解决：java.util.ConcurrentModificationException: null：异常

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public class Test {

    public static void main(String[] args) {

        List<String> list = new ArrayList<>();
        list.add("aa");
        list.add("bb");
        list.add("cc");
        list.add("dd");
        list.add("ee");
        list.add("ff");
        list.add("gg");
        list.add("hh");
        list.add("kk");
        list.add("yy");
        list.add("oo");
        list.forEach(s -> {
            list.add("tt");
        });
    }
}

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Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList.forEach(ArrayList.java:1260)
    at com.hzt.tool.Test.main(Test.java:28)

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    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }


   private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

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for (int i = 0; i < list.size(); i++) {
            list.add("tt");
}