使用CustomList适配器时，空指针异常

Question

I am trying to create a list view inside a fragment. The array which is causing a null pointer exception is being taken from a separate class called "getData".

To create the list view i am using the Custom List Adapter. It is only when I put the array into the custom list adapter that i get the error.

Fragment where error evolves:

package com.example.testapp;

import android.os.Bundle;

public class FragmentA extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    View V = inflater.inflate(R.layout.fragment_a, container, false);

    ListView listView = (ListView)V.findViewById(R.id.list);


        Integer[] imageId = {
                R.drawable.ic_launcher,
                R.drawable.ic_launcher,
               };

        getData data = getData.getMyData();

      CustomList adapter = new
            CustomList(getActivity(), data.myArray, imageId); //This is where i put the array.      
      listView.setAdapter(adapter);

    return V;
}
}

getData class (where the array is taken from):

package com.example.testapp;

import java.io.BufferedReader;

public class getData{ 

    private static getData _instance; 


    public String myArray[]; //Array set up


    public static getData getMyData() //This is what the fragment calls to get the array.
    {
        if(_instance == null)
            _instance = new getData();

        return _instance;
    }

public void runData(){
       getData data = getData.getMyData();

       data.myArray[0] = "test"; //Array given value

}

}

Answer 1

You are not setting up array you are just declaring it.

public String myArray[];

The below line will give NP because the array object is not initialized yet.

data.myArray[0] = "test";

you can create array object like this.

public String myArray[] = new String[10];

Here is updated getData class, this is not 100% encapsulated and singleton but it will work. learn more about encapsulating data and singleton.

package com.example.testapp;

import java.io.BufferedReader;

public class getData{ 

    private static getData _instance; 


    public String myArray[] = new String[10]; //Array set up


    public static getData getMyData() //This is what the fragment calls to get the array.
    {
        if(_instance == null)
            _instance = new getData();
            _instance.runData();

        return _instance;
    }

    private void runData(){       
           this.myArray[0] = "test"; //Array given value

    }
}

Answer 2

Change your GetData to

package com.example.testapp;

import java.io.BufferedReader;

public class GetData
{
    private static GetData _instance; 
    public List<String> myArray = null;

    public static GetData getMyData() //This is what the fragment calls to get the array.
    {
        if(_instance == null) _instance = new GetData();
        return _instance;
    }

    private GetData()
    {
        myArray = new LinkedList<String>();
    }

    public void runData()
    {
        GetData data = GetData.getMyData();
        myArray.add(0, "test"); //Array given value
    }
}

Your array was not correctly initialized. If you want to use a String[] array, you must specify a length, using new String[5] for example. But if you don't know the final length, I suggest you to use my above code.