doubts on questions #12, #13, #15, #16 and #17 self-test Chapter 13

[John Lerry]

I have great difficulty in understanding the operation of the code supplied with the exercises mentioned.

#12
Given this code:

public class TwoThreads {
     static Thread laure1, hardy;
     public static void main(String[] args) {
            laure1 = new Thread() [b]{
                        public void run() {
                                  System.out.println("A");
                                  try { 
                                            hardy.sleep(1000);
				  } catch (Exception e) {
				  	System.out.println("B");
				  }
				        System.out.println("C");

                        }
            };
	    hardy = new Thread() {
	    	public void run() {
			System.out.println("D");
			try {
				laure1.wait();
			} catch (Exception e) {
				System.out.println("E");
			}
			System.out.println("F");
		}
            };
  	    laure1.start();
	    hardy.start();			 
     }
}

Which letters will eventually appear somewhere in the output?

Correct answers:
A. A
C. C
D. D
E. E
F. F

#13
Given this code:

public class Starter implements Runnable {
	void go(long id) {
		System.out.println(id);
	}
	public static void main(String[] args) {
		System.out.print(Thread.currentThread.getId() + " ");
		// insert here
	}
	public void run() { go(Thread.currentThread().getId()); }
}

and given the following five fragments:

I. new Starter().run();
II. new Starter().start();
III. new Thread(new Starter());
IV. new Thread(new Starter()).run();
V. new Thread(new Starter()).start();

When the five fragments are inserted, one at a time at line 9, which are true?

Correct answers:
C. Only one might produce the output 4 2
D. Exactly two might produce the output 4 4

#15
Given this code:

class Dudes {
	static long flag = 0;
	// insert here
		if (flag == 0) flag = id;
		for(int x = 1; x < 3; x++) {
			if (flag == id) System.out.print("yo");
			else System.out.print("dude ");
		}
	}
}
public class DudesChat implements Runnable {
	static Dudes d;
	public static void main(String[] args) {
		new DudesChat().go();
	}
	void go() {
		d = new Dudes();
		new Thread(new DudesChat()).start();
		new Thread(new DudesChat()).start();
	}
	public void run() {
		d.chat(Thread.currentThread().getId());
	}
}

And given these two fragments:

I. synchronized void chat(long id) {
II. void chat(long id) {

When fragment I or fragment II is inserted at line 5, which are true?

Correct answer:
F. With fragment II, the output could be yo dude dude yo

#16
Given this code:

class Chicks {
	synchronized void yack(long id) {
		for(int x = 1; x < 3; x++) {
			System.out.print(id + " ");
			Thread.yield();
		}
	}
}
public class ChicksYack implements Runnable {
	Chicks c;
	public static void main(String[] args) {
		new ChicksYack().go();
	}
	void go() {
		c = new Chicks();
		new Thread(new ChicksYack()).start();
		new Thread(new ChicksYack()).start();
	}
	public void run() {
		c.yack(Thread.currentThread().getId());
	}
}

Which are true?

Correct answer:
F. An exception is thrown at runtime

#17
Given this code:

public class Chess implements Runnable {
	public void run() {
		move(Thread.currentThread().getId());
	}
	// insert code here
		System.out.print(id + " ");
		System.out.print(id + " ");
	}
	public static void main(String[] args) {
		Chess ch = new Chess();
		new Thread(ch).start();
		new Thread(new Chess()).start();
	}
}

And given these two fragments:

I. synchronized void move(long id) {
II. void move(long id) {

When either fragment I or fragment II is inserted at line 7, which are true?

Correct answers:
C. With fragment I, the output could be 4 2 4 2
E. With fragment II, the output could be 2 4 2 4

I just can not understand why the correct answers are those. Can you help me understand?
I apologize for the amount but I just can not understand.

[John Lerry]

I'm going back to apologize for the length but I have other 3 codes (taken from the questions on cd) of which I can not understand the operation.

-
Given this code:

public class Letters extends Thread {
       private String name;
       public Letters(String name) { this.name = name; }
       public void write() {
                 System.out.print(name);
                 System.out.print(name);
       }
       public static void main(String[] args) { 
                 new Letters("X").start();
                 new Letters("Y").start();
       }

We want to guarantee that the output can be either XXYY or YYXX, but never XYXY or any other combination. Which method definitions could be added to the Letters class to make this guarantee?

