List<String> oldList = new ArrayList<>();
oldList.add("a");
oldList.add("b");
oldList.add("c");
System.out.println("oldList ## " + oldList);

List<String> newList = new ArrayList<>();
newList.add("a");
newList.add("1");
newList.add("2");
System.out.println("newList ## " + newList);

oldList.retainAll(newList);
System.out.println("求交集 ## " + oldList);

newList.retainAll(oldList);
System.out.println("求交集 ## " + newList);

oldList.removeAll(newList);
System.out.println("oldList.removeAll(newList);  --> 求差集（ 在 oldList 集合中，刪掉 oldList 與 newList 中都有的元素）： ## " + oldList);

newList.removeAll(oldList);
System.out.println("newList.removeAll(oldList);  --> 求差集（ 在 newList 集合中，刪掉 oldList 與 newList 中都有的元素）： ## " + newList);

    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    } 

    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

oldList.addAll(newList);
System.out.println("求并集 ##" + oldList);

newList.addAll(oldList);
System.out.println("求并集 ##" + newList);

oldList.removeAll(newList); // 先求差集得到去除相同項后的 oldList
oldList.addAll(newList); // 再求并集
System.out.println("求并集 ## " + oldList);

newList.removeAll(oldList);
newList.addAll(oldList);
System.out.println("求并集 ## " + newList);