Print ab1, ab2, ab3 With Threads

[Keith Mattix]

I am working on some code for my Advanced Java class, however, I cannot figure out why this code is not giving me the output, ab1, ab2, ab3, ab4, etc. Could you guys please help me?

The code:

import java.util.concurrent.locks.*;

public class PrintNumAndChar implements Runnable {
	private char charToPrint;
	private int lastNumber;
	private int numOfTimes;
	public static StringBuilder outputString;
	private Lock methodLock;
	private Condition myCondition;

	public PrintNumAndChar(int lastNumber, int numOfTimes, char charToPrint) {
		this.lastNumber = lastNumber;
		this.numOfTimes = numOfTimes;
		this.charToPrint = charToPrint;
		outputString = new StringBuilder("");
		methodLock = new ReentrantLock();
		myCondition = methodLock.newCondition();

	}

	public void printNumAndChar() throws InterruptedException {
		methodLock.lock();
		if ((numOfTimes == 0 || charToPrint == '0') && lastNumber != 0) {
			for (int i = 1; i <= 100; i++) {
				if(i > 1) {
					methodLock.lock();
				}
				PrintNumAndChar.outputString.append(printInt(i) + " ");
				methodLock.unlock();
				if (!methodLock.tryLock()) {
					while (true) {
						if (methodLock.tryLock()) {
							break;
						}
					}
				}
			}

		} else if (lastNumber == 0 && charToPrint == 'a') {
			for (int i = 0; i < numOfTimes; i++) {
				try {
				if (!methodLock.tryLock()) {
					myCondition.await();
					PrintNumAndChar.outputString.append(printA() + " ");
					methodLock.lock();
				}
				else {
					PrintNumAndChar.outputString.append(printA() + " ");
					myCondition.signal();	
				}
			}catch(InterruptedException | IllegalMonitorStateException e) {
				
			}finally {
				methodLock.unlock();
			}
		} 
		}else if (lastNumber == 0 && charToPrint == 'b') {
			for (int i = 0; i < numOfTimes; i++) {
				try {
					if (methodLock.tryLock()) {
						PrintNumAndChar.outputString.append(printB() + " ");
					} else {
						myCondition.await();
						methodLock.lock(); 
						PrintNumAndChar.outputString.append(printB() + " ");
					}
					myCondition.signal();
				} catch (InterruptedException | IllegalMonitorStateException e) {
				} finally {
					methodLock.unlock();
				}
			}
		}
	}

	@Override
	public void run() {
		try {
			printNumAndChar();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public char printA() {
		return 'a';
	}

	public char printB() {
		return 'b';
	}

	public int printInt(int number) {
		return number;
	}

}

And I call it like this:

Runnable printA = new PrintNumAndChar(0,100,'a');
		Runnable printB = new PrintNumAndChar(0,100,'b');
		Runnable print100 = new PrintNumAndChar(100, 0, '0');
		
		Thread thread1 = new Thread(printA);
		Thread thread2 = new Thread(printB);
		Thread thread3 = new Thread(print100);
		
		thread3.start();
		thread1.start();
		thread2.start();

PS: I'm new to Java ranch, so if I do anything wrong, please tell me and I will correct it.

[Nam Ha Minh]

Keith Mattix wrote:I am working on some code for my Advanced Java class, however, I cannot figure out why this code is not giving me the output, ab1, ab2, ab3, ab4, etc. Could you guys please help me?

Because all the threads are running concurrently and the order is arbitrary.

[Keith Mattix]

While that is true, the point of the assignment is to produce the desired out put using Thread synchronization, Locks, or Semaphores.

[Henry Wong]

Keith Mattix wrote:While that is true, the point of the assignment is to produce the desired out put using Thread synchronization, Locks, or Semaphores.

Hint: How many synchronization locks do you have?

Henry

[Keith Mattix]

Ahh, so what you're saying is that I need a lock for each block of code I need to run?

[Henry Wong]

Keith Mattix wrote:Ahh, so what you're saying is that I need a lock for each block of code I need to run?

No.

And since, I don't have anymore hints to give, the reason is .... Your three threads are using three different locks. There is no synchronization going on between them.

Henry

[Keith Mattix]

It's the same lock. I instantiated it in the constructor. I just used the same lock.

[Henry Wong]

Keith Mattix wrote:It's the same lock. I instantiated it in the constructor. I just used the same lock.

Look again, You have three different threads, each with it's own runnable class (and each with it own lock).

Henry

[Keith Mattix]

So if I make the lock and condition static, then I shouldn't have that problem.

[Steve Luke]

Keith Mattix wrote:So if I make the lock and condition static, then I shouldn't have that problem.

It'll be better, there will be synchronization. There will still be a possible problem with the order of operations. Since all the Runnables use the same Condition then you will lose control over what order they get the signal to run in. Whenever you send the signal any one of the other threads waiting on the condition will be awoken, so there is no guarantee that they will go in the order of a -> b-> number. You will need to either use multiple conditions to ensure the correct order, or use an additional conditional to make sure only the intended thread wakes up.

[Keith Mattix]

It doesn't have to necessarily be in that order. It could be ab1, b1a, etc.

[Steve Luke]

how about ab1 ababab2 ?

[Keith Mattix]

No, it has to be where you see a and b with a number (in numerical order) in any order.