How to determine whether there is an intersection between two collections in java

Background:

The front-end passes the list collection, and the back-end fields are also in the form of (1,2,3,4). Without using sql, how to check whether the set passed by the front end is in the set of back-end fields?

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Code:

public static boolean judgeIntersection(List<String> list1,List<String> list2){
    boolean flag = false;
    // 使用retainAll会改变list1的值，所以写一个替代
    List<String> origin = new ArrayList<>();
    origin.addAll(list1);
    origin.retainAll(list2);
    // 有交集
    if(origin.size()>0){
        flag = true;
    }
    return flag;
}

boolean flag = origin.retainAll(Collection> ; c)

If there is data in origin that is not in set C, return false. No true is returned

At the same time, the origin set will change, and only the same data of the two sets will be retained. In other words, the origin set has deleted data, and false will be returned

How to judge whether there is Intersection?

1. Two sets, none of them are the same, origin is empty, and the return value is false

2. Two sets, by chance, the data in origin are all in c, origin No change, the return value is true

These two special cases lead to the fact that it is impossible to judge the intersection simply by relying on the return value true or false. Therefore, depending on the number of origin collections, >0, there is an intersection

##retainAll(Collection> c) source code

public boolean retainAll(Collection<?> c) {
        // 判断c集合是否为空
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }
 
    private boolean batchRemove(Collection<?> c, boolean complement) {
        // 得到调用该函数的集合。因为是引用类型，所以修改了都会有变化
        final Object[] elementData = this.elementData;
        // w:记录交集的数据都放到elementData前面，w是其位置分界线
        int r = 0, w = 0;
        // 返回值 用于判断elementData有没有被修改
        boolean modified = false;
        try {
            // 循环elementData集合，判断其中元素是否在c集合中
            for (; r < size; r++)
                // 若在集合中，则w自增，并将该值放到elementData[w]中，即是交集的数据都放到集合的前面
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            // 正常情况下，经过上面的循环，r==size。为防止出现循环异常，将由于异常导致的r到size是交集的数据但并没有放到对应w的位置的数据，都放到对应w之后的位置上
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            // 若elementData中有数据不在c集合中，就清理掉w位置之后的数据，便于垃圾回收
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                // 记录elementData集合被增删的次数，这里是删除
                modCount += size - w;
                // 赋值最新的size
                size = w;
                // elementData集合被清理，modified为true
                modified = true;
            }
        }
        return modified;
    }

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