Correct answers:
E. public void run() { synchronized(Letters.class) { write(); } }
F. public void run() { synchronized(System.out) { write(); } }

-
Given this code:

public class ThreadTest {
       class InnerRun implements Runnable {
               public void run() {
                       for(int x = 0; x < 100; x++) {
                               try {
                                      Thread.sleep(5);
                               } catch (Exception e) { }
                               System.out.print("Ren "); 
                       }
               }
       }
       class InnerRunTwo implements Runnable {
                 Thread other;
                 InnerRunTwo(Thread t) { other = t; }
                 public void run() {
                        try {
                                  other.join();
                        }  catch (Exception e) {  }
                        for(int x = 0; x < 100; x++) {
                                   try {
                                             Thread.sleep(5);
                                   } catch (Exception e) { }
                                   System.out.print("Stimpy ");
                        }
                  }
       }
       void start() {
                  InnerRun ir = new InnerRun();
                  Thread t = new Thread(ir);
                  InnerRunTwo irr = new InnerRunTwo(t);
                  Thread u = new Thread(irr);
                  t.start();
                  u.start();
       }
       public static void main(String[] args) {
                  ThreadTest tt = new ThreadTest();
                  tt.start();
       }
}

What is the result?

Correct answer:
A. Ren Ren Ren... ...Stimpy Stimpy Stimpy

-
Given that the following code is intended to create a single WidgetMaker that makes Widget objects and several WidgetUsers who wait for a Widget to be made:

import java.util.*;
class Widget { }
class WidgetMaker extends Thread {
	List<Widget> finishedWidgets = new ArrayList<Widget>();
	public void run() {
		try {
			while(true) {
				Thread.sleep(2000); 
				Widget w = new Widget();
			
				// INSERT SOMETHING HERE
			}
		} catch (InterruptedException e) { }
	}
	public Widget waitForWidget() {
   		synchronized (finishedWidgets) {
			if (finishedWidgets.size() == 0) {
				try {
					finishedWidgets.wait();
				} catch (InterruptedException e) { }
			}
			return finishedWidgets.remove(0);
		}
	}
}
public class WidgetUser extends Thread {
	private WidgetMaker maker;
	public WidgetUser(String name, WidgetMaker maker) {
		super(name);
		this.maker = maker;
	}
	public void run() {
		Widget w = maker.waitForWidget();
 		System.out.println(getName() + " got a widget");
	}
	public static void main(String[] args) {
		WidgetMaker maker = new WidgetMaker();
		maker.start();
		new WidgetUser("Lenny", maker).start();
		new WidgetUser("Moe", maker).start();
		new WidgetUser("Curly", maker).start();	
	}
}

If each WidgetUser should receive its own Widget, which no other WidgetUser is using, what code should be inserted in place of the comment // INSERT SOMETHING HERE ?

Correct answer:
C.
synchronized(finishedWidgets) {
finishedWidgets.add(w);
finishedWidgets.notify();
}

[John Lerry]

anyone help me?

[Henry Wong]

John Lerry wrote:anyone help me?

There are reasons that long posts don't get much attention, and apologizing generally doesn't get it more attention. Also, you need to ShowSomeEffort with each question, and not just have all your effort in cutting and pasting lots of questions.

Are any of these questions a priority? If so, pick one, show us that one question, explain us the answer you think it is, why you think so. And then we can give you a hint in the right direction.

Henry

[John Lerry]

you're right. I try to give an answer to the first question.

John Lerry wrote:
#12
Given this code:

public class TwoThreads {
     static Thread laure1, hardy;
     public static void main(String[] args) {
            laure1 = new Thread() {
                        public void run() {
                                  System.out.println("A");
                                  try { 
                                            hardy.sleep(1000);
				  } catch (Exception e) {
				  	System.out.println("B");
				  }
				        System.out.println("C");

                        }
            };
	    hardy = new Thread() {
	    	public void run() {
			System.out.println("D");
			try {
				laure1.wait();
			} catch (Exception e) {
				System.out.println("E");
			}
			System.out.println("F");
		}
            };
  	    laure1.start();
	    hardy.start();			 
     }
}

Which letters will eventually appear somewhere in the output?

Correct answers:
A. A
C. C
D. D
E. E
F. F

First you can not determine the order of output because there is no way to establish which of the two threads to run first, in any case, the thread "laure1" will print "A" and "C", with one second between the two results, while thread "hardy" will print "D", then "E" (the exception is thrown because there is NOT a method notify()) and then "F".

Is it correct?

[Henry Wong]

John Lerry wrote:
First you can not determine the order of output because there is no way to establish which of the two threads to run first, in any case, the thread "laure1" will print "A" and "C", with one second between the two results, while thread "hardy" will print "D", then "E" (the exception is thrown because there is NOT a method notify()) and then "F".

Is it correct?

Can you point to us a source/link that says that an exception will be thrown when you call wait() and no other thread calls notify()? If so, how do you think that the environment can tell that no notification will arrive in the future?

Henry

[John Lerry]

actually it makes no sense. The wait() method MUST be enclosed within a try/catch block to not generate an error in compilation but I can not understand because in this performance an exception would be thrown.