Develop Reference

How do i get myfirstapp to work on android studio?

i am new to android studio and trying to get the my first app running. The hello world function works fine but the app closes whenever i press the send button in the second application.
The website tutorial i have based this off can be found here: https://developer.android.com/training/basics/firstapp/starting-activity
Here are my codes:
Mainactivity.java
package com.example.myfirstapp;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import static android.provider.AlarmClock.EXTRA_MESSAGE;
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.edit_message);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);
startActivity(intent);
}
}
DisplayMessageActivity.java
package com.example.myfirstapp;
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);
textView.setText(message);
}
}
AndroidManifest.xml:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.myfirstapp">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/Theme.MyFirstApp">
<activity android:name=".DisplayMessageActivity"
android:parentActivityName=".MainActivity">
<!-- The meta-data tag is required if you support API level 15 and lower -->
<meta-data
android:name="android.support.PARENT_ACTIVITY"
android:value=".MainActivity" />
</activity>
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
Any help is much appreciated!
Thanks
Josh
EDIT
Here is the error in logcat
2021-06-22 13:36:31.716 6345-6345/com.example.myfirstapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myfirstapp, PID: 6345
java.lang.IllegalStateException: Could not execute method for android:onClick
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:414)
at android.view.View.performClick(View.java:7352)
at android.widget.TextView.performClick(TextView.java:14230)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:992)
at android.view.View.performClickInternal(View.java:7318)
at android.view.View.access$3200(View.java:846)
at android.view.View$PerformClick.run(View.java:27800)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7050)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:409)
at android.view.View.performClick(View.java:7352) 
at android.widget.TextView.performClick(TextView.java:14230) 
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:992) 
at android.view.View.performClickInternal(View.java:7318) 
at android.view.View.access$3200(View.java:846) 
at android.view.View$PerformClick.run(View.java:27800) 
at android.os.Handler.handleCallback(Handler.java:873) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:214) 
at android.app.ActivityThread.main(ActivityThread.java:7050) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
at com.example.myfirstapp.MainActivity.sendMessage(MainActivity.java:24)
at java.lang.reflect.Method.invoke(Native Method) 
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:409) 
at android.view.View.performClick(View.java:7352) 
at android.widget.TextView.performClick(TextView.java:14230) 
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:992) 
at android.view.View.performClickInternal(View.java:7318) 
at android.view.View.access$3200(View.java:846) 
at android.view.View$PerformClick.run(View.java:27800) 
at android.os.Handler.handleCallback(Handler.java:873) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:214) 
at android.app.ActivityThread.main(ActivityThread.java:7050) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965) 
2021-06-22 13:36:31.777 6345-6345/com.example.myfirstapp I/Process: Sending signal. PID: 6345 SIG: 9
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_marginLeft="16dp"
android:layout_marginTop="16dp"
android:ems="10"
android:hint="#string/edit_message"
android:inputType="textPersonName"
app:layout_constraintEnd_toStartOf="#+id/button"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginEnd="16dp"
android:layout_marginRight="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>
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">
<TextView
android:id="#+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="16dp"
android:fontFamily="casual"
android:text="TextView"
android:textAppearance="#style/TextAppearance.AppCompat.Large"
android:textColor="#00BCD4"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Cannot say causes the app to close without the logcat
but it could be because of it could not find the Edittext in the sendMessage method.
The id need to be the same in the xml and java files. In your case the edittext id edit_message
/** 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.edit_message);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);
startActivity(intent);
}
<EditText
android:id="#+id/edit_message"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginTop="16dp"
android:ems="10"
android:hint="#string/edit_message"
android:inputType="textPersonName"
app:layout_constraintEnd_toStartOf="#+id/button"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />

There is your xml layout file.

