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Remember the use of add method once

2022-07-02 10:07:00 【SpongeBob's nest】

I came across a relatively simple question when I was doing the question today , But the car overturned , After debugging for a long time, I didn't find the cause of the error , The output result is always empty

Suddenly I had a flash of inspiration , Could it be this add There is a problem with your method , After all, there is nothing wrong with the code logic .

So I checked the information, but I didn't get anything ……

I caught a glimpse of a search bar add Method is a copied reference or a value . I suddenly realized , Soon! ,

I left a set , The right set sets , And then run , Ha , That's true

public static void main(String[] args) {
        ArrayList<Integer>list = new ArrayList<>();
        ArrayList<ArrayList<Integer>>result = new ArrayList<>();
        result.add(list);
        for(int i = 0; i < 5; i++){
            list.add(i);
            System.out.println(" Results to be selected "+list);
            System.out.println(" Result set :"+result);
            System.out.println("========================");
        }
}

We can clearly see that we put list Has been added to result in , however list After adding elements in ,result The elements in have also changed , This explanation add What's coming in is list References to , It's not worth it .

public static void main(String[] args) {
        ArrayList<Integer>list = new ArrayList<>();
        ArrayList<ArrayList<Integer>>result = new ArrayList<>();
        result.add(list);
        for(int i = 0; i < 5; i++){
            list.add(i);
            System.out.println(" Results to be selected "+list);
            System.out.println(" Result set :"+result);
            list.remove(list.size()-1);
            System.out.println("result change "+result);
            System.out.println("========================");
        }
    }