AlexRogalskiy · October 25, 2024 08:48 · Mar 8, 2016 · Nov 20, 2013
diff --git a/Counter.java b/Counter.java
@@ -0,0 +1,20 @@
+public class Counter {
+
+    private int count = 0;
+
+    public int getCount() {
+        return count ;
+    }
+
+    /**
+     * The "synchronized" keyword is crucial here; removing it will lead to serious
+     * race conditions on multiple-core computers. Having it ensures that two threads
+     * can't enter this method at the same time, making sure that only one thread passes
+     * through this method at a time, which ensures that the increments are handled
+     * correctly.
+     */
+    public synchronized void incrementCount() {
+        ++count;
+    }
+
+}
diff --git a/SynchronizedIncrement.java b/SynchronizedIncrement.java
@@ -0,0 +1,34 @@
+public class SynchronizedIncrement implements Runnable {
+
+    private static final int NUM_THREADS = 4;
+    private static final int NUM_INCREMENTS = 10000;
+    private Counter counter;
+
+    public SynchronizedIncrement(Counter counter) {
+        this.counter = counter;
+    }
+
+    public static void main(String[] args) throws InterruptedException {
+        Thread[] threads = new Thread[NUM_THREADS];
+        Counter counter = new Counter();
+
+        for (int i=0; i<NUM_THREADS; ++i) {
+            threads[i] = new Thread(new SynchronizedIncrement(counter));
+            threads[i].start();
+        }
+
+        for (int i=0; i<NUM_THREADS; ++i) {
+            threads[i].join();
+        }
+
+        System.out.println("total count = " + counter.getCount() + " vs. expected = " + (NUM_THREADS * NUM_INCREMENTS));
+    }
+
+    @Override
+    public void run() {
+        for (int i=0; i<NUM_INCREMENTS; ++i) {
+            counter.incrementCount();
+        }
+    }
+
+}
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