I'm trying to populate a listview by using an intent

Question

I'm trying to have a user load my app to the main activity containing a listview, then move to the second activity and enter their "name" and "comments", pass it back to the main activity and show it in a toast as well as put it in the listview. However, when i try, my app crashes. Why?

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackground"
    tools:context="com.example.MainActivity">


    <Button
        android:id="@+id/contact"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Contact Us"
        android:layout_marginRight="46dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintVertical_bias="0.982"
        android:layout_marginEnd="46dp" />

    <Button
        android:id="@+id/comments"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add Comments"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/contact"
        android:layout_marginRight="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintHorizontal_bias="0.418"
        app:layout_constraintVertical_bias="0.982"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp" />

    <ListView
        android:id="@+id/ListViewMain"
        android:layout_width="207dp"
        android:layout_height="243dp"
        android:layout_marginLeft="8dp"
        android:textColor="@android:color/white"
        app:layout_constraintLeft_toLeftOf="parent"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintHorizontal_bias="0.503"
        app:layout_constraintVertical_bias="0.253" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:text="Comments"
        android:textColor="@android:color/white"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.062"
        app:layout_constraintHorizontal_bias="0.456" />

</android.support.constraint.ConstraintLayout>

MainActivity.java

package com.example;

import android.content.Context;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button comments;
    private Button contact;
    ListView lv;
    private String Name = "";
    private String Comments = "";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

         initViews();
        initListeners();
        lv = (ListView) findViewById(R.id.ListViewMain);
        Bundle extras = getIntent().getExtras();
                if (extras != null)
                {
                    Name = extras.getString("NAME");
                    Comments = extras.getString("COMMENTS");

                }


            String[] values = new String[] { Name, Comments};

            ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                    android.R.layout.simple_list_item_1, android.R.id.text1, values);

            lv.setAdapter(adapter);


    }

    private void initViews(){
        comments = (Button) findViewById(R.id.comments);
        contact = (Button) findViewById(R.id.contact);





    }

    private void initListeners(){
        comments.setOnClickListener(this);
        contact.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.comments:

                Context context = getApplicationContext();
                CharSequence text = "My name";
                int duration = Toast.LENGTH_SHORT;

                Toast toast = Toast.makeText(context, text, duration);
                toast.show();

                Intent intent2 = new Intent(getApplicationContext(), CommentsActivity.class);
                startActivity(intent2);

                break;
            case R.id.contact:
                Intent intent = new Intent(Intent.ACTION_SEND);
                intent.setType("text/plain");
                intent.putExtra(Intent.EXTRA_EMAIL, "myemail");
                intent.putExtra(Intent.EXTRA_SUBJECT, "Subject");
                intent.putExtra(Intent.EXTRA_TEXT, "Hello!");

                startActivity(Intent.createChooser(intent, "Send Email"));
                break;
        }
    }
}

activity_comments.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackground"
    tools:context="com.example.CommentsActivity">

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="Comments:"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.378"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintHorizontal_bias="0.087" />

    <EditText
        android:id="@+id/COMMENTS"
        android:layout_width="316dp"
        android:layout_height="53dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:inputType="text"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.432" />

    <EditText
        android:id="@+id/NAME"
        android:layout_width="316dp"
        android:layout_height="53dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:inputType="text"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.174" />

    <Button
        android:id="@+id/submit"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Submit"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintVertical_bias="0.894" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Name:"
        android:textColor="@android:color/white"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintVertical_bias="0.14"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintHorizontal_bias="0.079" />

</android.support.constraint.ConstraintLayout>

CommentsActivity.java

package com.example;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class CommentsActivity extends AppCompatActivity implements View.OnClickListener {

    private Button submit;
    private EditText edNAME;
    private EditText edCOMMENTS;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_comments);
        initViews();
        initListeners();
         edNAME  = (EditText)findViewById(R.id.NAME);
        edCOMMENTS  = (EditText)findViewById(R.id.COMMENTS);

    }

    private void initListeners(){
        submit.setOnClickListener(this);

    }

    private void initViews(){
        submit = (Button) findViewById(R.id.submit);

    }

    @Override
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.submit:



                CharSequence text = edNAME.getText().toString() + " - "  + edCOMMENTS.getText().toString();
                int duration = Toast.LENGTH_SHORT;
                Toast toast = Toast.makeText(getApplicationContext(), text, duration);
                toast.show();

                Intent intent = new Intent(getBaseContext(), MainActivity.class);
                intent.putExtra("NAME",  edNAME.getText());
                intent.putExtra("COMMENTS",  edCOMMENTS.getText());
                startActivity(intent);

                break;
        }

    }
}

error message:

12-22 14:31:58.765 1253-1253/com.example E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example, PID: 1253 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:445) at android.widget.ArrayAdapter.getView(ArrayAdapter.java:407) at android.widget.AbsListView.obtainView(AbsListView.java:2372) at android.widget.ListView.makeAndAddView(ListView.java:2052) at android.widget.ListView.fillDown(ListView.java:786) at android.widget.ListView.fillFromTop(ListView.java:847) at android.widget.ListView.layoutChildren(ListView.java:1826) at android.widget.AbsListView.onLayout(AbsListView.java:2171) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1197) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) at android.widget.FrameLayout.onLayout(FrameLayout.java:261) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:434) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) at android.widget.FrameLayout.onLayout(FrameLayout.java:261) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791) at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635) at android.widget.LinearLayout.onLayout(LinearLayout.java:1544) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) at android.widget.FrameLayout.onLayout(FrameLayout.java:261) at com.android.internal.policy.DecorView.onLayout(DecorView.java:758) at android.view.View.layout(View.java:19586) at android.view.ViewGroup.layout(ViewGroup.java:6053) at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2484) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2200) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1386) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6733) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911) at android.view.Choreographer.doCallbacks(Choreographer.java:723) at android.view.Choreographer.doFrame(Choreographer.java:658) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897) at android.os.Handler.handleCallback(Handler.java:789) at android.os.Handler.dispatchMessage(Handler.java:98) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

                                                                       [ 12-22 14:31:58.766  1253: 1347 D/         ]
                                                                       SurfaceInterface::setAsyncMode: set async mode 1

Answer 1

There must be at least one null value in the array that you passed in ArrayAdapter. Please log the values of the variables Name and Comments in MainActivity class before initializing the ArrayAdapter.