u04-1

.gitignore

@@ -0,0 +1 @@
+*.class

u04-1/Exchanger.java

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+class Exchanger<T> {
+	T ex1 = null;
+	T ex2 = null;
+
+	public synchronized T exchange(T obj) throws InterruptedException {
+		T res = null;
+
+		while (ex2 != null) wait();
+
+		if (ex1 == null) {
+			ex1 = obj;
+			while (ex2 == null) wait();
+
+			res = ex2;
+			ex2 = null;
+
+		} else {
+			ex2 = obj;
+
+			res = ex1;
+			ex1 = null;
+		}
+
+		notifyAll();
+
+		return res;
+	}
+}

u04-1/ExchangerTester.java

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+class Worker extends Thread {
+	public Worker(Exchanger<String> ex) {
+		this.ex = ex;
+	}
+	public void run() {
+		String received = null;
+		
+		try {
+			received = ex.exchange("World");
+		} catch (InterruptedException e) {
+			System.out.println("Worker interrupted :-(");
+		}
+		System.out.println("Worker received " + received + " in exchange for World");
+		
+		try {
+			received = ex.exchange("Hi");
+		} catch (InterruptedException e) {
+			System.out.println("Worker interrupted :-(");
+		}
+		System.out.println("Worker received " + received.hashCode() + " in exchange for " + "Hi".hashCode());
+
+	}
+
+	private Exchanger<String> ex;
+}
+
+public class ExchangerTester {
+	private static int N_EXCHANGES = 100;  // Set to larger number to test repeated exchange()'s
+	public static void main(String[] args) {
+		String received = null;
+		Exchanger<String> exchanger = new Exchanger<String>();
+		for (int i = 0; i < N_EXCHANGES; ++i) {
+			new Worker(exchanger).start();
+
+			try {
+				received = exchanger.exchange("Hello");
+			} catch (InterruptedException e) {
+				System.out.println("Main thread interrupted :-(");
+			}
+			System.out.println("Main thread received " + received + " in exchange for Hello");
+
+			try {
+				Thread.sleep((long) (Math.random()*10));
+			} catch (InterruptedException e) {
+				// Being interrupted in a random sleep is still random, so nothing to do here
+			}
+
+			try {
+				received = exchanger.exchange("Hi");
+			} catch (InterruptedException e) {
+				System.out.println("Main thread interrupted :-(");
+			}
+			System.out.println("Exchange #" + i + ": Main thread received " + received.hashCode() + " in exchange for " + "Hi".hashCode());
+		}
+}
+
+}