Android - Adapter NullPointerException

Question

I apologize in advance if this is a duplicate. I am still new to android development and tried looking for a resolution however could not find one that works.

I am creating a to-do app and getting this error in my adapter.

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String assignment.Model.getAssignment()' on a null object reference
at assignment.Adapter.getView(Adapter.java:39)    

and the line of code that it is referencing to is

assignment.setText(modelItems[position].getAssignment());

I believe that the position that I am setting it as is what is causing the error but I'm not sure how to fix it.

Here's part of the rest of my code for reference:

MainActivity.Java - onActivityResult

   @Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    String assignmentSentBack = data.getStringExtra("editAssignment");
    Integer monthSentBack = data.getIntExtra("month", 0);
    Integer daySentBack = data.getIntExtra("day", 0);

    modelItems = new Model[100];
    ArrayList<Model> modelArrayList = new ArrayList<>(Arrays.asList(modelItems));
    modelArrayList.add(new Model(assignmentSentBack, (monthSentBack + 1) + "/" + daySentBack, 0));
    lv = (ListView) findViewById(R.id.listAssignment);
    ListAdapter adapter = new Adapter(this, modelItems);
    lv.setAdapter(adapter);
}

Second Activity - onSendActivity (Button)

public void onSendAssignment (View view) {

    EditText editAssignmentET = (EditText)
            findViewById(R.id.editAssignment);

    String editAssignment = String.valueOf(editAssignmentET.getText());

    DatePicker datePickerDP = (DatePicker)
            findViewById (R.id.datePicker);

    Integer month = Integer.valueOf(datePickerDP.getMonth());

    Integer day = Integer.valueOf(datePickerDP.getDayOfMonth());

    Intent goingBack = new Intent();

    goingBack.putExtra("editAssignment", editAssignment);
    goingBack.putExtra ("month", month);
    goingBack.putExtra("day", day);

    setResult(RESULT_OK, goingBack);

    finish();
}

Adapter

public class Adapter extends ArrayAdapter {

Model[] modelItems = null;
Context context;

public Adapter(Context context, Model[] resource) {
    super(context, R.layout.row, resource);
    this.context = context;
    this.modelItems = resource;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
        LayoutInflater inflater = ((Activity) context).getLayoutInflater();
        convertView = inflater.inflate(R.layout.row, parent, false);
        TextView assignment = (TextView) convertView.findViewById(R.id.assignment);
        TextView dueDate = (TextView) convertView.findViewById(R.id.dueDate);
        CheckBox cb = (CheckBox) convertView.findViewById(R.id.checkBox);
        assignment.setText(modelItems[position].getAssignment());
        dueDate.setText(modelItems[position].getDueDate());
        if (modelItems[position].getValue() == 1)
            cb.setChecked(true);
        else
            cb.setChecked(false);

        return convertView;
    }
}

Model

public class Model {

String assignment;
String dueDate;
int value;

Model (String assignment, String dueDate, int value){
    this.assignment = assignment;
    this.dueDate = dueDate;
    this.value = value;
}

public String getAssignment(){
    return this.assignment;
}

public String getDueDate(){
    return this.dueDate;
}

public int getValue(){
    return this.value;
    }
}    

Any help is appreciated. Thank you.

Answer 1

you should try to wrap it in an inner Holder class and define the parameters in that class

public class Adapter extends ArrayAdapter {

Model[] modelItems = null;
Context context;

public Adapter(Context context, Model[] resource) {
    super(context, R.layout.row, resource);
    this.context = context;
    this.modelItems = resource;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
        LayoutInflater inflater = ((Activity) context).getLayoutInflater();
        if (convertView == null) {
            holder = new ViewHolder();

            convertView = vi.inflate(R.layout.list_layout, null);

            // Find the child views.
            holder.assignment= (TextView) convertView.findViewById(R.id.txt_name);
            holder.dueDate= (Button) convertView.findViewById(R.id.btn_invite);
            holder.cb= (Button) convertView.findViewById(R.id.btn_track);
            convertView.setTag(holder);

//....

        }
        // Reuse existing row view
        else {
            holder = (ViewHolder)convertView.getTag();

        }

        return convertView;
    }
 class ViewHolder {

        TextView assignment;
TextView dueDate;
CheckBox cb;

    }
}

Answer 2

You have created model array of size 100 here:

modelItems = new Model[100];

So, 100 models are being created but all 100 indexes have null value.

Then you have created ArrayList using that array:

ArrayList<Model> modelArrayList = new ArrayList<>(Arrays.asList(modelItems));

So again your modelArrayList has 100 null objects. Which BTW you are using no where.

You are passing modelItems into constructor of Adapter. So now since all you items are null, you are getting this exception.

Try to do something like this:

ArrayList<Model> modelArrayList = new ArrayList<>();
modelArrayList.add(new Model(assignmentSentBack, (monthSentBack + 1) + "/" + daySentBack, 0));

Similarly add more model objects like that. Pass this modelArrayList in your adapter's constructor and use this (instead of array) to display the list.

lv = (ListView) findViewById(R.id.listAssignment);
ListAdapter adapter = new Adapter(this, modelArrayList)

And thus your adapter will be like this:

ArrayList<Model> modelArrayList;
Context context;

public Adapter(Context context, ArrayList<Model> resource) {
    super(context, R.layout.row, resource);
    this.context = context;
    this.modelArrayList = resource;
}

Answer 3

change

public class Adapter extends ArrayAdapter {

to

public class Adapter extends ArrayAdapter<Model> {