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mock exam query

 
Ranch Hand
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hi,
Following are queries from mock exam on go4java.20msite.
Which one of the following statements will give you an Exception when
placed at line no.10?Select one correct answer.

1 class AA{}
2 class BB extends AA{}
3 class casting
4 {
5 public static void main(String arg[])
6 {
7 AA a=new AA();
8 BB b=new BB();
9 AA c=new BB();
10 //Here
11 }
11 }

1. b = (BB) a;
2. a = b ;
3. b = (BB) c;
4. c = b;

class Q20
2 {
3 public static void main(String arg[])
4 {
5 Q20 e=null;
6 System.out.println(e.equals(null));
7 Q20 e1;
8 System.out.println(e1.equals(null));
9 }
10 }

1. Error at line no. 6
2. Excepetion at line no. 6
3. Error at line no. 8
4. Excepetion at line no. 8

What will be the result of compiling and running the given program?
Select one correct answer.
1 class initialize
2 {
3 int y=0;
4 {
5 y=1;
6 }
7 initialize()
8 {
9 this(10);
10 y=3;
11 }
12 initialize(int s)
13 {
14 y=2;
15 }
16 public static void main(String arg[])
17 {
18 initialize i=new initialize();
19 System.out.println(i.y);
20 }
21 }


1. Compile time error
2 Program compiles correctly and prints 1 when executed.
3. Program compiles correctly and prints 2 when executed.
4. Program compiles correctly and prints 3 when executed.
5. Program compiles correctly and prints 0 when executed.

What is the answer?
Thankyou
 
Greenhorn
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For your second question the answer is option number 4
explanation:
first constructor initialize() is called and then this(10) call the constructor initialise(int) and then at last y=3 is executed.

So the output is 3
 
Ranch Hand
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Hi Ketki

Following is the explanation with answers -

Q1 -
1. b = (BB) a; is the answer.

explanation -
Here a super class reference is being assigned to child class reference (downcasting).
As per rules downcasting can be done only after Upcasting.
i.e. if b = (BB) a; is written after a = b; then no exception will be generated.

Q2 -
3. Error at line no. 8 is the answer.
explanation -
Here, 'e1' is not initialized yet and we are trying to call .equals(), thus its a compiler error.
Line# 6 is ok as 'e' is initialized to 'null'.

Q3 -
'4. Program compiles correctly and prints 3 when executed'. is the correct answer.
explanation -
First the initiazer block { y=1; } is called, then the parameterized constructor is called and finally non parameterized constructor is called and i.y becomes 3.

Hope this clarifies your doubts.
Cheers..
 
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Originally posted by Mohit Jain:
Q2 -
3. Error at line no. 8 is the answer.
explanation -
Here, 'e1' is not initialized yet and we are trying to call .equals(), thus its a compiler error.
Line# 6 is ok as 'e' is initialized to 'null'.



Since we don't know the exact question, 2 and 3 could be the answer.
 
Greenhorn
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For the second question,
class Q20 {
public static void main(String arg[]) {
Q20 e = null;
System.out.println(e.equals(null)); //6
Q20 e1;
System.out.println(e1.equals(null)); //8
}
}

@6 There will be run time NullPointerException
@8 It's compile tile error.

Sweta pillai
 
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