Welcome to Java Class: Learn Java One Bit at a Time

[chroot]

Does the Keyboard.class file have to exist within a package? Do you need to use the import statement to import it? Can you provide an example of a program which successfully uses this class?

- Warren

 

[Entropy]

We just did a coin java program like that in my computer science class.

 

[ek]

Ok, so it turns out that error message had to with the capitalization of the title of the keyboard.java document.

I really suck at java.

Ok so now the output just lists the denominations: Quarters, Dimes, Nickels, Pennies. How do I get the appropriate numbers to show up? Does it have something to do with the coding around this part...

change = DOLLAR - amount;
numQuarters = change / QUARTER;
remQuarters = change % QUARTER;
numDimes = remQuarters / DIME;
remDimes = remQuarters % DIME;
numNickels = remDimes / NICKEL;
remNickels = remDimes % NICKEL;
numPennies = remNickels / PENNY;

Do I need brackets or println's or something? Some sort of extra coding to get it to work?

How did you manage to get this done Entropy? Any pointers?

Edit: Fixed one problem. Getting there...

 

[gnome]

For my first foray into threads, I wrote a readers/writers (shared resource) simulation. The main() method has an int variable "clock", and it just keeps looping and incrementing clock on each loop. While it loops it creates reader and writer threads, according to a randomly generated schedule; each reader & each writer is a separate thread, and when each one is created it is passed a parameter "duration" telling it the length of time it should be active.

I want each reader or writer thread to release the shared resource after it has been active for its alloted amount of time, so in each constructor I pass "clock" as a parameter, and I set a member int variable of the thread class = to that parameter. I thought that would work since I understood that in Java parameters are passed by reference. But it's not working: the readerthreads and writerthreads are not seeing the updating of the clock value that's occurring in main(). Instead each of those threads just sees a constant clock value -- the value that clock had when the thread was created.

How can I enable the child threads to see the updating of a variable that's occurring in the parent (main) thread?

 

[Prometheus]

The main() method has an int variable "clock",
I understood that in Java parameters are passed by reference.

Objects are passed by reference. ints are not.

How can I enable the child threads to see the updating of a variable that's occurring in the parent (main) thread?

There are numerous ways, some more elegant than others. A simple way is not to pass the value at all. The child threads can access the clock value by ClassName.clock, assuming that the clock variable was declared:
public static int clock;

 

[gnome]

ek:

you only asked it to print the words:
for example,
System.out.println ("Quarters");
just says to print exactly what's inside the quotes, in this case the word Quarters.

What you want is
System.out.println (String.valueOf(numQuarters) + " Quarters");
and so on...

(String.valueOf(int i) is a static method of the String class that converts an int to a string.)

 

[Prometheus]

System.out.println (String.valueOf(numQuarters) + " Quarters");

I usually write this System.out.println (numQuarters + " Quarters");

 

[gnome]

originally posted by prometheus:
There are numerous ways, some more elegant than others. A simple way is not to pass the value at all. The child threads can access the clock value by ClassName.clock, assuming that the clock variable was declared:
public static int clock;

Does it need to be static? Clock is declared in the main function of the project.

 

[gnome]

Originally posted by Prometheus:
I usually write this System.out.println (numQuarters + " Quarters");

Thanks. I didn't realize println() was that smart.

 

[Prometheus]

Does it need to be static? Clock is declared in the main function of the project.

You mean the main method, not main function.

No, of course not. I said that there are many solutions. This is not a solution that I would use.

However, you are writing your code inside the main method. I suspect, although I may be wrong, that you are new to Java. Otherwise, why write your code in the main method? Since you are writing your code in the main method, you have no objects. By the way, I would declare this variable outside of the main method, at class scope.

TYpically, if I have a variable that I want other objects to see, I pass in the constructor a this reference. The constructed object can then call an accessor method in the creating object to determine the value of the variable. In order to do this, however, the creator must be an object. Your program in the main method does not qualify. Unless you have good reason, I recommend that you do not write Java programs inside of the main method.

 

[gnome]

Yes I am definitely new to Java: I wrote hello, world in Java 2 weeks ago for the 1st time.

