added ex Y

src/exercises/ex_y/RaceTester.java

@@ -0,0 +1,21 @@
+package exercises.ex_y;
+
+import java.util.concurrent.Phaser;
+
+public class RaceTester {
+    public static final int NUM_RUNNERS = 10;
+    public static final int NUM_STAGES = 5;
+
+    public static void main(String[] args) throws InterruptedException {
+        Phaser phaser = new Phaser();
+        phaser.register();
+        for (int i = 0; i < NUM_RUNNERS; i++) {
+            new Thread(new Runner(NUM_STAGES, phaser)).start();
+        }
+        for (int i = 0; i <= NUM_STAGES; i++) {
+            phaser.arriveAndAwaitAdvance();
+        }
+        System.out.println("Finished race");
+        phaser.arriveAndDeregister();
+    }
+}

src/exercises/ex_y/Runner.java

@@ -0,0 +1,43 @@
+package exercises.ex_y;
+
+import exercises.Utils;
+
+import java.util.concurrent.Phaser;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class Runner implements Runnable {
+    private static AtomicInteger nextId = new AtomicInteger(0);
+    private final int id;
+    private final int maxStep;
+    private final Phaser phaser;
+
+    public Runner(int maxStep, Phaser phaser) {
+        id = nextId.getAndIncrement();
+        this.maxStep = maxStep;
+        this.phaser = phaser;
+        phaser.register();
+    }
+
+    @Override
+    public void run() {
+        System.out.println(this + " is on the starting line");
+        phaser.arriveAndAwaitAdvance();
+
+        while (phaser.getPhase() <= this.maxStep) {
+            System.out.println(this + " started stage " + phaser.getPhase());
+            try {
+                Utils.randomSleep(1000, 5000);
+            } catch (InterruptedException e) {
+                break;
+            }
+            System.out.println(this + " finished stage " + phaser.getPhase());
+            phaser.arriveAndAwaitAdvance();
+        }
+        phaser.arriveAndDeregister();
+    }
+
+    @Override
+    public String toString() {
+        return "Runner " + id;
+    }
+}

src/exercises/ex_y/answers.txt

@@ -0,0 +1 @@
+We should use a Phaser as it allows for threads to wait on each other before moving to the next step