java – ArrayList在每个索引处返回0

private static int LIST_TRAVERSE_LIMIT = 10000; // size of problem

private boolean problem_3_failed = false; // set to true if failed

private List<Integer> data = new ArrayList<Integer>(); // data shared between threads,traversed and modified

private int negative_hits = 0; // counter for how many modifications are found during traversal

private void test_parallel_iteration() {
    for(int i=0; i<LIST_TRAVERSE_LIMIT; i++) data.add(i);
    // Define threads and run them
    Thread t1 = new Thread(() -> { // modify the list in just a few places
        synchronized(data){
            for(int i=0; i<LIST_TRAVERSE_LIMIT; i++) 
                if(data.get(i)%(LIST_TRAVERSE_LIMIT/3)==0) data.add(0,-data.get(i));
        }
    });
    Thread t2 = new Thread(() -> { // iterate the list
        try {
            synchronized(data){
                for(int i: data) 
                    if(i<0) {
                        System.out.println("Found negative number: "+i);
                        negative_hits++;
                    }
            }
        } catch(ConcurrentModificationException exn) {
            System.out.println("Problem 3,thread 2 failed: concurrent modification");
            problem_3_failed = true;
        }
    });
    finish(t1,t2);
    // What happened?
    System.out.println("#Negative hits: "+negative_hits);
    if(problem_3_failed) System.out.println("Problem 3 failed");
    else System.out.println("Problem 3 succeeded");
}

private void finish(Thread t1,Thread t2) {
    t1.start();
    t2.start();
    try {
        t1.join();
        t2.join();
    } catch (InterruptedException e) {
        throw new Error("Internal error: interrupted!");
    }
}

输出：

#Negative hits: 0
Problem 3 succeeded