In this case my main method is just a test driver that instantiates the various ReaderThread and WriterThread objects, and "ticks" the clock. Everything else is in 5 separate class files:
ReaderThread
WriterThread
ReaderCounter
WriterCounter
BinarySemaphore

I tried changing the declaration of clock but I keep getting an "illegal start of expression" error, whether I call it public static int clock; or public int clock; or static int clock;
It only let's me do int clock;
Why?


edit: Got it. I had to move the declaration outside of the main method.

 

[gnome]

Thanks Prometheus. It looks like it's running correctly now, but it'll take a while to analyze all the starting and finishing messages from the threads to make sure they're all behaving nicely.

 

[Prometheus]

I tried changing the declaration of clock but I keep getting an "illegal start of expression" error, whether I call it public static int clock; or public int clock; or static int clock;
It only let's me do int clock;
Why?

edit: Got it. I had to move the declaration outside of the main method.

The main method is static, so that all variables declared inside of it are static by context: hence, static is invalid. The main method is a method, so that all variables inside of it are local; hence, public is invalid.

If you have a question, it helps to post your code if it is not too large.

 

[gnome]

I believe it's now working the way I intended, but if you don't mind wading through it, I'd welcome any comments, criticisms or suggestions you may offer. Anyway, thanks for your help.

//	test driver for readers/writers problem
//	There can be multiple readers, or a single writer, active at any time.
//		This solution gives priority to writers: active readers are allowed to
//		finish, but no new readers can start if there is a waiting writer.

import java.util.Random;
import java.util.Arrays;

public class testReadWrite{

	public static int clock;

	public static void main(String[] args){

		final int trial_len = 200;
		final int num_readers = 10;
		final int num_writers = 10;
		final int max_read_time = 10;
		final int max_write_time = 4;

		Random randnum = new Random();
		ReaderCounter rc = new ReaderCounter();
		WriterCounter wc = new WriterCounter();
		BinarySemaphore writing = new BinarySemaphore();
		ReaderThread r;
		WriterThread w;

//		int clock;	//	used to count iterations of the while loop (below)
		clock = 0;
		String s = new String();

		int[] read_arrive = new int[num_readers];
		int[] read_len = new int [num_readers];
		int[] write_arrive = new int [num_writers];
		int[] write_len = new int [num_writers];

		for (int i = 0; i < num_readers; i++){
			read_arrive[i] = randnum.nextInt(100);
			read_len[i] = randnum.nextInt(30) + 5;	//	don't want any 0 durations
		}
		for (int i = 0; i < num_writers; i++){
			write_arrive[i] = randnum.nextInt(100);
			write_len[i] = randnum.nextInt(5) + 5;	//	don't want any 0 durations
		}

		Arrays.sort(read_arrive);
		Arrays.sort(write_arrive);

// next 10 lines are just for evaluating the results
		for (int i = 0; i<num_readers; i++){
			System.out.print("reader" + String.valueOf(i) + " ");
			System.out.println("arr: " + String.valueOf(read_arrive[i]) + " len: "
											+ String.valueOf(read_len[i]));
		}
		for (int i = 0; i<num_writers; i++){
			System.out.print("writer" + String.valueOf(i) + " ");
			System.out.println("arr: " + String.valueOf(write_arrive[i]) + " len: "
											+ String.valueOf(write_len[i]));
		}

		while (clock<trial_len){
			System.out.println("clock = " + String.valueOf(clock));
			for (int i = 0; i < num_readers; i++){
				if (read_arrive[i]==clock){
						// form name of new reader
					s = "reader";
					s = s.concat(String.valueOf(i));
						// create new reader, specifying its read duration
					r = new ReaderThread(s,read_len[i],rc,wc,clock);					
					r.start();
				}
			}
			for (int i = 0; i < num_writers; i++){
				if (write_arrive[i]==clock){
						// form name of new writer
					s = "writer";
					s = s.concat(String.valueOf(i));
						// create new reader, specifying its read duration
					w = new WriterThread(s,write_len[i],rc,wc,writing,clock);					
					w.start();
				}
			}
			
			try{
				Thread.sleep(0,50);
			} catch(InterruptedException e){
			}
			clock++;
		}
	}
}

