Sushant Kaushik

Hello Ranchers,

Today I gave my SCJP 5 and cleared it with 97% marks.
Thanks to all of you for being such a great help and last but not the least many thanks to K & B for such a wonderful book.

Cheers,
Sushant

Thanks Ruben, Its clear now

This code is from ExamLab-

class A { public static void main(String[] args) { A a = new B(); B b = new B(); C c = new C(); C[] ca = null; a = b; b = c; A a1 = (B) c; Runnable r1 = (Runnable)a; Runnable[] ra1 = null; A r2 = (A) r1; A[] aa2 = (A[]) ra1; ra1 = (Runnable[])ca; //Line 1 } } class B extends A { } final class C extends B { }

Line1 is giving compilation error.

When I am removing final keyword from class C then there is no compilation error. How final keyword is affecting Line1.

Thanks,
Sushant

Thanks Henry for the explanation. This makes sense.

Thanks Sunil for the prompt response.

But List<Integer> can be assigned to both List<? super Integer> & List<? extends Integer>. Is it because that List<Number> can't hold reference of List<Integer> and that is the reason its complaining?

Hello Rancher,

This is a test program.

Why the below line of code doesn't compiles?

List<? super Integer> l1 = new ArrayList<Integer>(); List<? extends Integer> l2 = new ArrayList<Integer>(); l1 = l2;

Thanks,
Sushant

Punit Singh wrote:

This is confusing. As far as i know Protected means Available to subclasses even outside Package. If outside pacakage then we can access the protected members via inheritance. Thats what i am doing. Isn't it? Then why compiler error?

This means you can use protected member using instance of subclass only, not the instance of superclass.
As protected member is inheritable, you can use thinking it as a member of subclass, but not a member of superclass if there is a package difference.

subclassinstance.protectedmember;//ok as subclass has this member via inheritence.
superclassinstance.protectedmember;//compiler error as accessing super class protected member outside the package.

@Punit, in the given program above, pack is an instance of SuperClass, then how compiler is not complaining when we are trying to access x2 from subclass using reference of SuperClass?

Thanks all. My doubt got clarified

This is test Program -
class TestInteger { public static void main(String[] args) { Integer i = new Integer(10); Integer j =i; j = j.intValue(); //Line 1 System.out.println(j == i); } }

Output is - false

If I comment Line 1 then output is - true.

Can someone please explain why output is false when Line1 is not commented :roll:
What is the type of j after Line 1? is it still Integer or int? If its Integer then I can understand that output will be false. If its int then Output should be true. correct??

Got it...Thanks Henry & Alex..

Oops...I missed that...but still in Class B we can assume that runNow() method is overrideen even though its private in class A, isn't it??

This is from Diagnostic Test from Exam Lab -

public class A{ private String runNow(){ return "High"; } static class B extends A{ public String runNow(){ return "Low"; } } public static void main(String args[]){ A[] a=new B[]{new B(),new C()}; for(A aa:a) System.out.print(aa.runNow()+" "); } } class C extends A.B{ public String runNow(){ return "Out"; } }

Output is - High,High

I though the output will be Low,Out as run time Polymorphism will come into picture and runNow method in B & C would have been called respectively.

Thanks in advance.

Thanks Shweta

Thanks Arie..now it got clarified

Source - K &B chapter 6, Page 455

class SuperNotSerial{ public static void main (String[] args) { Dog d = new Dog(35, "Fido"); System.out.println("before: " + d.name + " " + d.weight); try { FileOutputStream fs = new FileOutputStream("testSer.ser"); ObjectOutputStream os = new ObjectOutputStream(fs); os.writeObject(d); os.close(); } catch (Exception e) { e.printStackTrace(); } try { FileInputStream fis = new FileInputStream("testSer.ser"); ObjectInputStream ois = new ObjectInputStream(fis); d = (Dog) ois.readObject(); ois.close(); } catch (Exception e) { e.printStackTrace(); } System.out.println("after: " + d.name + " "+ d.weight); } } class Dog extends Animal implements Serializable { String name; Dog(int w, String n) { weight = w; // inherited name = n; // not inherited } } class Animal{ transient int weight = 42; }

The Output is -
before: Fido 35
after: Fido 42

This is fine as Animal class didn't implement Serializable interface so weight got its intialized value of 42.
My question is if I modify Animal class as -
class Animal implements Serializable{ transient int weight = 42; }
The output is
before: Fido 35
after: Fido 0

Can somebody please explain why in this case I am getting 0 for weight instead of 42?

Thanks,
Sushant