Android program stopped working as soon as button is clicked

I am very new to Android app making, but I'm able to understand java. The program which I'm making, should return relative frequency of characters that are in the text that person will write in the program (input text is meant to be written in EditText field with id inputText, and as soon as person will click button with id continueButton, onClick the program should continue with onClickFindRelevancy method). The code seems fine to me, but the problem is still occuring. I am able to input the text, but as soon as I click the button, then the app stops working and I just can't figure out what is wrong... What do I need to change in code (what is wrong), for the program to do what is intended?
I really tried to find a solution to my problem, but didn't find something like I have (just seeing the code snippets that someone else had problems with), so I hope that this topic won't be closed, because I tried and really didn't find the solution. Have been trying for some time now.
MainActivity.java
package com.example.<theprogrammnameihave>;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
import android.widget.EditText;
import java.util.Map;
import java.util.TreeMap;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void onClickFindRelevancy(View view)
{
EditText inputText = (EditText) findViewById(R.id.inputText);
String input = inputText.getText().toString();
TextView result = (TextView) findViewById(R.id.result);
Map<Character, Float> k = new TreeMap<Character, Float>();
for (char b : input.toCharArray()) {
if (k.containsKey(b)) {
k.put(b, k.get(b) + 1);
}
else {
k.put(b, 1f);
}
}
for (char b : input.toCharArray()) {
float freq = k.get(b) / input.length();
result.setText(getString(b) + " " + freq);
}
}
}
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">
<TextView
android:id="#+id/greetingText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/greeting_message"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.109" />
<Button
android:id="#+id/continueButton"
android:layout_width="wrap_content"
android:layout_height="48dp"
android:layout_marginStart="136dp"
android:layout_marginTop="32dp"
android:layout_marginEnd="137dp"
android:layout_marginBottom="340dp"
android:onClick="onClickFindRelevancy"
android:text="#string/button_send"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/inputText" />
<EditText
android:id="#+id/inputText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="100dp"
android:layout_marginTop="10dp"
android:layout_marginEnd="101dp"
android:layout_marginBottom="191dp"
android:ems="10"
android:hint="#string/edit_message"
android:inputType="textPersonName"
app:layout_constraintBottom_toTopOf="#+id/continueButton"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/greetingText" />
<TextView
android:id="#+id/result"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="205dp"
android:layout_marginTop="418dp"
android:layout_marginEnd="206dp"
android:layout_marginBottom="294dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.<theprogrammnameihave>">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
The error I get when the app stops working -
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.kriptoprogramma, PID: 13645
java.lang.IllegalStateException: Could not execute method for android:onClick
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:402)
at android.view.View.performClick(View.java:6256)
at android.view.View$PerformClick.run(View.java:24701)
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$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:397)
at android.view.View.performClick(View.java:6256) 
at android.view.View$PerformClick.run(View.java:24701) 
at android.os.Handler.handleCallback(Handler.java:789) 
at android.os.Handler.dispatchMessage(Handler.java:98) 
at android.os.Looper.loop(Looper.java:164) 
at android.app.ActivityThread.main(ActivityThread.java:6541) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 
Caused by: android.content.res.Resources$NotFoundException: String resource ID #0x54
at android.content.res.Resources.getText(Resources.java:338)
at android.content.res.Resources.getString(Resources.java:432)
at android.content.Context.getString(Context.java:556)
at com.example.kriptoprogramma.MainActivity.onClickFindRelevancy(MainActivity.java:36)
at java.lang.reflect.Method.invoke(Native Method) 
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:397) 
at android.view.View.performClick(View.java:6256) 
at android.view.View$PerformClick.run(View.java:24701) 
at android.os.Handler.handleCallback(Handler.java:789) 
at android.os.Handler.dispatchMessage(Handler.java:98) 
at android.os.Looper.loop(Looper.java:164) 
at android.app.ActivityThread.main(ActivityThread.java:6541) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

You should use error stack trace, as suggested in comments. See this link.
The mistake is that you use getString method to convert a char to String.
In Android, this is the method of Activity (and Context) to get a string from resources (i. e. when you are localizing your app).
You should use String.valueOf(char) instead.