//	writer class for readers/writers problem
//	test driver must instantiate ReaderCounter rc, WriterCounter wc
//		BinarySemaphore writing, int clock


public class WriterThread extends Thread{
	 public WriterThread(String s, int d, ReaderCounter rctr, WriterCounter wctr,
														BinarySemaphore bs, int clk){
		super(s);
		name = s;
		duration = d;
		rc = rctr;
		wc = wctr;
		writing = bs;
		clock = clk;
		System.out.println(s + "created.");
	}

	public void run(){
//	reversed the order of the following 2 lines 
		wc.addWriter(); // add to count of waiting writers
		rc.writerPass(); // check ReaderCounter for any active readers
		writing.acquire(); // wait for semaphore to start writing
		finish_time = testReadWrite.clock + duration;
		while(finish_time > testReadWrite.clock)	//	changed from <
{
			try{
				System.out.println(name + ": clock = " + testReadWrite.clock +
							"; finish_time = " + finish_time);
				sleep(0,10); // writer is writing
			} catch(InterruptedException e){
			}
		}
		writing.release(); // finished writing
		wc.delWriter();
	}


	private String name;

	private int clock;
	private int duration;
	private int finish_time;
	private ReaderCounter rc;
	private WriterCounter wc;
	private BinarySemaphore writing;
}

//	reader class for readers/writers problem
//	test driver must instantiate ReaderCounter rc, WriterCounter wc
//		BinarySemaphore writing, int clock

public class ReaderThread extends Thread{
	public ReaderThread(String s, int d, ReaderCounter rctr,
											 WriterCounter wctr, int clk){
		super(s);
		name = s;
		duration = d;
		rc = rctr;
		wc = wctr;
		clock = clk;
		System.out.println(s + "created.");
	}

	public void run(){
		wc.readerPass(); // readers not allowed to start if a writer is waiting
		rc.addReader(); // makes writers wait while any readers are reading
		finish_time = testReadWrite.clock + duration;
		while(finish_time > testReadWrite.clock)	//	changed from <
{
			try{
				System.out.println(name + ": clock = " + testReadWrite.clock +
										"; finish_time = " + finish_time);
				sleep(0,10);	//	reader is reading
			} catch(InterruptedException e) {
			}
		}
		rc.delReader();		// finished
	}


	private String name;

//	duration  and finish_time will be used by test driver to determine when
//		this thread finishes reading
	private int clock;
	private int duration;
	private int finish_time;
	private ReaderCounter rc;
	private WriterCounter wc;

}

//	utility class for reader/writers problem
//	test driver will instantiate one ReaderCounter named "rc"

public class ReaderCounter{

	public ReaderCounter(){
		count = 0;
	}

//	used by reader starting to read
	public synchronized void addReader(){
		count++;
		System.out.println(Thread.currentThread().getName() + " starting to read.");
		notifyAll();	//in case another reader tried to start at same time
	}


//	used by reader finished reading
	 public synchronized void delReader(){
		count--;
		System.out.println(Thread.currentThread().getName() + " finished reading.");
		notifyAll();	//in case a writer is waiting
	}

//	used by writer wanting to write
	public synchronized void writerPass(){
		while(count > 0){
			try{
				wait();
			} catch (InterruptedException e){
			}
		}
	System.out.println(Thread.currentThread().getName() + " checking for active readers.");
	}

//	unsynchronized: used by main test loop to see when to terminate
	public int getCount(){
		return count;
	}

	private int count;

}

//	utility class for reader/writers problem
//	test driver will instantiate one WriterCounter named "wc"

public class WriterCounter{

	public WriterCounter(){
		count = 0;
	}

//	used by writer waiting to write
	public synchronized void addWriter(){
		count++;
		System.out.println(Thread.currentThread().getName() + " added to WriterCounter.");
		notifyAll();	//in case another writer is trying
	}


//	used by writer finished writing
	 public synchronized void delWriter(){
		count--;
		System.out.println(Thread.currentThread().getName() + " removed from WriterCounter.");
		notifyAll();	//in case other writers or readers are waiting
	}

//	used by reader wanting to read
	public synchronized void readerPass(){
		while(count > 0){
			try{
				wait();
			} catch (InterruptedException e){
			}
		}
	}

//	unsynchronized: used by main test loop to see when to terminate
	public int getCount(){
		return count;
	}


	private int count;

