Broader exceptions thrown by methods that implement interfaces

[Sandra Bachan]

Question from Enthuware

Consider these two interfaces:

interface I1
{
void m1() throws IOException;
}
interface I2
{
void m1() throws SQLException;
}

What methods have to be implemented by a class that says it implements I1 and I2 ?

I selected

public void m1() throws Exception

which is wrong

Is this because m1() is throwing a broader Exception?

Then please explain why the class that implements interface I1 and I2 can throw Exception. Please see Enthuware's explanation below:

When interfaces are involved, more than one method declaration may be implemented by a single method declaration. In this case, the overriding declaration must have a throws clause that is compatible with ALL the overridden declarations. The declaration public void m1(){} satisfies both the declarations. Example...

interface I1{   void m1() throws IOException;}
interface I2{   void m1() throws SQLException;}
public class TestClass implements I1, I2
{
   public static void main(String args[])  throws Exception
   {
     TestClass tc = new TestClass();
     I1 i1 = (I1) tc; //This is valid.
     i1.m1();

     I2 i2 = (I2) tc; //This is valid too.
     i2.m1();

   }
   public void m1()
   {
     System.out.println("Hi there");
   }
}

[Ankit Garg]

Then please explain why the class that implements interface I1 and I2 can throw Exception.

A class never throws an exception, methods do. In the code the overriding m1 method is actually not throwing any exception. Thats the point. The reason for throwing Exception from the main method is completely different and has nothing to do with TestClass implementing I1 and I2...

[Abimaran Kugathasan]

Sandra Bachan wrote:
Is this because m1() is throwing a broader Exception?

Yes, and you are mixing two different things here. Declaring the main() method as it throws Exception and overriding the method m1() are two different things!

[Abimaran Kugathasan]

Oh, Ankit answered!

[Sandra Bachan]

Thank you