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Android app crashes with button click

I'm creating a simple quiz according to this tutorial, but the app crashes with the button "Next answer" which is related to a checked radio button.
Main code:
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;
public class Display extends Activity {
TextView tv;
Button btnNext;
RadioGroup rg;
RadioButton rb1, rb2,rb3;
String questions[]={"Capital of Portugal?", "Capital of Spain?", "Capital of France?"};
String answers[]={"Lisbn","Madrid","Paris"};
String options[]={"Zuriq", "London","Lisbon","Manchester","Buenos Aires","Madrid", "NY", "Paris","Moscow"};
int flag=0;
public static int marks,correct,wrong;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.dispaly);
tv=(TextView) findViewById(R.id.tvque);
btnNext=(Button) findViewById(R.id.btnNext);
rb1= (RadioButton) findViewById(R.id.radioButton);
rb2= (RadioButton) findViewById(R.id.radioButton2);
rb3= (RadioButton) findViewById(R.id.radioButton3);
tv.setText(questions[flag]);
rb1.setText(options[flag*3]);
rb2.setText(options[(flag*3)+1]);
rb3.setText(options[(flag*3)+2]);
Toast.makeText(this, "Negative Marks: " + MainActivity.tbflag, 1000).show();
btnNext.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
//The error comes from the line bellow, i believe:
RadioButton user_answer= (RadioButton)findViewById(rg.getCheckedRadioButtonId());
String answerText=user_answer.getText().toString();
if(answerText.equalsIgnoreCase(answers[flag])){
correct++;
}else{
wrong++;
flag++;
if(flag<questions.length){
tv.setText(questions[flag]);
rb1.setText(options[flag*3]);
rb2.setText(options[(flag*3)+1]);
rb3.setText(options[(flag*3)+2]);
}
else
{
if(MainActivity.tbflag)
{
marks=correct-wrong;
}
else
{
marks=correct;
}
Intent in =new Intent(getApplicationContext(), ResultActivity.class);
startActivity(in);
}
}
}
});
}
}
Log message:
java.lang.NullPointerException: Attempt to invoke virtual method 'int android.widget.RadioGroup.getCheckedRadioButtonId()' on a null object reference
I don't understand this error, why isn't the method being called? Can someone help?
XML:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="com.example.vtorferreira.anew.MainActivity">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="#+id/tvque"
tools:text="Questions" />
<RadioGroup xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:id="#+id/group">
<RadioButton
android:text="RadioButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/textView"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginTop="32dp"
android:id="#+id/radioButton" />
<RadioButton
android:text="RadioButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/radioButton"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:id="#+id/radioButton2" />
<RadioButton
android:text="RadioButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/radioButton3"
android:layout_below="#+id/radioButton2"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
</RadioGroup>
<Button
android:text="Next Question"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginLeft="60dp"
android:layout_marginStart="60dp"
android:layout_marginBottom="125dp"
android:id="#+id/btnNext" />
</RelativeLayout>

The reason you are getting the first NullPointerException is because you must inflate rg before it is used.
The second NullPointerException is possibly due to the fact that you are pressing the "Next" button with no radio button selected. To prevent this, you can add a null check inside the OnClickListener or select one of the radio buttons by default. To select a radio button by default, add the following to your XML:
<RadioGroup
...
android:checkedButton="#+id/some-radio-button-id">
...
</RadioGroup>
Alternatively, you can set the default checked RadioButton in your Activity:
rg.check(R.id.radioButton)