My App crashed. Unfortunately app has stopped

I'm a new Android developer working on the JustJava app on Udacity Beginners course. Using my phone to debug my code, when I clicked on the button to update price it gives the error: "Unfortunately JustJava has stopped".
AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.android.justjava">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="justjava"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
MainActivity.java
import android.icu.text.NumberFormat;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.RequiresApi;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
/**
* This app displays an order form to order coffee.
*/
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
/**
* This method is called when the order button is clicked.
*/
public void submitOrder(View view) {
display(2);
displayPrice(2 * 5);
}
/**
* This method displays the given quantity value on the screen.
*/
private void display(int number) {
TextView quantityTextView = (TextView)
findViewById(R.id.quantity_text_view);
quantityTextView.setText("" + number);
}
/**
* *This method displays the given price on the screen.
*/
private void displayPrice(int number) {
TextView priceTextView = (TextView)
findViewById(R.id.price_text_view);
priceTextView.setText
(NumberFormat.getCurrencyInstance().format(number));
}
}
activity_main.xml
<?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="com.example.android.justjava.MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="quantity"
android:textAllCaps="true"
android:padding="16dp" />
<TextView
android:id="#+id/quantity_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0"
android:textSize="16sp"
android:layout_marginLeft="16dp"
android:layout_marginStart="16dp"
android:textColor="#android:color/black"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Price"
android:textAllCaps="true"
android:padding="16dp" />
<TextView
android:id="#+id/price_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="NGN0"
android:textSize="16sp"
android:layout_marginLeft="16dp"
android:layout_marginStart="16dp"
android:textColor="#android:color/black"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="ORDER"
android:layout_margin="16dp"
android:onClick="submitOrder"/>
</LinearLayout>
Logcat error:
04-25 09:22:04.944 449-449/com.example.android.justjava E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.android.justjava, PID: 449
java.lang.IllegalStateException: Could not execute method for android:onClick
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
at android.view.View.performClick(View.java:4470)
at android.view.View$PerformClick.run(View.java:18773)
at android.os.Handler.handleCallback(Handler.java:808)
at android.os.Handler.dispatchMessage(Handler.java:103)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5341)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:830)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:646)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
at android.view.View.performClick(View.java:4470) 
at android.view.View$PerformClick.run(View.java:18773) 
at android.os.Handler.handleCallback(Handler.java:808) 
at android.os.Handler.dispatchMessage(Handler.java:103) 
at android.os.Looper.loop(Looper.java:193) 
at android.app.ActivityThread.main(ActivityThread.java:5341) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:830) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:646) 
at dalvik.system.NativeStart.main(Native Method) 
Caused by: java.lang.NoClassDefFoundError: android.icu.text.NumberFormat
at com.example.android.justjava.MainActivity.displayPrice(MainActivity.java:49)
at com.example.android.justjava.MainActivity.submitOrder(MainActivity.java:33)
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515) 
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
at android.view.View.performClick(View.java:4470) 
at android.view.View$PerformClick.run(View.java:18773) 
at android.os.Handler.handleCallback(Handler.java:808) 
at android.os.Handler.dispatchMessage(Handler.java:103) 
at android.os.Looper.loop(Looper.java:193) 
at android.app.ActivityThread.main(ActivityThread.java:5341) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515)

The error is rather specific. The root cause is at the end of the stacktrace :
Caused by: java.lang.NoClassDefFoundError: android.icu.text.NumberFormat
at com.example.android.justjava.MainActivity.displayPrice(MainActivity.java:49)
at com.example.android.justjava.MainActivity.submitOrder(MainActivity.java:33)
The classloader could have not found the android.icu.text.NumberFormat class definition you are using in your app.
You should check that the Android version installed on your phone is compatible with this class that is available only since the API 24 (Android 7.0).
https://developer.android.com/reference/android/icu/text/NumberFormat.html

Attempt to invoke virtual method 'void android.widget.TextView.append(java.lang.CharSequence)' on a null object reference

I'm unable to get data(text) from an EditText to a TextView via a Button.
Logic should be fine and I can't really find the mistake.
I get the error as soon as I click the button which triggeres "sendMessage(View v)".
I can't append text to "chat" but I don't know why.
MainActivity.java:
package com.chattr.neonardo.chattr;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
TextView chat;
EditText message;
Button send;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
chat = (TextView) findViewById(R.id.chat);
message = (EditText) findViewById(R.id.message);
send = (Button) findViewById(R.id.send);
Log.d("test", "test2");
/* send.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
chat.append("\n" + message.getText().toString());
}
});*/
}
public void onMatchTap(View v) {
Toast myToast = Toast.makeText(getApplicationContext(), R.string.searching, Toast.LENGTH_LONG);
myToast.show();
setContentView(R.layout.chat_main);
Log.d("test", "test4");
}
public void sendMessage(View v) {
Log.d("test", "test6");
chat.append("\n" + message.getText().toString());
Log.d("test", "test7");
}
}
I've tried something with the onClickListener but it didn't work so I commented it out.
main_activity.xml:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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: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.chattr.neonardo.chattr.MainActivity">
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:srcCompat="#drawable/chattr"
android:id="#+id/imageView"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="91dp"
android:onClick="onMatchTap"
android:text="#string/start_chat" />
I'm working with two different xml's because I'm changing scenes.
chat_main.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:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:weightSum="1">
<TextView
android:id="#+id/chat"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1.00"
android:autoLink="web"
android:text="#string/chat_connection"
android:visibility="visible" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal">
<EditText
android:id="#+id/message"
android:layout_width="301dp"
android:layout_height="wrap_content"
android:ems="10"
android:inputType="text"
android:text="#string/defaultMessage" />
<Button
android:id="#+id/send"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:elevation="0dp"
android:onClick="sendMessage"
android:text="Send" />
</LinearLayout>
That's the error:
Process: com.chattr.neonardo.chattr, PID: 24340
java.lang.IllegalStateException: Could not execute method for android:onClick
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
at android.view.View.performClick(View.java:5637) 
at android.view.View$PerformClick.run(View.java:22429) 
at android.os.Handler.handleCallback(Handler.java:751) 
at android.os.Handler.dispatchMessage(Handler.java:95) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6119) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.append(java.lang.CharSequence)' on a null object reference
at com.chattr.neonardo.chattr.MainActivity.sendMessage(MainActivity.java:47)
at java.lang.reflect.Method.invoke(Native Method) 
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
at android.view.View.performClick(View.java:5637) 
at android.view.View$PerformClick.run(View.java:22429) 
at android.os.Handler.handleCallback(Handler.java:751) 
at android.os.Handler.dispatchMessage(Handler.java:95) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6119) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Remove setContentView(R.layout.chat_main);
You should use Intent to open a new activity inside onClick method.
Intent intent = new Intent(CurrentActivity.this,NextActivity.class);
startActivity(intent);
Should be
public void onMatchTap(View v) {
Toast myToast = Toast.makeText(getApplicationContext(), R.string.searching, Toast.LENGTH_LONG).show();
Intent intent = new Intent(MainActivity.this,ChatMain.class);
startActivity(intent);
Log.d("test", "test4");
}

