【Java】ArrayList线程不安全的坑

    public static void main(String[] args) {
        List<Integer> a = new ArrayList<>();
        for (int i = 0; i < 20; i++) {
            a.add(i);
        }
        List<String> a1 = new ArrayList<>();
        a.parallelStream().forEach(x -> {
            a1.add(String.valueOf(x));
        });
        System.out.println(a1);
    }

[12, 6, 14, 5, 13, 8, 11, 9, 10, 7, 2, 17, 1, 4, 0, null, 19, 18, 16, 15]

    private void add(E e, Object[] elementData, int s) {
        if (s == elementData.length) {
            elementData = this.grow();
        }

        elementData[s] = e;
        this.size = s + 1;
    }

    public boolean add(E e) {
        ++this.modCount;
        this.add(e, this.elementData, this.size);
        return true;
    }