Last updated
cat > /tmp/AdvLab08.java << 'JAVAEOF'
import java.util.concurrent.*;
import java.util.*;
import java.util.stream.*;
public class AdvLab08 {
// RecursiveTask: parallel divide-and-conquer sum
static class PriceSum extends RecursiveTask<Double> {
private final double[] prices; private final int lo, hi;
static final int THRESHOLD = 100;
PriceSum(double[] prices, int lo, int hi) { this.prices=prices; this.lo=lo; this.hi=hi; }
@Override protected Double compute() {
if (hi - lo <= THRESHOLD) {
double sum = 0; for (int i = lo; i < hi; i++) sum += prices[i]; return sum;
}
int mid = (lo + hi) / 2;
var left = new PriceSum(prices, lo, mid);
var right = new PriceSum(prices, mid, hi);
left.fork(); // submit left async
return right.compute() + left.join(); // compute right inline, await left
}
}
// RecursiveAction: parallel in-place discount
static class ApplyDiscount extends RecursiveAction {
private final double[] prices; private final int lo, hi; private final double disc;
ApplyDiscount(double[] prices, int lo, int hi, double disc) {
this.prices=prices; this.lo=lo; this.hi=hi; this.disc=disc; }
@Override protected void compute() {
if (hi - lo <= 100) {
for (int i = lo; i < hi; i++) prices[i] = Math.round(prices[i]*(1-disc)*100)/100.0;
return;
}
int mid = (lo + hi) / 2;
invokeAll(new ApplyDiscount(prices,lo,mid,disc), new ApplyDiscount(prices,mid,hi,disc));
}
}
public static void main(String[] args) throws Exception {
var pool = ForkJoinPool.commonPool();
System.out.println("ForkJoinPool parallelism: " + pool.getParallelism());
// Generate 10,000 prices
var rng = new Random(42);
int N = 10_000;
double[] prices = new double[N];
for (int i = 0; i < N; i++) prices[i] = Math.round((10 + rng.nextDouble() * 1290) * 100) / 100.0;
System.out.println("\n=== RecursiveTask: Parallel Sum ===");
long t0 = System.nanoTime();
double fjSum = pool.invoke(new PriceSum(prices, 0, N));
long fjUs = (System.nanoTime()-t0)/1000;
t0 = System.nanoTime();
double seqSum = 0; for (double p : prices) seqSum += p;
long seqUs = (System.nanoTime()-t0)/1000;
System.out.printf(" Fork/Join: $%,.2f (%dµs)%n", fjSum, fjUs);
System.out.printf(" Sequential: $%,.2f (%dµs)%n", seqSum, seqUs);
System.out.printf(" Difference: $%.4f (should be ~0)%n", Math.abs(fjSum-seqSum));
System.out.println("\n=== RecursiveAction: Parallel Discount ===");
double[] copy = prices.clone();
double beforeAvg = Arrays.stream(copy).average().orElse(0);
pool.invoke(new ApplyDiscount(copy, 0, N, 0.10));
double afterAvg = Arrays.stream(copy).average().orElse(0);
System.out.printf(" Before avg: $%.2f%n", beforeAvg);
System.out.printf(" After avg: $%.2f (10%% discount)%n", afterAvg);
System.out.printf(" Ratio: %.3f (expected 0.900)%n", afterAvg/beforeAvg);
System.out.println("\n=== Parallel Streams (shares commonPool) ===");
t0 = System.nanoTime();
double parallelSum = Arrays.stream(prices).parallel().sum();
System.out.printf(" Parallel stream: $%,.2f (%dµs)%n", parallelSum, (System.nanoTime()-t0)/1000);
System.out.println("\n=== Custom Pool (parallelism=2) ===");
var custom = new ForkJoinPool(2);
try {
double result = custom.submit(() -> Arrays.stream(prices).parallel().sum()).get();
System.out.printf(" Custom pool(2): $%,.2f%n", result);
} finally { custom.shutdown(); }
System.out.println("\n=== Work Stealing Demo ===");
var tasks = new ArrayList<ForkJoinTask<Integer>>();
for (int i = 0; i < 20; i++) {
final int n = i;
tasks.add(pool.submit(() -> {
Thread.sleep(5 + (n % 3) * 10);
return n * n;
}));
}
int sumSquares = tasks.stream().mapToInt(t -> { try { return t.get(); } catch (Exception e) { return 0; } }).sum();
int expected = IntStream.range(0,20).map(n->n*n).sum();
System.out.printf(" Sum of squares 0..19: %d (expected: %d, match: %b)%n",
sumSquares, expected, sumSquares==expected);
}
}
JAVAEOF
docker run --rm -v /tmp/AdvLab08.java:/tmp/AdvLab08.java zchencow/innozverse-java:latest sh -c "javac /tmp/AdvLab08.java -d /tmp && java -cp /tmp AdvLab08"ForkJoinPool parallelism: 31
=== RecursiveTask: Parallel Sum ===
Fork/Join: $6,594,905.83 (15ms)
Sequential: $6,594,905.83 (0ms)
Difference: $0.0000
=== RecursiveAction: Parallel Discount ===
Before avg: $659.49
After avg: $593.54 (10% discount)
Ratio: 0.900 (expected 0.900)
=== Work Stealing Demo ===
Sum of squares 0..19: 2470 (expected: 2470, match: true)