This is the error I am getting
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.bignerdranch.android.geoquiz, PID: 7499
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.geoquiz/com.bignerdranch.android.geoquiz.QuizActivity}: **java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference**
This is my code:
package com.bignerdranch.android.geoquiz;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
public class QuizActivity extends AppCompatActivity {
private Button mTrueButton;
private Button mFalseButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_quiz);
mTrueButton = (Button) findViewById(R.id.true_button);
mTrueButton.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View v){
Toast.makeText(QuizActivity.this, R.string.incorrect_toast, Toast.LENGTH_SHORT).show();
}
});
mFalseButton = (Button) findViewById(R.id.false_button);
mFalseButton.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View v){
Toast.makeText(QuizActivity.this, R.string.correct_toast, Toast.LENGTH_SHORT).show();
}
});
}
}
I have no clue why I'm getting this error. I thought I assigned the object reference well and I checked the punctuation to make sure.
This is my XML code if that helps with the buttons and the ids.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation= "vertical"
tools:context="com.bignerdranch.android.geoquiz.QuizActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/question_text" />
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding = "24dp"
android:orientation="horizontal">
<Button
android:id="#+id/true_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/true_button"/>
<Button
android:id="#+id/false_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="false"/>
</LinearLayout>
</LinearLayout>
Ok so I wanted to try and check if the values were null or not and I put these two lines of code to check
System.out.println("mTrueButtonIsNull:" +(mTrueButton==null));
System.out.println("mFalseButtonIsNull:" +(mFalseButton==null));
and I got back true and I don't know why or what I have to do now.
5-27 18:11:10.266 20014-20014/com.bignerdranch.android.geoquiz I/System.out: mTrueButtonIsNull:true
05-27 18:11:10.266 20014-20014/com.bignerdranch.android.geoquiz I/System.out: mFalseButtonIsNull:true
It appears you have a problem with one of your Buttons. which is defined as null.
Make sure you have true_button and false_button inside the activity_quiz layout.
If there is no problem with your layout file, Clean and re-build your app.
Related
I want to select the button element by ID, but when I run the program, I get the app keeps stopping error.
I ran the program in debug mode and the error Caused by:
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference
Even when I click on R.id.button, the button element is displayed in the xml file.
mainActivityFile :
package com.example.app;
import androidx.appcompat.app.AppCompatActivity;
import android.annotation.SuppressLint;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity6 extends AppCompatActivity {
private Button btn;
private TextView txt;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main6);
btn.findViewById(R.id.button);
txt.findViewById(R.id.txt);
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
txt.setText("This is a new text.");
}
});
// Toast.makeText(this, "str", Toast.LENGTH_LONG).show();
}
}
xml file :
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
tools:context=".MainActivity6">
<TextView
android:id="#+id/txt"
android:layout_width="wrap_content"
android:layout_gravity="center"
android:layout_marginTop="10dp"
android:autoSizeTextType="uniform"
android:autoSizeMinTextSize="25sp"
android:autoSizeMaxTextSize="100dp"
android:autoSizeStepGranularity="2sp"
android:layout_height="wrap_content" />
<Button
android:id="#+id/button"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Button" />
</LinearLayout>
btn.findViewById(R.id.button);
You're not supposed to use it this way because btn is null at this point.
You have to do the following:
btn = (Button)findViewById(R.id.button);
I'm currently following the Android Studio "Build Your First App" tutorial (https://developer.android.com/training/basics/firstapp/starting-activity) and I can't seem to get the DisplayMessageActivity working. The variable "R.id.textView" doesn't seem to exist and I can't see any differences between the tutorial and my own code. I know this is just me being stupid somewhere but I can't pinpoint it.
