为什么列表大小返回0？

Question

why the list size return 0 in toast?

Activity Class

List<Models> models= new ArrayList<>();
List<String> Images = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
        putData();
        Toast.makeText(this, "" + models.get(0).Images.size(), Toast.LENGTH_SHORT).show();
    }

    void putData() {


        for (int x = 1; x <= 6; x++) {
            Images.add(String.valueOf(x) + ".png");
        }
        models.add(new Model(Images));
        Images.clear();



    }

Model Class

public class Models {


    List<String> Images;

    public Models(List<String> images) {
        Images = images;
    }
}

I hope you help me...........................................................

Answer 1

Because of this line:

Images.clear();

which clears the list Images .
Maybe you think that Images and models.get(0).Images are 2 different lists.
They are not as they both point to the same list.
Remove this

List<String> Images = new ArrayList<>();

from your activity class declarations and change putData() :

void putData() {
    List<String> Images = new ArrayList<>();
    for (int x = 1; x <= 6; x++) {
        Images.add(String.valueOf(x) + ".png");
    }
    models.add(new Model(Images));
}

Answer 2

You've called clear() on the images. They are gone by the point you call size().

Answer 3

Because you're retaining the instance of Images and then calling clear() on it.

When you create a new ArrayList in Java, something like:

ArrayList<Something> somethings = new ArrayList<>();

and then pass it to another class:

SomeClass someClass = new SomeClass(somethings);

the ArrayList contained by SomeClass is the exact same one referenced by somethings .

When you call putData() , you're passing a single instance of your ArrayList to your Models class.

You do fill it, but then you immediately clear it. Since you clear the Images list in your putData() class, you also clear the Images list in your Models instance, because the two lists are the same.

Make it a local variable and remove the clear() :

void putData() {
    ArrayList<String> Images = new ArrayList<>();
    for (int x = 1; x <= 6; x++) {
        Images.add(String.valueOf(x) + ".png");
    }
    models.add(new Model(Images));
}