}

//	semaphore class for reader/writers problem

public class BinarySemaphore{

	public BinarySemaphore(){
		value = 1;
	}

	public synchronized void acquire(){
		while (value == 0){
			try{
				wait();
			} catch(InterruptedException e){
			}
 		}
		value = 0;
		System.out.println(Thread.currentThread().getName() + " now writing.");
	}

	 public synchronized void release(){
		System.out.println(Thread.currentThread().getName() + " finished writing.");
		value = 1;
		notifyAll();
	}

	private int value;

}

 

[ek]

Thanks a lot guys, I really appreciate it. After the time I spent on this, it was pretty satisfying to finally get it to work.

Thanks for the help!

 

[Prometheus]

I believe it's now working the way I intended, but if you don't mind wading through it, I'd welcome any comments, criticisms or suggestions you may offer. Anyway, thanks for your help.

Looks pretty good for someone new to Java. I don't have my Java JVM on this machine to load and test it, so I won't comment on it absent a question.

Note that your String.valueOf() are not necessary, and I prefer to omit them.
This method is called automatically when a primitive is concatenated to a string.

 

[Prometheus]

I believe it's now working the way I intended, but if you don't mind wading through it, I'd welcome any comments, criticisms or suggestions you may offer.

I noticed that your ReaderCounter and WriterCounter classes are 100% identical, except for slight differences in method names. I recommend that you just use the same class for both. If you were to have differences in behavior, an interface or an abstract superclass would be better than two classes that are basically identical. In this case, however, I see no difference in functionality at all.

 

[gnome]

Point well taken. Thanks again.

 

[ek]

I'm supposed to write an applet to draw a chessboard using two nested "if" statements. How would I even go about starting this?

Something like attaching numerical values to each square and write "if int=even color black else color white"?

What would the other if statement be? How would you even get the chessboard to look like a chessboard?

This java class is getting to me. It's just getting harder and harder.

Any hints?

 

[TenaliRaman]

the simplest way to draw a chessboard use
fillRect(int,int,int,int) along with setColor(Color)

choose any start position x,y
choose a neat size for the square as squareSize
run a for loop and change the x as x+squareSize
and at each x,y draw a square (change color as a mod 2 operation)
and once u have drawn 8 squares then change y as y+squareSize

Thus, draw Squares row wise till 8 rows ... and u have a chess board ...

-- AI

 

[ek]

Ok this is what I got so far, it's pretty hopeless, I know.

How do I choose a start position?
How do I get the squares to start repeating? (using a for loop I'm guessing, but how)

import java.applet.Applet;
import java.awt.*;
import java.util.Random;

public class Evan4 extends Applet
{

//stuff here

final int NUM_SQUARES = 64;
final int SQUARE_WIDTH = 30;
int x = 5;
int y = 5;



public void paint (Graphics g)
{


for (int count = 0; count < NUM_SQUARES; count++);

{g.setColor (Color.black);
g.fillRect(x,y,SQUARE_WIDTH,SQUARE_WIDTH);

x += SQUARE_WIDTH;
y += SQUARE_WIDTH;
}
}
}

 

[TenaliRaman]

public final int INITIAL_X = /*somevalue*/
public final int INITIAL_Y = /*somevalue*/
for(x=INTIAL_X;x<INITIAL_X+SQUARE_WIDTH*8;x+=SQUARE_WIDTH)
for(y=INITIAL_Y;y<INITIAL_Y+SQUARE_WIDTH*8;y+=SQUARE_WIDTH)
{
/*draw rectangle here*/
}

-- AI

 

[chroot]

BTW, the notation:

for (int count = 0; count < NUM_SQUARES; count++);

is equivalent to this


for (int count = 0; count < NUM_SQUARES; count++)
;

and actually means an EMPTY for loop. Lose the semicolon.

