Re: Thinking in Java 4th edition - help with inner class exercise
cy wrote:
Trying to solve this exercise: (Chapter Innerclasses, Exercise 6,
page 353)
Create an interface with at least one method, in its own package.
Create
a class in a separate package. Add a protected inner class that
implements the interface. In a third package, inherit from your class
and
inside a method, return an object of the protected inner class,
upcasting to the interface during the return.
The following solution compiles and runs, but I had to use a public
method to instantiate the protected class in the 2nd package. Is
You don't actually instantiate the inner class in the second class (not
package), nor should you.
there a solution using the protected class directly? Shouldn't a
child class have access to protected members, including inner class
members, of parent class from another package.
_____________________________
/* // in first package:
* public interface Ex6Interface {
* String say();
* }
*
* // and in second package:
* public class Ex6Base {
* protected class Ex6BaseInner implements Ex6Interface {
* public String say() { return "Hi"; }
* }
* public Ex6BaseInner getEx6BaseInner() {
The instructions say
In a third package, inherit from your class
and
inside a method, return an object of the protected inner class,
upcasting to the interface during the return.
You did this in the second package. Furthermore, your code for the second
class doesn't show the "import" statement.
* return new Ex6BaseInner();
* }
* }
*/
// in third package:
import innerclasses.ex6Interface.*;
Where was this import in the second class?
import innerclasses.ex6Base.*;
public class Ex6 extends Ex6Base {
private Ex6Base e6b;
You're inheriting from Ex6Base. Why also include an instance variable of the
type?
Here's your problem - you are creating a second object of the Ex6Base type
instead of using 'this'.
Ex6() {
super();
You completely do not need this - the super() call is always implicitly present.
e6b = new Ex6Base();
You completely do not need this. Ex6 /is-a/ Ex6GBase already.
}
Ex6Interface getBaseInner() {
// Error: Ex6BaseInner has protected access:
// return e6b.new Ex6BaseInner();
Sure. You aren't referring to this.Ex6BaseInner, but e6b.Ex6BaseInner. You
need to use 'this'.
// to create protected class use public method:
return e6b.getEx6BaseInner();
}
public static void main(String[] args) {
Ex6 ex = new Ex6();
// Error: Ex6BaseInner has protected access:
// Ex6Base.Ex6BaseInner ebi = ex.new Ex6BaseInner();
// new Ex6Base().new Ex6BaseInner();
Don't post comments pretending to be code. Post code. How can we see the
errors you get if you paraphrase, rephrase, comment-out or otherwise corrupt
the example?
System.out.println(ex.getBaseInner().say());
}
}
Inheritance - extended type /is-a/ base type
Composition - new type /has-a/ other type
Your bug came from using composition when the exercise called for inheritance.
--
Lew