synchronizedList needs manual locking to iterate

Each individual call is synchronized, but a compound action such as iteration is not. Hold the lock yourself, or use a concurrent collection instead.

Code

List<Integer> shared = Collections.synchronizedList(new ArrayList<>());
var pool = Executors.newFixedThreadPool(4);

for (int i = 0; i < 200; i++) { int v = i; pool.submit(() -> shared.add(v)); }
pool.shutdown();
pool.awaitTermination(10, TimeUnit.SECONDS);

System.out.println("size = " + shared.size());
synchronized (shared) {                       // required for safe iteration
    System.out.println("sum  = " + shared.stream().mapToInt(Integer::intValue).sum());
}

Output

size = 200
sum  = 19900

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Published 2026-07-30