- Warren

 

[ek]

Ok, I got some help at school today, but it still won't draw properly. It's a problem inside the for loops. It isn't resetting itself to y=0 when the row reaches the LIMIT (8 squares). It just draws them consecutively. Why? How do I fix this?

import java.applet.Applet;
import java.awt.*;

public class Evan4 extends Applet
{

//variables and values

final int LIMIT = 8;
int x = 20;
int y = 20;
int height = 20;
int width = 20;
int ypos;
int xpos;


public void paint (Graphics g)
{

//colour of background
setBackground (Color.gray);

// initial box color
g.setColor (Color.white);
//loop for x
for (int count = 0; count < LIMIT; count++)
//y for loop
{
for (int county = 0; county < LIMIT; count++)
{
//x and y values changing
xpos = x+width*count;
ypos = y+height*county;
//if box is even count draw box black, else white
if ((count + county) % 2 ==0)
g.setColor (Color.black);

else
g.setColor (Color.white);

g.fillRect (xpos, ypos, width, height);


}
}
}
}

 

[TenaliRaman]

You have made one small but highly annoying error ...

Look at this statement :-
for (int county = 0; county < LIMIT; count++)

Do u see something wrong here?

-- AI

 

[ek]

You have made one small but highly annoying error ...

Look at this statement :-
for (int county = 0; county < LIMIT; count++)

Do u see something wrong here?

-- AI

No I don't. I suck at java.

 

[ek]

It just won't go back to the y origin and draw 8x8. That's pretty much the only problem. It just draws on one y value.

I'm missing something. And it's really making me mad. It's so frustrating.

 

[chroot]

No I don't. I suck at java.

You are incrementing count in a loop that tests county.

- Warren

 

[ek]

You are incrementing count in a loop that tests county.

- Warren

Awesome. Thanks.

Amazing that such a small error can screw you over so badly.

You guys are great.

 

[chroot]

Next time you have such an error, consider adding some print statements in strategic places to watch what your program is doing.

I have come across many instances in my career where one single character makes the difference between a perfect program and a disastrous one. Programming requires almost neurotic attention to detail.

- Warren

 

[ek]

Ok, one last thing.

How would I turn my applet for drawing a chessboard into one that randomly produces chessboards at time intervals at random places on the page? With random colours to boot.

 

[ek]

Alright so I gave it a try, but this code gets lots of compiling errors.

What is fundamentally wrong with this code? What's missing or in the wrong place? It says the if/else statement is screwed up as well.

Edit: I know some of it is off, it's cut/pasted. (like the black/white stuff)

import java.applet.Applet;
import java.awt.*;
import java.util.Random;
;

public class Evan4c extends Applet
{

//variables and values


final int WIDTH = 20;
final int LENGTH = 20;
final int LIMIT = 8;
int x = 20;
int y = 20;
int height = 20;
int width = 20;
int ypos;
int xpos;
int n = 150; // wait n milliseconds
int w = (int) (Math.random () * 41);


public void paint (Graphics g)
{

//colour of background
setBackground (Color.cyan);

// initial box color
g.setColor (Color.white);
//loop for x
for (int count = 0; count < LIMIT; count++)
//y for loop
{
for (int county = 0; county < LIMIT; county ++)
{
//x and y values changing
xpos = x+width*count;
ypos = y+height*county;
//if box is even count draw box black, else white
if ((count + county) % 2 ==0)
Color c = new Color;


(int)(Math.random () * 256);); // red = 0..255
(int)(Math.random () * 256), // green = 0..255
(int)(Math.random () * 256)); // blue = 0..255
g.setColor (c);

else
Color c = new Color (
(int)(Math.random () * 256), // red = 0..255
(int)(Math.random () * 256), // green = 0..255
(int)(Math.random () * 256)); // blue = 0..255
g.setColor (c);

g.fillRect (xpos, ypos, width, height);


}
}
}
}

 

[Hurkyl]

You forgot to use braces to wrap the two clauses in your if statement.
Try looking at your first "Color c = ..." statement.

But most importantly, I thought you were trying to draw black and white rectangles, but for both cases you're trying to draw a randomly colored rectangle.

 

[ek]

Here's my error log, what's going on here?

C:\Documents and Settings\Evan\Desktop\Evan4c.java:47: '.class' expected
(int)(Math.random () * 256), // red = 0..255
^
C:\Documents and Settings\Evan\Desktop\Evan4c.java:48: ')' expected
(int)(Math.random () * 256); // green = 0..255
^
C:\Documents and Settings\Evan\Desktop\Evan4c.java:49: not a statement
(int)(Math.random () * 256); // blue = 0..255
^
C:\Documents and Settings\Evan\Desktop\Evan4c.java:55: '.class' expected
(int)(Math.random () * 256), // red = 0..255
^
C:\Documents and Settings\Evan\Desktop\Evan4c.java:57: ')' expected
(int)(Math.random () * 256); // blue = 0..255

Never mind. I figured it out.