Android Studio: Error: <identifier> expected error

I'm a rookie to programming. I've been trying to build an app and been getting these errors as seen in the screenshot. I've tried the "Invalid Caches/Restart" but it didn't help. What am I doing wrong here?
EDIT: Sorry for earlier ignorance. I've added the code snippets.
Login.Java:
package com.example.ankit.mrestro;
import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
public class Login extends AppCompatActivity implements View.OnClickListener {
Button blogin;
EditText etUsername, etPassword;
TextView RegisterHere,Skip;
UserLocalStorage userLocalStorage;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
etUsername = (EditText) findViewById(R.id.etUserName);
etPassword = (EditText) findViewById(R.id.etPassword);
RegisterHere = (TextView) findViewById(R.id.RegisterHere);
Skip = (TextView) findViewById(R.id.Skip);
blogin = (Button) findViewById(R.id.blogin);
blogin.setOnClickListener(this);
RegisterHere.setOnClickListener(this);
Skip.setOnClickListener(this);
userLocalStorage= new UserLocalStorage(this);
}
#Override
public void onClick(View v) {
switch (v.getId()){
case R.id.blogin:
User user= new User(null, null);
userLocalStorage.storeUserData(user);
userLocalStorage.SetUserLoggedIn(true);
break;
case R.id.RegisterHere:
startActivity(new Intent(this,Register.class));
break;
case R.id.Skip:
startActivity(new Intent(this,MainActivity.class));
break;
}
}
}
activity_login.xml
<?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="fill_parent"
android:layout_height="fill_parent"
android:gravity="center"
android:background="#drawable/screen1"
android:orientation="vertical"
tools:ignore="HardcodedText" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1.2"
android:orientation="vertical">
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.3"
android:orientation="vertical">
<EditText
android:layout_width="350dp"
android:layout_height="35dp"
android:layout_gravity="center"
android:background="#drawable/text"
android:textColor="#A9A9A9A9"
android:textStyle="normal"
android:textSize="20sp"
android:textAlignment="center"
android:text="Username"/>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.4"
android:orientation="vertical">
<EditText
android:layout_width="350dp"
android:layout_height="35dp"
android:layout_gravity="center"
android:background="#drawable/text"
android:textColor="#A9A9A9A9"
android:textStyle="normal"
android:textSize="20sp"
android:textAlignment="center"
android:text="Password"/>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.8"
android:orientation="vertical">
<Button
android:layout_width="#android:dimen/thumbnail_width"
android:layout_height="#android:dimen/app_icon_size"
android:layout_gravity="center"
android:background="#drawable/button"
android:textColor="#ffffff"
android:textStyle="bold"
android:textSize="20dp"
android:text="Login"/>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.2"
android:orientation="vertical">
<Button
android:layout_width="120dp"
android:layout_height="25dp"
android:layout_gravity="center"
android:background="#null"
android:textColor="#ffffff"
android:textStyle="bold"
android:textSize="15dp"
android:text="Register"/>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.4"
android:orientation="vertical">
<Button
android:layout_width="120dp"
android:layout_height="25dp"
android:layout_gravity="center"
android:background="#null"
android:textColor="#ffffff"
android:textStyle="bold"
android:textSize="15dp"
android:text="Skip"/>
</LinearLayout>
</LinearLayout>
Thanks for help guys!

Look at your activity_login.xml to ensure that the ID's you are trying to reference are linked to your UI elements in the layout file. It seems like you are trying to link the class level variable to itself in the findViewById() instead of linking the variable to the ID's in the layout file of the corresponding UI elements.
EDIT:
None of your UI elements (EditText fields, Buttons, or even LinearLayouts) have ID's associated with them. You can either double click the elements when viewing activity_login.xml in the 'Desgin' view to set an ID for the specific element. Otherwise, you can define an ID for the element in the 'Properties' window or explicitly write out android:id="#+id/yourIDHere" in the 'Text' view of the layout file. Whatever you decide to make your ID, you then must call that in name in findViewById() like so: findViewById(R.id.yourIDHere);.
Hope this helps!

XML / Java Button Click to change TextView

I am realitavely new to Java coding and am trying to use a button press (in an Android App) to update a text view. The code I am currently using is causing my app to crash whenever I try to press the button.
Here is my XML
<LinearLayout>
<TextView
android:id="#+id/myTextView_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="calculateNewNumber"
android:text="Calculate New Number" />
</LinearLayout>
And Here is my Java
package com.example.android.myapp;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
int startNumber = 0;
public void calculateNewNumber(View view) {
startNumber = startNumber + 1;
display(startNumber);
}
private void display(int number) {
TextView myTextView = (TextView) findViewById(
R.id.myTextView_text_view);
myTextView.setText("" + number);
}
}
Thanks for any help you can give.

The code itself works fine so the problem must be with your XML, assuming you have provided it in full here.
Using this XML, your MainActivity code works fine for me.
<RelativeLayout 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:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:paddingBottom="#dimen/activity_vertical_margin" tools:context=".MainActivity">
<LinearLayout
android:layout_height="wrap_content"
android:layout_width="fill_parent">
<TextView
android:id="#+id/myTextView_text_view"
android:layout_width="60dp"
android:layout_height="wrap_content"
android:text="0" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="calculateNewNumber"
android:text="Calculate New Number" />
</LinearLayout>
</RelativeLayout>

