Whats wrong about this code ?

[Kaxhif Khan]

Hi, i am preparing for OCPJP exam (iz0-804) from Apress's "A comprehensive OCPJP7 certification guide". I found this statement in Chapter 11 (Exceptions and Assertions)

"If a method is declared in two or more interfaces, and if that method declares to throw different exceptions in the throws clause, the implementation should list all of these exceptions". I decided to practically check and see what happens

interface A{
	public void doNothing() throws FileNotFoundException;
}
interface B{
	public void doNothing() throws ParseException;
}

class AA implements A, B{



	@Override
	
        //Method Signature Line
        public void doNothing() throws ParseException,FileNotFoundException{
		// TODO Auto-generated method stub
		
	}
	
}

1 = public void doNothing(){
2= public void doNothing() throws ParseException{
3 = public void doNothing() throws FileNotFoundException{
4 = public void doNothing() throws ParseException,FileNotFoundException{
5 = public void doNothing() throws FileNotFoundException, ParseException{

I replace method sigature line with options from 1 -5 and below is what happened
1, = gave no compilation error
2 = gave compile error saying "doNothing() is not compatible with the throws clause in A.doNothing()
3 = gave compile error saying "doNothing() is not compatible with the throws clause in B.doNothing()
4 = gave compile error saying "doNothing() is not compatible with the throws clause in A.doNothing()
5 = gave compile error saying "doNothing() is not compatible with the throws clause in B.doNothing()

What wrong here ? Is the statement in the book wrong or my understanding is wrong ? How can one remove compilation error if he/she face situation like i placed above ...

[Girish Bal]

Having Generic Exception at interface level and Specific one at implementation level solves the issue. But I am not sure whether you are looking at such a solution.

This compiles fine:

import java.io.FileNotFoundException;
import java.text.ParseException;

interface A{
    public void doNothing() throws Exception;
}
interface B{
    public void doNothing() throws Exception;
}
 
public class AB implements A, B {
 
    @Override
    public void doNothing() throws FileNotFoundException, ParseException{
    	
         
    }
     
}

I am using Java 7.

[Mikalai Zaikin]

Some points:
1) I would not expect such ambiguous questions on real exam - same method on several interfaces, and both implemented by same class
2) Regarding this piece: "If a method is declared in two or more interfaces, and if that method declares to throw different exceptions in the throws clause, the implementation should list all of these exceptions" I would say that it's wrong.
General rule which needs to be remembered: "Implementation MAY list [checked] exception declared in the interface, OR the exception's subclasses, but NOT their superclasses".

Quick example:

interface MyIntf {
    void doIt() throws IOException;
}

// GOOD
class Class1 implements myIntf {
    public void doIt() throws FileNotFoundException { // subclass of IOException
        ...
    }
}

// BAD
class Class2 implements myIntf {
    public void doIt() throws Exception { // superclass of IOException
        ...
    }
}

// GOOD
class Class3 implements myIntf {
    public void doIt() { // we can choose not to throw anything
        ...
    }
}

// GOOD
class Class4 implements myIntf {
    public void doIt() throws IOException { // same as in the interaface - obviously OK
        ...
    }
}

// GOOD
class Class5 implements myIntf {
    public void doIt() throws ArrayIndexOutOfBoundsException { // the "subclass" rule is only for checked exception, runtime exceptions may be declared in any combination in any implementation method
        ...
    }
}

[Roel De Nijs]

Kaxhif Khan wrote:"If a method is declared in two or more interfaces, and if that method declares to throw different exceptions in the throws clause, the implementation should list all of these exceptions". I decided to practically check and see what happens

Really like your attitude! Writing little code snippets to see what's happening, is always a good thing to do as it's very instructive.

Kaxhif Khan wrote:What wrong here ? Is the statement in the book wrong or my understanding is wrong ? How can one remove compilation error if he/she face situation like i placed above ...

The statement in the book is completely wrong!

Only if both method declarations in the interfaces define unchecked (runtime) exceptions, the statement in the book is true. Example:

interface I1 {
    void method() throws IllegalArgumentException;
}
interface I2 {
    void method() throws ArrayIndexOutOfBoundsException;
}
class A12A implements I1,I2 {
    public void method() throws IllegalArgumentException, ArrayIndexOutOfBoundsException {}
}

But as you know, with runtime exceptions you don't have to list the exceptions in the method declaration, so this will compile as well:

class A12B implements I1,I2 {
    public void method() {}
}

If both method declarations in the interfaces define a checked exception from a different class hierarchy (like in your original post), your code will only compile if the implementing method defines no checked exceptions in the throws clause (as you discovered yourself):

interface I3 {
    void method() throws InterruptedException;
}
interface I4 {
    void method() throws FileNotFoundException;
}
class A34A implements I3,I4 {
    public void method() {}
}

Every other combination of checked exception(s) will NOT compile (and that's why the statement in the book is completely wrong). As you know the throws clause of the implementing method can list as many runtime exceptions as you want:

class A34B implements I3,I4 {
    public void method() throws IllegalArgumentException, ArrayIndexOutOfBoundsException {}
}

If both method declarations in the interfaces define a checked exception from the same class hierarchy (like in your original post), your code will only compile if the implementing method defines no checked exceptions in the throws clause or the most specific checked exception (or one of its subclasses). A few examples to illustrate:

interface I5 {
    void method() throws Exception;
}
interface I6 {
    void method() throws IOException;
}
class A56A implements I5,I6 {
    public void method() throws IllegalArgumentException {}
}
class A56B implements I5,I6 {
    public void method() throws IOException {}
}
class A56C implements I5,I6 {
    public void method() throws FileNotFoundException {}
}
class A56D implements I5,I6 {
    public void method() throws Exception {}
}

Class A56A compiles because no checked exceptions are listed in the throws clause. Class A56B compiles as well, because IOException IS-A Exception (and the most specific exception). Class A56C compiles too, because FileNotFoundException IS-A IOException (and a subclass of the most specific exception). And finally class A56D will not compile, because Exception isn't the most specific exception.

What if the method declaration in the first interface defines a checked exception and the method declaration in the second interface defines a runtime exception? Your code will only compile if the implementing method defines no checked exceptions in the throws clause.

That was easy, wasn't it?

Hope it helps!
Kind regards,
Roel

[Roel De Nijs]

Girish Bal wrote:Having Generic Exception at interface level and Specific one at implementation level solves the issue. But I am not sure whether you are looking at such a solution.

Your code snippet is totally different than the code in the original post as your code list exactly the same method declaration in both interfaces. So you can't compare them at all.

[Girish Bal]

Roel De Nijs wrote:

Girish Bal wrote:Having Generic Exception at interface level and Specific one at implementation level solves the issue. But I am not sure whether you are looking at such a solution.

Your code snippet is totally different than the code in the original post as your code list exactly the same method declaration in both interfaces. So you can't compare them at all.

It was the change I made to the original code to get it work. That's why it is "totally different" ...

[Kaxhif Khan]

Thanks all for quick and informative feedback, i tried earlier to post reply but i always got "aah you got us with our pants down ==> Busy Server" ...!!