 

[ek]

Ok, I have a new problem.

This time I'm trying to make a program that prints all the prime factors of a given number and the 29 numbers one less than it. These numbers iare 432731 -> 432702.

So here is my program:

public class evan5ccc
{
/* the method tests whether a number is prime*/

public static boolean Primes(int input)
{
for (int x=2; x < input; x++)
if (input % x == 0)
return false;
return true;
}

/*
computes the smallest prime factor of the input number,
returns that number divided by its smallest prime factor
*/

public static int factor (int input)
{
if (input == 1)
return input;
else
{
for (int x=2; x <= input; x++)
if (input % x == 0)
{
System.out.print(" " + x );
return (input / x);
}
}
return 0;
}

public static void main(String args[])
{
int number; // number to be decomposed

for (number=432731; number > 432701; number--)

System.out.println("Prime factors of " + number + ":");


while (number != 1)
{
number = factor(number);
}
}
}

And here is the output:

Prime factors of 123456760:
Prime factors of 123456761:
Prime factors of 123456762:
Prime factors of 123456763:
Prime factors of 123456764:
Prime factors of 123456765:
Prime factors of 123456766:
Prime factors of 123456767:
Prime factors of 123456768:
Prime factors of 123456769:
Prime factors of 123456770:
Prime factors of 123456771:
Prime factors of 123456772:
Prime factors of 123456773:
Prime factors of 123456774:
Prime factors of 123456775:
Prime factors of 123456776:
Prime factors of 123456777:
Prime factors of 123456778:
Prime factors of 123456779:
Prime factors of 123456780:
Prime factors of 123456781:
Prime factors of 123456782:
Prime factors of 123456783:
Prime factors of 123456784:
Prime factors of 123456785:
Prime factors of 123456786:
Prime factors of 123456787:
Prime factors of 123456788:
Prime factors of 123456789:
3 3 3607 3803

This is copied and pasted from the sample output, so the numbers are wrong, but it basically prints just the last number's prime factors, instead of all 30. Also, it prints it on the next line, because i used println, but if I use print then it all goes into a big mess.

The output is supposed to look like this:

Prime factors of 123456760: 2 2 2 5 7 271 1627
Prime factors of 123456761: 123456761
Prime factors of 123456762: 2 3 3 11 13 47963
Prime factors of 123456763: 4021 30703
Prime factors of 123456764: 2 2 617 50023
Prime factors of 123456765: 3 5 523 15737
Prime factors of 123456766: 2 1051 58733
Prime factors of 123456767: 7 17636681
Prime factors of 123456768: 2 2 2 2 2 2 2 2 3 160751
Prime factors of 123456769: 53 283 8231
Prime factors of 123456770: 2 5 29 425713
Prime factors of 123456771: 3 3 3 17 268969
Prime factors of 123456772: 2 2 30864193
Prime factors of 123456773: 11 83 135221
Prime factors of 123456774: 2 3 7 7 419921
Prime factors of 123456775: 5 5 13 19 19993
Prime factors of 123456776: 2 2 2 79 195343
Prime factors of 123456777: 3 5779 7121
Prime factors of 123456778: 2 23 1531 1753
Prime factors of 123456779: 109 173 6547
Prime factors of 123456780: 2 2 3 3 5 47 14593
Prime factors of 123456781: 7 41 149 2887
Prime factors of 123456782: 2 61728391
Prime factors of 123456783: 3 2797 14713
Prime factors of 123456784: 2 2 2 2 11 11 43 1483
Prime factors of 123456785: 5 24691357
Prime factors of 123456786: 2 3 20576131
Prime factors of 123456787: 31 31 128467
Prime factors of 123456788: 2 2 7 13 17 71 281
Prime factors of 123456789: 3 3 3607 3803

So how do I make all 30 prime factorizations happen? I've been staring at this code for some time now and I can't figure it out.