Java 8 - throw multiple generic checked exceptions in lambda

Question

In a project I am working at, I have found a class which wraps all methods of its super-class in some elaborate exception handling. It looks similar to that:

public void method1() throws ExceptionA {
    String exceptionString = "";
    try {
        super.method1();
    } catch (ExceptionA e) {
         exceptionString = // <convert the exception to string in an elaborate way>
         throw e;
    } finally {
         // <an elaborate logger call which uses value of exceptionString>
    }
}

public void method2() throws ExceptionB, ExceptionC {
    String exceptionString = "";
    try {
        super.method2();
    } catch (ExceptionB | ExceptionC e) {
         exceptionString = // <convert the exception to string in elaborate way>
         throw e;
    } finally {
         // <an elaborate logger call which uses value of exceptionString>
    }
}

// ... <a bunch of other methods like this>

I immediately though "Wow, how could would it be to have one generic wrapper and just call it in every of these methods. The class would be like 10x shorter!". So I got to work.

This is where I got stuck:

private interface ThrowingMethod<E extends Exception> {
    void run() throws E;
}

public <E extends Exception> void wrapMethod(ThrowingMethod<E> method) throws E {
    String exceptionString = "";
    try {
        method.run();
    } catch (Exception e) {
         exceptionString = // <convert the exception to string in an elaborate way>
         throw e;
    } finally {
         // <an elaborate logger call which uses value of exceptionString>
    }
}

public void method1() throws ExceptionA {
    wrapMethod(super::method1); // works
}

public void method2() throws ExceptionB, ExceptionC {
    wrapMethod(super::method2); // Error in Eclipse: "Unhandled exception type Exception"
}

// ... <a bunch of other methods like this>

In conclusion, this approach works for methods that throws only one type of checked exception. When method throws multiple checked exceptions, Java assumes that the exception type is Exception.

I tried to add more generic parameters to ThrowingMethod and wrapMethod but it doesn't change anything.

How can I get a functional interface to work with multiple generic exceptions?

I'd say one of the problems would be that your wrapMethod() would need to support multiple exceptions in its throws-clause in order to call it like in method2() and have that throw ExceptionB and/or ExceptionC. So you might need to provide overloads for more than one exception (probably not that many since IMO a method with 20 exceptions in its throws-clause is bad design anyways ). Since you'd need multiple wrapMethod()'s you'd probably need to extract the conversion and logging parts into separate methods - if possible. — Thomas
@Thomas I tried method with 5 generic exceptions instead of 1. It didn't change a thing. Wrapping method1 still works and wrapping method2 still defaults to Exception. So it won't work, unless you are suggesting that I should make a separate wrapper for each possible number of exceptions but this would defeat the point of this exercise which is the make the code shorter. — NO_NAME
You should cleanup the syntax. It’s interface ThrowingMethod<E extends Exception>, not interface <E extends Exception> ThrowingMethod. Further, there is no need to declare interfaces as static. That’s always implied. — Holger
Well, if all your methods have a different number of exceptions they throw then yes the exercise would be moot. If not then it might still be worth it, especially since the interface declarations aren't that huge and could be extracted into separate files (and reused in other situations as well). — Thomas

Holger Holger · Accepted Answer · 2018-09-28T16:30:49

When you expand your interface to use two type variables, i.e.

private static interface ThrowingMethod<E1 extends Exception,E2 extends Exception> {
    void run() throws E1, E2;
}

public <E1 extends Exception,E2 extends Exception>
void wrapMethod(ThrowingMethod<E1,E2> method) throws E1,E2 {
    // same as before
}

the rules regarding the type inference do not change and they are the same for both type variables. E.g. you can still use

public void method1() throws ExceptionA {
    wrapMethod(super::method1);
}

as before, as the compiler simply infers the same single exception type for both type variables.

For the method declaring two exceptions, it won’t pick up one for the first type variable and the other for the second; there is no rule which could tell the compiler which exception to use for which type variable.

But you can help the compiler out in this case, e.g.

public void method2() throws ExceptionB, ExceptionC {
    wrapMethod((ThrowingMethod<ExceptionB, ExceptionC>)super::method2);
}

which is the best you can get with this approach.

Java 8 - throw multiple generic checked exceptions in lambda

2 Answers