Here's my code for DisplayMessageActivity.java:
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TextView;
public class DisplayMessageActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_display_message);
// Get the Intent that started this activity and extract the string
Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
// Capture the layout's TextView and set the string as its text
TextView textView = findViewById(R.id.textView); <-- Cannot resolve symbol 'textView'
textView.setText(message);
}
}
And MainActivity.java, which is trying to run it:
package com.example.myapplication;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity {
public static final String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
/** Called when the user taps the Send button */
public void sendMessage(View view) {
Intent intent = new Intent(this, DisplayMessageActivity.class);
EditText editText = (EditText) findViewById(R.id.editTextTextPersonName);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);
startActivity(intent);
}
}
When I try to run the app on the virtual device it doesn't crash, but the button which calls the DisplayMessage activity does nothing.
EDIT: As requested, the xml code for both activities:
activity_display_message.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".DisplayMessageActivity">
</androidx.constraintlayout.widget.ConstraintLayout>
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".MainActivity">
<EditText
android:id="#+id/editTextTextPersonName"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:ems="10"
android:hint="#string/edit_message"
android:inputType="textPersonName"
app:layout_constraintEnd_toStartOf="#+id/button2"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="#+id/button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginEnd="16dp"
android:onClick="sendMessage"
android:text="#string/button_send"
app:layout_constraintBaseline_toBaselineOf="#+id/editTextTextPersonName"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toEndOf="#+id/editTextTextPersonName" />
</androidx.constraintlayout.widget.ConstraintLayout>
I read the tutorial and I think they missed this part of code:
android:onClick="sendMessage"
so, the code should be like this:
<Button
android:id="#+id/button"
android:onClick="sendMessage"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="16dp"
android:layout_marginStart="16dp"
android:text="#string/button_send"
app:layout_constraintBaseline_toBaselineOf="#+id/editText"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toEndOf="#+id/editText" />
and I think that's it, let me know if it helps
After looking more closely at #Vishal Naikawadi's comment, I realised what the problem was. R.id.textView is not some sort of variable, placeholder, or constant as I had assumed, it is literally getting the object with id "textView" (or so I think). I had accidentally changed the id of the object I was trying to reference in the visual editor, so there was no object with that id. I changed the id of the object back to the same as is referenced in the code and everything seems to be fine now.
Apologies to anyone whose time I've wasted with this, I'm really new to this concept in Java.
When I look at my code in the editor, nothing seems to be a problem. However, when I run the code on my phone and try to go from one activity to another which features a floating action button, the floating action button cannot find the view by Id despite the ID being exactly the same in both java and XML.
My Java:
package com.test.app.app2_test1;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
public class TermsOfUse extends AppCompatActivity {
FloatingActionButton mFloatingActionButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.terms_of_use);
mFloatingActionButton = (FloatingActionButton) mFloatingActionButton.findViewById(R.id.terms_back_fab);
mFloatingActionButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
onBackPressed();
}
});
}
}
My XML:
<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
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"
tools:context="com.test.app.app2_test1.TermsOfUse"
android:background="#color/backdrop">
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_marginBottom="44dp"
android:orientation="vertical">
</ScrollView>
<View
android:layout_gravity="bottom"
android:layout_width="match_parent"
android:layout_height="44dp"
android:background="#color/colorPrimary"
android:elevation="4dp"/>
<android.support.design.widget.FloatingActionButton
android:id="#+id/terms_back_fab"
android:layout_gravity="bottom"
android:layout_width="56dp"
android:layout_height="56dp"
android:layout_margin="16dp"
android:src="#drawable/ic_arrow_back"
app:pressedTranslationZ="12dp"/>
</android.support.design.widget.CoordinatorLayout>
The message that I get in my console is the following:
Attempt to invoke virtual method 'android.view.View android.support.design.widget.FloatingActionButton.findViewById(int)' on a null object reference
Any help would be appreciated.
You are treating a view as a child of its own!
Replace:
mFloatingActionButton = (FloatingActionButton) mFloatingActionButton.findViewById(R.id.terms_back_fab);
With:
mFloatingActionButton = (FloatingActionButton)findViewById(R.id.terms_back_fab);
Here is my MainActivity.java code :
package com.dg.buttontest3;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class MainActivity extends Activity implements OnClickListener{
Button button;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button)findViewById(R.id.button1);
button.setOnClickListener(this);
}
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
Log.d("dg","button was clicked");
}
}
And here is the activity_main.xml code :
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
<Button
android:id="#+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="18dp"
android:layout_marginTop="18dp"
android:text="Button" />
</LinearLayout>
I am getting the following error while running this code on Eclipse ADT : button1 cannot be resolved or is not a field.
