Errata - Sybex 829 - Chapter 6

[João Lopes]

Hi,

I think there is an errata on Chapter 6, page 309 on the TIP box.

This rule applies to primitive types and object types alike.

You can't override a method that returns double with a method that returns int.

for example:
On class A:

    public double example() {
        return 1;
    }

On class B that extends class A:

    @Override
    public int example() {
        return 1;
    }

The method on class B will not compile.

Thanks!
João Lopes

[João Lopes]

I think I found another one on page 316:

An abstract method can only be declared in an abstract class.

Shoudn't it be "An abstract method can only be declared in an abstract class or an interface." ?

Thanks,
João Lopes

[Stephan van Hulst]

Maybe in place of writing abstract class, they can just write abstract type. Interfaces are implicitly abstract.

[João Lopes]

João Lopes wrote:I think I found another one on page 316:

An abstract method can only be declared in an abstract class.

Shoudn't it be "An abstract method can only be declared in an abstract class or an interface." ?

Thanks,
João Lopes

Regarding this you have the same reference on the Summary topic but this time includes the interface.

An abstract method can only be placed inside an abstract class or interface.

But on the Exam Essentials topic is missing mentioning interfaces again:

Abstract methods follow all the method override rules and may be defined only within abstract classes

Thanks,
João

[Jeanne Boyarsky]

João Lopes wrote:Hi,

I think there is an errata on Chapter 6, page 309 on the TIP box.

This rule applies to primitive types and object types alike.

You can't override a method that returns double with a method that returns int.

for example:
On class A:

    public double example() {
        return 1;
    }

On class B that extends class A:

    @Override
    public int example() {
        return 1;
    }

The method on class B will not compile.

Thanks!
João Lopes

The book doesn't say it does. Just that you can't mix a primitive and void.

[Jeanne Boyarsky]

João Lopes wrote:I think I found another one on page 316:

An abstract method can only be declared in an abstract class.

Shoudn't it be "An abstract method can only be declared in an abstract class or an interface." ?

Thanks,
João Lopes

While this is true, the text you quoted is in chapter 6 and interfaces are covered in chapter 7.  So true as of point in time.

[Jeanne Boyarsky]

I will make a note about consistency. Odd we included it in the one place.

[João Lopes]

Jeanne Boyarsky wrote:

João Lopes wrote:Hi,

I think there is an errata on Chapter 6, page 309 on the TIP box.

This rule applies to primitive types and object types alike.

You can't override a method that returns double with a method that returns int.

for example:
On class A:

    public double example() {
        return 1;
    }

On class B that extends class A:

    @Override
    public int example() {
        return 1;
    }

The method on class B will not compile.

Thanks!
João Lopes

The book doesn't say it does. Just that you can't mix a primitive and void.

The book on the TIP box says the following:

A simple test for covariance is the following: given an inherited return type A and an overriding return type B, can you assign an instance of B to a reference variable for A without a cast? If so, then they are covariant. This rule applies to primitive types and object types alike. If one of the return types is void, then they both must be void, as nothing is covariant with void except itself.

Isn't it saying that the test for covariance described applies to primitive types and object types? Or I interpreted it wrong?

Regards,
João

[Jeanne Boyarsky]

We didn't mean primitive and object types together though.