Exceptions thrown in interfaces and their implementations

[Ioan Damian Sirbu]

Hello everyone,

I am trying to understand a situation that occurs when implementing a method in an interface and throwing a wider exception type.

Declaring an exception in SudetImpl does not compile and it's OK,because the overriding method can NOT declare wider exceptions

public interface Student {
    void speak();
}

public class StudentImpl implements Student {
    public void speak() throws Exception {
        System.out.println("Speaking...");
    }
}

The weird thing comes next: when StudentImpl.speak() throws RuntimeException, the code compiles

public interface Student {
    void speak();
}

public class StudentImpl implements Student {
    public void speak() throws RuntimeException {
        System.out.println("Speaking...");
    }

In my opinion, there is absolutely no dfference - StudentImpl.speak() throws RuntimeException, while Student.speak() does not.
It should throw a compiler error... and yet, it does not


If someone could enlighten me regarding this, I would be really grateful.

Thanks

[Jesper de Jong]

That's because RuntimeException (and any subclass of RuntimeException) is an unchecked exception. You don't even have to declare that your method throws RuntimeException - you can even throw it from inside the method without declaring it.

See Lesson: Exceptions in Sun's Java Tutorials to learn about checked and unchecked exceptions.

[Harpreet Singh janda]

This is not required to catch RunTimeException. Your code will compile fine even if you are not catching this exception or subclass of this exception.

[Prasad Kharkar]

RuntimeException can be thrown anything because that is an unchecked exception and Unchecked Exceptions need not be declared or caught
the difference between checked and unchecked exceptions:
Checked : Compiler checks whether the exceptions thrown are caught in the program somewhere or not
Unchecked: Compiler does not check the exceptions are caught somewhere or not, means they occur at runtime , like ArrayIndexOutOfBoundsException

[Ioan Damian Sirbu]

Thank you, I suspected it but I was not sure.
So basically, adding an unchecked exception in a method's declaration has absolutely no effect.

[Seetharaman Venkatasamy]

Damian Sarbu wrote:So basically, adding an unchecked exception in a method's declaration has absolutely no effect.

Exactly. and you *shouldn't*

[Ioan Damian Sirbu]

I would expected that the compiler would give at least a warning

Thank you all for your help!