    }
}

As in pleaseThrow method throws SQLException still it gives Compilation error.

Error :

Unreachable catch block for SQLException. This exception is never thrown from the try 
 statement body

Answer 1

The problem is because you are throwing a RuntimeException , but trying to catch SQLException .

In your method,

private void pleaseThrow(final Exception t) throws T{
    throw (T)t;
}

You are casting the argument SQLException in your case to T (which is RuntimeException in your case and throwing it.

So, the compiler is expecting a RuntimeException to be thrown and not SQLException .

Hope this is clear.

Answer 2

you would be able to write this when your pleaseThrow method actually throws SQLException .

Currently what you are doing is just passing a object of type SQlExcetion as parameter to this method.

Currently what Compiler observes is you are calling a method and it does not throw any SQLException , so compiler deems the catch clause as a problem and shows this compilation problem

Answer 3

Your pleaseThrow() does not throw an SQLException . You have some choices: make your catch to catch a generic Exception

        catch (final Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }

or make pleaseThrow(..) actually throw an SQLException

    private void pleaseThrow(final Exception t) throws SQLException{
        throw (SQLException)t;
    }

or actually throw an SQLException

new Mocker<SQLException>().pleaseThrow(new SQLException());