使用 ViewPager 的 Layout Inflater 抛出 NullPointerException

Question

我正在开发一个基于测验的应用程序。每个问题有 4 个选项。问题是一个文本视图，而对于选项我使用的是 viewpager，以便用户可以滑动将从服务器动态加载的 4 个选项。
我收到了 NullPointerException 的 Layout inflater。
这是我得到空值的行：

LayoutInflater inflater = (LayoutInflater)  
context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View itemView = inflater.inflate(R.layout.viewpager_item, container, false);
class MyAdapter extends PagerAdapter { 
    int NumberOfPages = 4;
    Context context;
    ArrayList<String> option1List; .....

我们非常感谢您的帮助和指导。

Error :

E/AndroidRuntime: FATAL EXCEPTION: main java.lang.NullPointerException 在 quizapp.android.mohit.com.quizapp.MyAdapter.instantiateItem(Questions.java:485)

这是我的 Mainactivity 代码：

  public class Questions extends ActionBarActivity {
  protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_questions);
  dbHelper = new DataBaseHelper(getApplicationContext());

viewPager = (ViewPager) findViewById(R.id.viewPager);
adapter = new MyAdapter(option1List, option2List, option3List,option4List);
viewPager.setAdapter(adapter);

class MyAdapter extends PagerAdapter {
    int NumberOfPages = 4;
    Context context;
    LayoutInflater inflater;
    View itemView;
    ArrayList<String> option1List;
    ArrayList<String> option2List;
    ArrayList<String> option3List;
    ArrayList<String> option4List;

    public MyAdapter(ArrayList<String> option1List, ArrayList<String> option2List, ArrayList<String> option3List, ArrayList<String> option4List) {
        this.option1List = option1List;
        this.option2List = option2List;
        this.option3List = option3List;
        this.option4List = option4List;
    }
    public int getCount(){
        return NumberOfPages;
    }

    public boolean isViewFromObject(View container, Object obj) {
        return container == obj;
        // return container == (View) obj;
    }

    public Object instantiateItem(ViewGroup container, int position) {
        LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View itemView = inflater.inflate(R.layout.viewpager_item, container, false);
        // Declare Variables
        TextView option1, option2, option3, option4;
        // Locate the TextViews in viewpager_item.xml
        option1 = (TextView) itemView.findViewById(R.id.tv_option1);
        option2 = (TextView) itemView.findViewById(R.id.tv_option2);
        option3 = (TextView) itemView.findViewById(R.id.tv_option3);
        option4 = (TextView) itemView.findViewById(R.id.tv_option4);
        // Capture position and set to the TextViews
        option1.setText(option1List.get(0));
        option2.setText(option2List.get(0));
        option3.setText(option3List.get(0));
        option4.setText(option4List.get(0));
        // Add viewpager_item.xml to ViewPager
        container.addView(itemView);
        return itemView;
    }

    public void destroyItem(ViewGroup container, int position, Object obj) {
       container.removeView((LinearLayout)obj);
    }
}

Answer 1

您应该尝试以另一种方式膨胀视图，例如使用 LayoutInflater。您还应该停止使用应用程序上下文，而应该使用您的活动作为上下文。

代码：-

View v = LayoutInflater.from(context).inflate(R.layout.viewpager_item, container, false);

Answer 2

您创建 Adapter 对象并在其中传递四个 ArrayList 但在您的适配器中您没有定义您的构造函数.....请定义它。参见下面的代码

试试这个.....

public class MyAdapter extends PagerAdapter {
    int NumberOfPages = 4;
    LayoutInflater inflater;

public MyAdapter(ArrayList<type you define> option1List, ArrayList<type you define>  option2List, ArrayList<type you define>  option3List,ArrayList<type you define> option4List) {

}
    public int getCount(){
        return NumberOfPages;
    }

    public boolean isViewFromObject(View container, Object obj) {
        return container == obj;
        // return container == (View) obj;
    }

    public Object instantiateItem(ViewGroup container, int position) {
        inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View itemView = inflater.inflate(R.layout.viewpager_item, container, false);
        // Declare Variables
        TextView option1, option2, option3, option4;
        // Locate the TextViews in viewpager_item.xml
        option1 = (TextView) itemView.findViewById(R.id.tv_option1);
        option2 = (TextView) itemView.findViewById(R.id.tv_option2);
        option3 = (TextView) itemView.findViewById(R.id.tv_option3);
        option4 = (TextView) itemView.findViewById(R.id.tv_option4);
        // Capture position and set to the TextViews
        option1.setText(option1List.get(0));
        option2.setText(option2List.get(0));
        option3.setText(option3List.get(0));
        option4.setText(option4List.get(0));
        // Add viewpager_item.xml to ViewPager
        container.addView(itemView);
        return itemView;
    }

    public void destroyItem(ViewGroup container, int position, Object obj) {
       container.removeView((LinearLayout)obj);
    }
}

Answer 3

创建一个MyAdapter 构造函数，其中Context 作为其参数之一，例如

private Context context;
private ArrayList<String>arrayList;
public MyAdapter(ArrayList<String>, Context context){
this.context = context;
this.arrayList = arrayList;

在你的活动中，只需通过传入数组列表和上下文来初始化适配器

myAdapter = new MyAdapter(myList, getApplicationContext());