IrixTech Mock Exam

[Duran Harris]

Mock Questions from Practice test 1 @ http://www.irixtech.com

Hi sorry this post is so long...I'm just really dubious of this mock exam..

What is the output of the following code when compiled and run ?

01. class A {
02. int j;
03. public synchronized void ace( ) {
04. System.out.print( ++j + " " );
05. }
06. }
07.
08. public class R implements Runnable {
09. A a = new A( );
10.
11. public void run( ) {
12. for ( int i = 0; i < 3; i++ )
13. a.ace( );
14. }
15.
16. public static void main(String[ ] args) throws InterruptedException {
17. Thread t1 = new Thread( new R( ) );
18. Thread t2 = new Thread( new R( ) );
19. t1.start( );
20. t1.join( );
21. t2.start( );
22. }
23. }

Answer:123123

How can this be when the join method has no parameter??

Also:

Given the following,

01. class A {
02. public int x;
03. }
04. public class B extends A {
05. private int y;
06. public void setValue( int p ) {
07. y = p;
08. }
09. }

which is true about the classes depicted above ?

I said that class B is tightly encapsulated.The answer said neither.Isn't B's instance variable marked as private??

Another one:
What will happen if run the following code?

Boolean[ ] b1 = new Boolean[10];
boolean[ ] b2 = new boolean[10];
 
System.out.print("The value of b1[1] = " + b1[1] );
System.out.print(" while the value of b2[1] = " + b2[1] );

I said the output will be false and false.They said false null.Why null!?!?
They were both declared and accessed in exacty the same way...

Another one:

class Foo {
public static void main (String[ ] args) {
(new Foo()).showValue();
}
 
final int x;
public void showValue() {
System.out.println(" x has a value of " + ++x );
}
}

Here they say that there is a compiler error because x is not initialised.I'm not too sure about this one...

Another:
A source code file contains the following lines:

01. class Bear {}
02. class Deer {}
03. class Sheep {}
04. class Tiger {}

Doesn't there need to be a public class matching the name of the java file?

And another:

01. package pkg0;
02. public class Father {
03. protected byte f = 10;
04. }
 
01. package pkg1;
02. import pkg0.Father;
03.
04. class Son extends Father {
05. public void sayItOut( ) {
06. System.out.print("f is " + f );
07. Father F = new Father( );
08. System.out.print(" while F is " + F . f );
09. }
10. public static void main(String[ ] args) {
11. new Son( ).sayItOut( );
12. }
13. }

They say compiler error on line 08...Cannot access protected member f.
But doesn't protected allow a subclass to access the protected member as long as you dont modify it??
[ December 23, 2008: Message edited by: Duran Harris ]

[Ankit Garg]

Duran you should Use One Thread Per Question

Anyways let me try to solve your queries

the join method has no parameter??

What's the problem with join having no parameters. The code starts a thread and then joins it

19. t1.start( );
20. t1.join( );
21. ...

So the execution will reach to statement 21 when the thread t1 has completed execution i.e. its run method completes.

I said that class B is tightly encapsulated.

But B is-a A. Since A is not encapsulated, so neither is B. The field x breaks the encapsulation.

Boolean[ ] b1 = new Boolean[10];
boolean[ ] b2 = new boolean[10];

They were both declared and accessed in exacty the same way...

Note that one of the array is a Boolean i.e. array of references. Elements in an array of booleans are automatically initialized to false. And elements in an array of references are always initianlized to null.

Here they say that there is a compiler error because x is not initialised.I'm not too sure about this one...

If your class has a final field, it must be initialized before or in the last statement of every constructor (if a constructor calls another constructor then that constructor doesn't need and in fact cannot initialize it).

Doesn't there need to be a public class matching the name of the java file?

Not necessarily. The rule says that if you have a public class, it must be stored in a .java file with the same name as the public class. So you cannot have two public top level classes in the same source file.

But doesn't protected allow a subclass to access the protected member??

You can access a protected inherited member outside the package only through inheritance. You cannot use an instance of the super class to access it...

[M Srilatha]

Reg 1st question: How can this be when the join method has no parameter??

First of all:
t1 and t2 are created two different objects of R. So they wont block each other.

And there is a call t1.join() just after starting t1. That means main thread waits till the time t1 finishes its task. So run() method will be executed once and prints 123. After run() method is executed, main thread resumes and starts t2. As t2 is created with a different object of R, run() method will print 123. Hence the output is: 123123.

I am not sure why you are confused with join() with no parameters.
if you call join() with no parameters, the waiting thread will become runnable just after the other thread has finished.
if you specify the maximum time that a thread can wait for another thread to finish its task, the waiting thread will become runnable after the time has elapsed if the other thread has not finished before that.

Hope this is clear!

[Punit Singh]

How can this be when the join method has no parameter??

Why, join() does should take any parameter.

t1.join() internally calls join(0).

public final void join() throws InterruptedException {
join(0);
}

Here 0 parameter indicates that current thread should wait for infinite time until t1 thread does not complete its execution.

[Duran Harris]

Aaah...wow I made dumb mistakes.
Okay so calling t.join() joins the currently executing thread to t?I forgot about main..?Doesn't join have a parameter of(Thread t) or is it optional...I really must go over threads tonight

If your class has a final field, it must be initialized before or in the last statement of every constructor (if a constructor calls another constructor then that constructor doesn't need and in fact cannot initialize it).

So what exactly happens when you declare an instance reference variable as final like this=> final int x;
=> does it only become immutable once a value is
=> assigned to it??
[ December 23, 2008: Message edited by: Duran Harris ]

[James Tharakan]

it becomes a compile time constant.
Compiler replaces that variable with its value once it is compiled

[Ankit Garg]

If you declare a constant in your class, it must be initialized before constructors are over. So this code will not compile as z is not initialized in the no-arg constructor

class Test {
   final int x = 10; //initialized at declaration
   final int y;
   final int z;
   Test() {     //z is not initialized if this constructor is called in any way
      y = 20;
   }
   Test(int i) {  // no need to initialize y in this constructor as it calls another constructor which initializes it
      this();
      z = 10;
   }
}

[Ruben Soto]

But if you declare a final AND static variable, then it must be initialized either as part of its declaration, or in a static initializer block exclusively.