数组列表 <DisplayMode> Java中的空指针异常

Question

Inside of this class I have

public DisplayMode resolutions[] = {
    new DisplayMode(1920, 1080, 32, 0),
    new DisplayMode(1680, 1050, 32, 0),
};

public ArrayList<DisplayMode> list;

and the constructor

14 - public myClass(){
15 -     list.add(resolutions[0]);
16 - }

I'm getting a null pointer exception on line 15. What's wrong?

Answer 1

You can't add elements to your list without instantiating it (it's null )

public ArrayList<DisplayMode> list = new ArrayList<>();

I suggest you use the List interface. Also, Java naming convention would be MyClass and you could initialize list in the constructor with Arrays.asList(T...) (which I suggest you not make public ) like

private List<DisplayMode> list;
public MyClass() {
  list = new ArrayList<>(Arrays.asList(resolutions));
}