From 81be577d016aec4a8ea97c0244a3eadc78be2106 Mon Sep 17 00:00:00 2001
From: Selebrator <l.panneke@googlemail.com>
Date: Tue, 18 Jun 2019 23:07:54 +0200
Subject: [PATCH] =?UTF-8?q?Alternative=20f=C3=BCr=2011.2?=
MIME-Version: 1.0
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---
 .../java/_11/_2/alternative/Barriers.java     | 16 ++++++++++
 .../java/_11/_2/alternative/Checkpoint.java   | 31 +++++++++++++++++++
 .../java/_11/_2/alternative/NumberRunner.java | 27 ++++++++++++++++
 3 files changed, 74 insertions(+)
 create mode 100644 src/main/java/_11/_2/alternative/Barriers.java
 create mode 100644 src/main/java/_11/_2/alternative/Checkpoint.java
 create mode 100644 src/main/java/_11/_2/alternative/NumberRunner.java

diff --git a/src/main/java/_11/_2/alternative/Barriers.java b/src/main/java/_11/_2/alternative/Barriers.java
new file mode 100644
index 0000000..03f81f4
--- /dev/null
+++ b/src/main/java/_11/_2/alternative/Barriers.java
@@ -0,0 +1,16 @@
+package _11._2.alternative;
+
+public class Barriers {
+
+	public static final int NUMBER_OF_RUNNERS = 3;
+
+	public static void main(String[] args) {
+		NumberRunner[] runners = new NumberRunner[NUMBER_OF_RUNNERS];
+		for(int i = 0; i < NUMBER_OF_RUNNERS; i++) {
+			runners[i] = new NumberRunner(i);
+		}
+		for(int i = 0; i < NUMBER_OF_RUNNERS; i++) {
+			runners[i].start();
+		}
+	}
+}
diff --git a/src/main/java/_11/_2/alternative/Checkpoint.java b/src/main/java/_11/_2/alternative/Checkpoint.java
new file mode 100644
index 0000000..c8e3f25
--- /dev/null
+++ b/src/main/java/_11/_2/alternative/Checkpoint.java
@@ -0,0 +1,31 @@
+package _11._2.alternative;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class Checkpoint {
+
+	private static AtomicInteger GATHERED = new AtomicInteger(0);
+
+	private final Object lock = new Object();
+	private int groupSize;
+
+	public Checkpoint(int groupSize) {
+		this.groupSize = groupSize;
+	}
+
+	/**
+	 * Blocks until called by {@link #groupSize} different Threads,
+	 * then wakes all of them up again.
+	 */
+	public void gather() throws InterruptedException {
+		synchronized(this.lock) {
+			if(GATHERED.incrementAndGet() != this.groupSize) {
+				this.lock.wait();
+			} else {
+				GATHERED.set(0);
+				System.out.println("Group gathered");
+				this.lock.notifyAll();
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/src/main/java/_11/_2/alternative/NumberRunner.java b/src/main/java/_11/_2/alternative/NumberRunner.java
new file mode 100644
index 0000000..a9017e1
--- /dev/null
+++ b/src/main/java/_11/_2/alternative/NumberRunner.java
@@ -0,0 +1,27 @@
+package _11._2.alternative;
+
+public class NumberRunner extends Thread {
+
+	private static final Checkpoint CHECKPOINT = new Checkpoint(Barriers.NUMBER_OF_RUNNERS);
+
+	private final int id;
+
+	public NumberRunner(int id) {
+		this.id = id;
+	}
+
+	@Override
+	public void run() {
+		for(int i = 0; i < 1000; i++) {
+			if(i % 10 == 0) {
+				try {
+					CHECKPOINT.gather();
+				} catch(InterruptedException e) {
+					throw new IllegalStateException("Group can never be complete again, because one of the members got lost", e);
+				}
+			}
+
+			System.out.println("Thread " + this.id + ": " + i);
+		}
+	}
+}