如何从一个活动和另一个活动向数组列表添加元素

Question

I am making an app that consists of multiple choice questions. Each question has its own activity, for example QuestionTwoActivity . Every time the user clicks on a button, for example a button for answer A, it moves them to the next activity, for example from QuestionTwoActivity to QuestionThreeActivity . If the answer is correct, it increases their mark by 3. It has ten questions and a total possible mark of 30.

That part of my app worked well, but the problem started when I wanted to make a final preview that appears on the GradeActivity which initially only showed the final grade of the user. I wanted to add a list to the grade activity that showed the answer of the answer of the user for each question, and if the user answered correctly or not, so I made a custom arrayAdapter to populate the list, and then tried to use the onclick method of each answer to add an element to the arraylist of the arrayadapter based on the question answered, but it caused the app to crash

Here is the code for the gradeActivity which opens only after the tenth question is answered:

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ListView;
import android.widget.TextView;
import java.util.ArrayList;

public class Grade extends AppCompatActivity{

    examark mExamGrade; // declaring an instance of the examark class to call getMark method
    double mark; // an int object to store the exam mark in it
    TextView mGradeTextView;
    TextView mNameTextView;
    ListView mPreviewListView;
    static ArrayList<answers_preview_data> mPreviewArrayList;
    PreviewAdapter mPreviewAdapter;
    examark name;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_grade);

        int colorRed = getResources().getColor(R.color.colorRed);
        int colorOrange = getResources().getColor(R.color.colorOrange);
        int colorYellow = getResources().getColor(R.color.colorYellow);
        int colorGreen = getResources().getColor(R.color.colorGreen);

        mExamGrade = new examark();
        name = new examark();

        mark = mExamGrade.getExamMark();// storing the exam mark in the mark variable

        mGradeTextView = (TextView) findViewById(R.id.grade); // initializing the view responsible for showing the final mark

        mGradeTextView.setText(String.valueOf(mark));

        mNameTextView = (TextView) findViewById(R.id.name_text_view);

        String nameOfQuizzer = name.getNameOfQuizTaker();

        mNameTextView.setText(nameOfQuizzer);

        mPreviewArrayList = new ArrayList<answers_preview_data>();
        mPreviewAdapter = new PreviewAdapter(this, mPreviewArrayList);
        mPreviewListView = (ListView) findViewById(R.id.preview_list);
        mPreviewListView.setAdapter(mPreviewAdapter);

        // a conditional statement to change the color of the grade based on how good the preformence
        if ( mark > 0.0 && mark < 10.0 ) {

            mGradeTextView.setTextColor(colorRed);

        } else if (mark > 10.0 && mark <= 15.0) {

            mGradeTextView.setTextColor(colorOrange);

        } else if (mark > 15.0 && mark < 20.0) {

            mGradeTextView.setTextColor(colorYellow);
        } else if (mark > 20.0) {

            mGradeTextView.setTextColor(colorGreen);
        }
    }
}

Here is the code for one of the answers onclickListner:

mAnswerATextView.setText("A-Labour Party");

    mAnswerATextView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Grade.mPreviewArrayList.add(new answers_preview_data("1", "the answer of question A" , " the true answer of  question 1", R.color.colorGreen, R.drawable.correct));
            Intent openQuestion = new Intent(QuestionOne.this ,QuestionTwo.class);
            startActivity(openQuestion);
            mExamark.incrementMark();
        }
    });

Here is what the debugger menu says when the answer button is clicked:

E/AndroidRuntime: FATAL EXCEPTION: main  
Process: com.example.admin.xams, PID: 16254
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
    at com.example.admin.xams.QuestionOne$1.onClick(QuestionOne.java:52)
    at android.view.View.performClick(View.java:4848)
    at android.view.View$PerformClick.run(View.java:20270)
    at android.os.Handler.handleCallback(Handler.java:815)
    at android.os.Handler.dispatchMessage(Handler.java:104)
    at android.os.Looper.loop(Looper.java:194)
    at android.app.ActivityThread.main(ActivityThread.java:5667)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:962)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)

Is there a way to add an element to an arraylist from other activities before the activity that contains the arraylist is opened (set to onCreate)?

Answer 1

You can implement one of the following options:

• Bad but quick - initialize the list inside your Activity 's constructor instead of onCreate() method
• Proper but slow - store your values in the Database and fetch once you need them

Answer 2

You could follow the Singleton pattern. In essence, you want to make one global list that you can access from anywhere. You could put this in a few places, I would suggest extending Application and putting it there for a good start.

You may have your own Application , but if not, you'll also need to register it in your manifest.

So, I created an app called MyApp , which extends from Application . This will contain the list of answers that we can access from anywhere.

public class MyApp extends Application {

    // A public static list of answers
    public static ArrayList<answers_preview_data> answers = new ArrayList<>();

    @Override
    public void onCreate() {
        super.onCreate();
        // do other app setup ...
    }
}

Now, within your questions, we can easily add a new answer to this list.

public class QuestionOne extends AppCompatActivity implements View.OnClickListener {

    @Override
    public void onClick(View v) {
        // Adding the new answer to your list.
        MyApp.answers.add(new answers_preview_data());
    }

}

And within your Grade class, we can fetch the list and use it locally.

public class Grade extends AppCompatActivity {

    private ArrayList<answers_preview_data> mPreviewArrayList;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mPreviewArrayList = MyApp.answers;
        // use your list ...
    }
}