// 初始化 list 为 { 1, 2, 3, 4, 5 }
List<Integer> list = new ArrayList<>();
for (int i = 1; i <= 5; i++) {
    list.add(i);
}

// 取前 3 个元素作为 subList，操作 subList
List<Integer> subList = list.subList(0, 3);
subList.add(6);

System.out.println(list.size());

/**
 * Returns a view of the portion of this list between the specified
 * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.  (If
 * <tt>fromIndex</tt> and <tt>toIndex</tt> are equal, the returned list is
 * empty.)  The returned list is backed by this list, so non-structural
 * changes in the returned list are reflected in this list, and vice-versa.
 * The returned list supports all of the optional list operations supported
 * by this list.<p>
 *
 * This method eliminates the need for explicit range operations (of
 * the sort that commonly exist for arrays).  Any operation that expects
 * a list can be used as a range operation by passing a subList view
 * instead of a whole list.  For example, the following idiom
 * removes a range of elements from a list:
 * <pre>{@code
 *      list.subList(from, to).clear();
 * }</pre>
 * Similar idioms may be constructed for <tt>indexOf</tt> and
 * <tt>lastIndexOf</tt>, and all of the algorithms in the
 * <tt>Collections</tt> class can be applied to a subList.<p>
 *
 * The semantics of the list returned by this method become undefined if
 * the backing list (i.e., this list) is <i>structurally modified</i> in
 * any way other than via the returned list.  (Structural modifications are
 * those that change the size of this list, or otherwise perturb it in such
 * a fashion that iterations in progress may yield incorrect results.)
 *
 * @param fromIndex low endpoint (inclusive) of the subList
 * @param toIndex high endpoint (exclusive) of the subList
 * @return a view of the specified range within this list
 * @throws IndexOutOfBoundsException for an illegal endpoint index value
 *         (<tt>fromIndex &lt; 0 || toIndex &gt; size ||
 *         fromIndex &gt; toIndex</tt>)
 */
List<E> subList(int fromIndex, int toIndex);

private class SubList extends AbstractList<E> implements RandomAccess {
    private final AbstractList<E> parent;
    // ...

    SubList(AbstractList<E> parent,
            int offset, int fromIndex, int toIndex) {
        this.parent = parent;
        // ...
        this.modCount = ArrayList.this.modCount;
    }

    public E set(int index, E e) {
        // ...
        checkForComodification();
        // ...
        ArrayList.this.elementData[offset + index] = e;
        // ...
    }

    public E get(int index) {
        // ...
        checkForComodification();
        return ArrayList.this.elementData(offset + index);
    }

    public void add(int index, E e) {
        // ...
        checkForComodification();
        parent.add(parentOffset + index, e);
        this.modCount = parent.modCount;
        // ...
    }

    public E remove(int index) {
        // ...
        checkForComodification();
        E result = parent.remove(parentOffset + index);
        this.modCount = parent.modCount;
        // ...
    }

    private void checkForComodification() {
        if (ArrayList.this.modCount != this.modCount)
            throw new ConcurrentModificationException();
    }

    // ...
}

list.add(0, 0);
System.out.println(subList);