Unable to run code for Simple Counter App for Android

I am trying to make an application on Android Studio with the help of a tutorial. I managed to get the User Interface right and I think that I have assigned the correct buttons too. I am unable to get where I am going wrong with my code. I am new to Java and so I am unable to pinpoint the error I have committed. I am posting below the code from the files I was asked to edit in the tutorial.
package com.example.to_dolistapplication.app;
import android.os.Bundle;
import android.util.TypedValue;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.app.Activity;
import android.graphics.Color;
public class MainActivity extends Activity implements OnClickListener {
Button btn1;
Button btn2;
Button btn3;
TextView textTitle;
EditText scoreText;
int counter = 0;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn1 = (Button)findViewById(R.id.button);
btn2 = (Button)findViewById(R.id.button2);
btn3 = (Button)findViewById(R.id.button3);
scoreText = (EditText)findViewById(R.id.textView);
textTitle = (TextView)findViewById(R.id.editText);
//---set on click listeners on the buttons-----
btn1.setOnClickListener(this);
btn2.setOnClickListener(this);
btn3.setOnClickListener(this);
// change font size of the text
textTitle.setTextSize(TypedValue.COMPLEX_UNIT_SP, 24);
}
#Override
public void onClick(View v) {
if (v == btn1){
counter++;
scoreText.setText(Integer.toString(counter));
scoreText.setBackgroundColor(Color.CYAN);
}
if (v == btn2){
counter--;
scoreText.setText(Integer.toString(counter));
scoreText.setBackgroundColor(Color.GREEN);
}
if (v == btn3){
counter = 0;
scoreText.setText(Integer.toString(counter));
scoreText.setBackgroundColor(Color.RED);
}
}
}
Above is the File from MainActivity.java
<RelativeLayout 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:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:paddingBottom="#dimen/activity_vertical_margin"
tools:context="com.example.to_dolistapplication.app.MainActivity">
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="+1"
android:id="#+id/button"
android:onClick="#string/intro"
android:layout_below="#+id/editText"
android:layout_toRightOf="#+id/textView"
android:layout_marginTop="79dp"
android:layout_alignRight="#+id/editText"
android:layout_alignEnd="#+id/editText" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="-1"
android:id="#+id/button2"
android:layout_centerVertical="true"
android:layout_alignLeft="#+id/button"
android:layout_alignStart="#+id/button"
android:layout_alignRight="#+id/editText"
android:layout_alignEnd="#+id/editText" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Reset"
android:id="#+id/button3"
android:layout_below="#+id/button2"
android:layout_marginTop="64dp"
android:layout_alignRight="#+id/button2"
android:layout_alignEnd="#+id/button2"
android:layout_alignLeft="#+id/button2"
android:layout_alignStart="#+id/button2" />
<TextClock
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/textClock"
android:layout_alignParentTop="true"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_toRightOf="#+id/textView2"
android:layout_alignBottom="#+id/textView2" />
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="numberDecimal"
android:ems="10"
android:id="#+id/editText"
android:layout_below="#+id/textView2"
android:layout_centerHorizontal="true"
android:layout_marginTop="29dp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="Score"
android:id="#+id/textView"
android:layout_alignBottom="#+id/editText"
android:layout_toLeftOf="#+id/editText" />
This is from the file activity_main.xml.
The app, when run on emulator, displays Unfortunately Counter App has stopped.
what might be the reason for the app not working? Please help.

**EditText** scoreText = (EditText)findViewById(R.id.textView);
**TextView** textTitle = (TextView)findViewById(R.id.editText);
you missmathed with types: android:id="#+id/editText" is EditText, but in Activity you wrote his id to TextView.
And you missmathed with types: android:id="#+id/textView" is TextView, but but in Activity you wrote his id as EditText.

You've mismatched id's in onCreate() method.
Use the debug mode or just look at your console output

Android AVD is giving an "Unfortunately [App] Has Stopped Working" error

I have just started learning to develop Android apps. In this project, I have used a Menu whose second option leads to a class called TextPlay. This works just fine till the time I add the line,
display.setText(input.getText().toString());
in TextPlay.java.
TextPlay.java
package com.example.myfirstapp;
import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.ToggleButton;
public class TextPlay extends Activity{
#Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.text);
Button chkCmd = (Button)findViewById(R.id.bResults);
ToggleButton passTog = (ToggleButton)findViewById(R.id.tbPassword);
EditText input = (EditText)findViewById(R.id.etCommands);
TextView display = (TextView)findViewById(R.id.tvDisplay);
display.setText(input.getText().toString());
}
}
Text.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:padding="25sp" >
<EditText
android:id="#+id/etCommands"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:hint="Type a Command"
android:password="true" />
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:weightSum="100" >
<Button
android:id="#+id/bResults"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="50"
android:text="Button" />
<ToggleButton
android:id="#+id/tbPassword"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="50"
android:checked="true"
android:text="ToggleButton" />
</LinearLayout>
<TextView
android:id="#+id/tvResults"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="TextView" />
</LinearLayout>

TextView display = (TextView) findViewById(R.id.tvDisplay);
Your TextView seems to be called R.id.tvResults but in your findViewById() you named it R.id.tvDisplay, thus it doesn't exists and returns null, that's why you're seeing that exception.

There's no TextView in your layout that has the id "tvDisplay".

In Text.xml,
you have saved it with different name:
android:id="#+id/tvResults"
correct it with
android:id="#+id/tvDisplay
you are getting this error because their is no tvDisplay id in R.java file. that is why it is sending null exception.