x = y使x指向y吗？

Question

I am sorry for such a stupid question, but I cant describe my problem any better. I have 2 classes:

    //First Class
    public class VKtracks extends Fragment {

        RecyclerView list;
        SongListAdapter listAdapter;
        private ArrayList<song> songs = new ArrayList<>();
        MainActivity mainActivity;

        public VKtracks(){

        }

        @Override
        public void onActivityCreated (Bundle savedInstanceState){
            super.onActivityCreated(savedInstanceState);
            mainActivity = (MainActivity)getActivity();
            songs = mainActivity.songs;
            Iterator<song> itr = songs.iterator();
            while (itr.hasNext()) {
                song element = itr.next();
                if(element.getSource() == 0){
                    itr.remove();
                }
            }
        }

    //Second class

public class Tracks extends Fragment {


    RecyclerView list;
    private MediaPlayer mp = new MediaPlayer();
    SongListAdapter listAdapter;
    private ArrayList<song> songs = new ArrayList<>();
    ImageButton playButton;
    boolean playerActive = false;
    MainActivity mainActivity;
    int mLastFirstVisibleItem = 0;


    public Tracks() {
    }

    @Override
    public void onActivityCreated (Bundle savedInstanceState){
        super.onActivityCreated(savedInstanceState);
        mainActivity = (MainActivity)getActivity();
        songs = mainActivity.songs;
        listAdapter = new SongListAdapter(getContext(),songs, mainActivity);
        listAdapter.notifyDataSetChanged();
        list.setHasFixedSize(true);
        LinearLayoutManager llm = new LinearLayoutManager(getActivity());
        list.setLayoutManager(llm);
        list.setAdapter(listAdapter);
    }
}

In both classes I am getting a list of songs from MainActivity. In the Tracks class I am just filling the RecyclerView with it, and in the VKtracks I am deliting unnecessary items with iterator and filling another RecyclerView. At the end i have 2 similar filtred lists. If I comment filtering part in the VKTracks class I am getting 2 similar unfiltred lists.

So, why this happens?

The only reasong I can suppose is that both songs ArrayLists are poiners to a songs ArrayList in the MainActivity.

Answer 1

Your reasoning is correct. The pointers Tracks.songs , VKTracks.songs , and MainActivity.songs are all point to the same list, so modifying either will change all three. Try using the ArrayList copy constructor in Tracks and VXTracks to create three unique array lists. To do this, replace the line

songs = mainActivity.songs;

with

songs = new ArrayList(mainActivity.songs);

in both Tracks and VKTracks .

Answer 2

It looks like you have two subclasses of Fragment and use its "getActivity()" method with after initialising with "savedInstance".

My guess is that in both cases the "songs" attribute gives you excatly the same pointer. So if you modify the list in one place, this modifies it everywhere.

To stop this behavior try to copy this list before modifying.

Answer 3

You are right, fragment.songs = mainActivity.songs points fragment.songs to the list maintained in mainActivity.

Thus, fragment.songs.remove(song) removes the song from that list.

Also, altering a song in fragment.songs (eg adding a comment) would in fact alter that object in the mainActivity.songs list.

To avoid this behaviour, you'd have to create a copy of the list and its items.

Answer 4

I am not exactly sure what you are asking, but I will try my best to give you a brief explanation of objects vs primitive values.

Consider this code:

int x;
int y = 1;
x = y; //x is now 1
y = 2; //y is now 2, but x is still 1
System.out.print(y); //prints 2
System.out.print(x); //prints 1

Since int is a primitive value and not an object, writing x = y; does not make x point to y since y is not an object, there is nothing to point to. x just gets the value of 2 . This is the same as writing x = 2; .

Now consider this code:

Person x;
Person y = new Person("Johnny");
x = y; //x is now pointing to the same object as y
y.changeName("Felicity"); //this changes the name of the object to Felicity
y.printName(); //prints Felicity
x.printName(); //also prints Felicity

Since a Person is an object, writing x = y; does make these two variables point to the same object. Any change made to the object via any variable is visible via the other variable as well.

Answer 5

In short,

For primitive variable (eg int, double, char, boolean etc): No

when you use x=y,

x then hold the value of y, change in y later on will not affect the value in x

For object (even Integer, Double): Yes

since

A x;
A y = new A();// here y holds the reference of a new object of class A created.
x = y // x then holds the reference of the same object of y

so x and y 'point to' the same object (They are called reference in Java's term)

Side notes:

When you say 'point to', if you are talking about c++ pointers, then the answer is Yes for both primitive and object, but well, it is a Java question.