Prajakta Vaidya

public class Test1 { public static void main(String a[]) { Set s =new TreeSet(); s.add(new Person(20)); s.add(new Person(10)); System.out.println(s); } } class Person { Person(int i){} }

This code compiles fine, but at runtime gives ClassCastException.

Is it because the class Person does not have equals() and hashcode() methods defined ?

Given:

11. Runnable r = new Runnable() { 12. public void run() { 13. System.out.print(�Cat�); 14. } 15. }; 16. Threadt=new Thread(r) { 17. public void run() { 18. System.out.print(�Dog�); 19. } 20. }; 21. t.start(); What is the result? A. Cat B. Dog C. Compilation fails. D. The code runs with no output. E. An exception is thrown at runtime.
Answer: B

I guessed the answer as B and it is correct, but I don't know the reasoning behind it.

Can anyone please explain ?

I am preparing for SCJP, I think I am good at other topics (this conclusion is from the mock tests I have solved) but I am not very comfortable with questions on "Threads" topics. Is there any tutorial available online that explains the concepts thoroughly.

I have gone through the chapter on threads in many books, I understand the theory but when it comes to code-based questions I am not able to guess the o/p

here is a sample code from 1 of the mock exams , the start() method of the Thread class in not static, here the method is called just as start() . so the code should give compiler error, is that rite ?


public class Tux extends Thread
{
static String sName = "Techno";
public static void main(String argv[])
{
Tux t = new Tux();
t.name(sName);
System.out.println(sName);
}
public void name(String sName)
{
sName = sName + " park";
start();
}
public void run()
{
for(int i=0;i < 4; i++)
{
sName = sName + " " + i;
}
}
}

Thnx

Actually I thought only from the no-args constructor of the subclass the super() gets called, and in this case no object of Sub class is created like Sub sb = new Sub(), compiler won't complain

Here is 1 Q in 1 of the SCJP mock tests on the net -

class Super{

String name;

Super(String s){

name =s ;

} }

class Sub extends Super{

String name;

Sub(String s){

name=s;

}

public static void main(String args[]){

Super sup = new Super("First");

Sub sub = new Sub("Second");

System.out.println(sub.name + " " + sup.name);

} }

I tried to run the code it gives compiler error for Super() constructor.

When I added no-args constructors for both the classes it worked fine, I don't get the reason behind the error.

Can anyone please help me ?

Thanks,

- Prajakta