java - Is it possible to tell the compiler that a method always throws an Exception

Question

Generally speaking, the Java compiler does not propagate the information that a method "always" throw an Exception, and therefore, does not detect that all code paths are complete.

(This is due to the fact that Java compiler compiles each class independently).

It's a problem when you want to write something like that.

public class ErrorContext?{
    public void fatalISE(String message)?{
        String context = "gather lots of information about the context of the error";
        throw new IllegalStateException(context +": " + message);
    }
}

public class A {
    public MyObject myMethod()?{
        if (allIsGood())?{
            return new MyObject();
        }
        ErrorContext.fatalISE("all is not good");
    }
}

(ie, a kind of "assertion helper" that gathers context information).

Because the compiler will complain that myMethod does not always return a MyObject.

To my knowledge, there is no specific annotation to indicate that a method always throws.

Answer 1

A simple workaround is to let your fatalISE method not throw the exception, but only create it:

public class ErrorContext {
    public IllegalStateException fatalISE(String message) {
        String context = "gather lots of information about the context of the error";
        return new IllegalStateException(context +": " + message);
    }
}

public class A {
    public MyObject myMethod() {
        if (allIsGood()) {
            return new MyObject();
        }
        throw ErrorContext.fatalISE("all is not good");
    }
}

This way the compiler will know not to complain about a missing return. And forgetting to use the throw is unlikely, exactly because the compiler will usually complain.