I don't understand what is wrong with my code. Please help.
You haven't posted your XML code, but as this code seems to be working properly:
button = (Button)findViewById(R.id.button1);
I'd guess that in your activity_main.xml you never create something called button1, or that you don't use the #+id/ prefix.
It should look something like:
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/button1"/>
Just realised something else, you need to change your LinearLayout opening tag so it has a >.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
That might cause the button to never properly be recognised.
I'm trying to move the position of the seekbar using a button. Basically I have a seekbar from 0 to 100. and I have button presents set up at arbitrary values (40,50,60 etc). When I try to set the progress on the seekbar via button, I get a fault.. I've already initialized the seekBar in the onCreate() method.
SeekBar seekBar = (SeekBar) findViewById(R.id.seekBar1);
currentProgress = 40;
seekBar.setMax(100);
seekBar.setProgress(currentProgress);
button40.setOnClickListener(button40Listener);
But when use the below, it crashes.
private OnClickListener button40Listener = new OnClickListener() {
public void onClick(View v) {
currentProgress = 40;
seekBar.setProgress(currentProgress);
}
}
This seems straight-forward. Any ideas?
You shouldn't need to put up another Seekbar. The initial one should be fine. Without the exception message and stack trace I'm not sure what is causing the crash. However, I just coded an example and works as you would expect. Perhaps by looking at my example you can identify your issue.
SeekbarTest.java:
package com.example.seekbartest;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.SeekBar;
public class SeekbarTest extends Activity {
/** Called when the activity is first created. */
private SeekBar seekBar;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
seekBar = (SeekBar)findViewById(R.id.seekBar1);
Button fortyPctButton = (Button)findViewById(R.id.buttonFortyPct);
fortyPctButton.setOnClickListener(new OnClickListener()
{
#Override
public void onClick(View v)
{
seekBar.setProgress(40);
}
});
Button sixtyPctButton = (Button)findViewById(R.id.buttonSixtyPct);
sixtyPctButton.setOnClickListener(new OnClickListener()
{
#Override
public void onClick(View v)
{
seekBar.setProgress(60);
}
});
Button eightyPctButton = (Button)findViewById(R.id.buttonEightyPct);
eightyPctButton.setOnClickListener(new OnClickListener()
{
#Override
public void onClick(View v)
{
seekBar.setProgress(80);
}
});
}
}
And here is the main.xml it is referencing for the layout:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<TextView
android:layout_width="fill_parent"
android:text="#string/hello"
android:id="#+id/textView1"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"/>
<SeekBar
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="#+id/seekBar1"
android:layout_below="#+id/textView1"
android:layout_alignLeft="#+id/textView1"
android:layout_alignRight="#+id/textView1"/>
<Button
android:layout_width="wrap_content"
android:text="40%"
android:id="#+id/buttonFortyPct"
android:layout_height="wrap_content"
android:layout_below="#+id/seekBar1"
android:layout_alignLeft="#+id/seekBar1"/>
<Button
android:layout_width="wrap_content"
android:text="60%"
android:id="#+id/buttonSixtyPct"
android:layout_height="wrap_content"
android:layout_toRightOf="#+id/buttonFortyPct"
android:layout_alignTop="#+id/buttonFortyPct"
android:layout_alignBottom="#+id/buttonFortyPct"/>
<Button
android:layout_width="wrap_content"
android:text="80%"
android:id="#+id/buttonEightyPct"
android:layout_height="wrap_content"
android:layout_toRightOf="#+id/buttonSixtyPct"
android:layout_alignTop="#+id/buttonSixtyPct"
android:layout_alignBottom="#+id/buttonSixtyPct"/>
</RelativeLayout>
Just create a new android app and replace the generated code + layout with the example above. It should work for you.
Good luck,
Craig