u06-4

u06-4/AtomicCounter.java

@@ -0,0 +1,22 @@
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.IntUnaryOperator;
+
+class AtomicCounter {
+	private AtomicInteger i;
+	
+	public AtomicCounter() {
+		this.i = new AtomicInteger(0);
+	};
+
+	public int updateAndGet(IntUnaryOperator op) {
+		// not safe if op is not a pure function
+
+		int oldVal;
+
+		do {
+			oldVal = i.get();
+		} while (!i.compareAndSet(oldVal, op.applyAsInt(oldVal)));
+
+		return op.applyAsInt(oldVal);
+	}
+}

u06-4/AtomicCounterTest.java

@@ -0,0 +1,30 @@
+public class AtomicCounterTest {
+	private static final int N_THREADS = 100;
+	private static final int INCREMENT = 1_000_000;
+	private static void incCounter(AtomicCounter atomicCounter) {
+		for (int i= 0; i < INCREMENT; ++i) {
+			atomicCounter.updateAndGet(x -> x+1);
+		}
+	}
+	public static void main(String[] args) {
+		AtomicCounter atomicCounter = new AtomicCounter();
+		Thread[] threads = new Thread[N_THREADS];
+		for (int i = 0; i < N_THREADS; ++i) {
+			Thread incrementer = new Thread(()-> incCounter(atomicCounter));
+			incrementer.start();
+			threads[i] = incrementer;
+		}
+		for (Thread incrementer: threads) {
+			try {
+				incrementer.join();
+			} catch (InterruptedException e) {
+				e.printStackTrace();
+			}
+		}
+		System.out.println("Counter value = " + 
+		               		atomicCounter.updateAndGet(x -> x)  + // only to get, not to change
+							" and that is" + 
+		               		(atomicCounter.updateAndGet(x -> x) == N_THREADS * INCREMENT ? "" : "NOT") + 
+		               		" correct.");
+	}
+}