similar named methods in interface

[Fran Kindred]

I tried a simple test case of a class that implements 2 interfaces that have similar method signatures but different exceptions that are thrown.

I seem to have a problem compiling it.

However if I take out the exceptions that are thrown it seems to work. Can someone explain this.

[Barry Gaunt]

AFAIK the methods in the interfaces, if they have the same signatures, must be declared as throwing the same exceptions. And the class implementing the overridden method must declare that the method throws those exceptions (or a subclass of them).

[Win jones]

Hi Barry,
Can you confirm if it is mandatory for an overridden method in a subclass also to declare the exception thrown by the parent class?? Even though using a code sample this does not seems to be mandatory but I am remember reading about it somewhere? or a similar kind of requirement...

class A{
void method1() throws Exception {
}
}
 
class B extends A{
void method1(){
}
}

compiles fine

[Steven Bell]

In a subclass of a class you may declare that your overridden method throws the same exception as the superclass method, one or more subclasses of the exception, or no exception at all. You may not throw a checked exception that the superclass does not throw. It is generally a good idea to declare the throws even if you are not going to throw and exception, otherwise you restrict subclasses of your class in their ability to throw exceptions.

If you are implementing two interfaces with the same method signature other than the exceptions, you will be restricted to throwing only exceptions that both interfaces throw.

[Fran Kindred]

I'm still confused if you have 2 interface that have similar method signatures but throw different exceptions how do I handle it if I implement both.

I tried to throw IOException on one and IllegalArgument on another.

For some reason I only can throw IllegalArgument but if I put both on the overriden method I get compiler error.

[Barry Gaunt]

As I previously said, you will have to put both throws clauses into both interfaces' method declarations.

[Richard Vagner]

Can you post you code here?

[Steven Bell]

Are the signatures the same or are the similar.

If they are the same you cannot throw all the Exceptions that both interfaces throw.

You can only throw Exceptions that would be legal from both. I think an example would best explain.

public interface A{
  public Object someMethod() throws SQLException
}
 
public interface B{
  public Object someMethod() thows SAXException
}
 
public class C implements A, B{
  //this method cannot throw any checked Exceptions
  public Object someMethod(){
    //some code
  }
}
 
public interface D{
  public Object otherMethod() throws IOException
}
 
public interface E{
  public Object otherMethod() throws FileNotFoundException
}
 
public class F implements D, E{
  //FileNotFoundException is a subclass of IOException
  //this method may throw a FileNotFoundException or 
  //a subclass of FileNotFoundException
  public Object otherMethod() throws FileNotFoundException {
    //some code
  }
}

Think about it this way.

If you create an instance of class C as such

A a = new C();

When you call

a.someMethod()

it would be illegal for it to throw a SAXException while when

B b = new C();
b.someMethod()

it would be illegal for it to throw a SQLException.

[Fran Kindred]

Your example cleared it up. I thought its possible to throw both distinct exemptions once you implement both of them. I tried it but coul'dnt get it to compile but I understand the confusion the compiler is having. Thanks!!!