chat = (TextView) findViewById(R.id.chat);
This TextView is Define in your chat_main.xml file and you try to bind it in your Main Activity.that's why it will gives you NullPointerException.

WebView crashing

Im writing a Webbrowser using WebView and everything worked fine until now. I dont know what i did that caused the app to crash each time I open it. Android Studio is not giving me any errors in the code so i cant find whats wrong.
Here is my code:
MainActivity.java
package riveras.kasparsweblser;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.KeyEvent;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.webkit.WebViewClient;
public class MainActivity extends AppCompatActivity {
WebView webview;
private WebView mWebView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
String url = "http://www.google.com";
webview = (WebView) findViewById(R.id.webView);
webview.getSettings().setJavaScriptEnabled(true);
webview.loadUrl(url);
webview.setWebChromeClient(new WebChromeClient());
webview.setWebViewClient(new WebViewClient());
}
#Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
mWebView = (WebView) findViewById(R.id.webView);
if (event.getAction() == KeyEvent.ACTION_DOWN) {
switch (keyCode) {
case KeyEvent.KEYCODE_BACK:
if (mWebView.canGoBack()) {
mWebView.goBack();
} else {
finish();
}
return true;
}
}
return super.onKeyDown(keyCode, event);
}
}
activity_main.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" android:fitsSystemWindows="true"
tools:context=".MainActivity">
<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"
tools:context=".MainActivity" >
<WebView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:id="#+id/wv"
android:layout_below="#+id/toolbar"
android:layout_alignParentBottom="true" />
<android.support.v7.widget.Toolbar android:id="#+id/toolbar"
android:layout_width="match_parent" android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary" app:popupTheme="#style/AppTheme.PopupOverlay"
android:clickable="true"
android:contextClickable="true"
android:elevation="1dp"
android:focusable="false"
android:max="#integer/status_bar_notification_info_maxnum"
android:indeterminate="false"
android:layout_alignParentTop="true"
android:layout_alignParentStart="true"
android:layout_alignParentEnd="true" />
</RelativeLayout>
</android.support.design.widget.CoordinatorLayout>
content_main.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"
xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
tools:showIn="#layout/activity_main" tools:context=".MainActivity"
android:layout_height="match_parent">
<WebView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/webView"
android:layout_alignParentTop="true"
android:layout_alignParentStart="true"
android:layout_alignParentEnd="true"
android:layout_alignParentBottom="true" />
</RelativeLayout>
AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="riveras.kasparsweblser" >
<uses-permission android:name="android.permission.INTERNET"/>
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:supportsRtl="true"
android:theme="#style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="#string/app_name"
android:theme="#style/AppTheme.NoActionBar" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
The error i get is:
11-23 18:46:56.045 32505-32505/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: riveras.kasparsweblser, PID: 32505
java.lang.RuntimeException: Unable to start activity ComponentInfo{riveras.kasparsweblser/riveras.kasparsweblser.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.webkit.WebSettings android.webkit.WebView.getSettings()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2356)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418)
at android.app.ActivityThread.access$900(ActivityThread.java:154)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5291)
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:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.webkit.WebSettings android.webkit.WebView.getSettings()' on a null object reference
at riveras.kasparsweblser.MainActivity.onCreate(MainActivity.java:26)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2309)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418) 
at android.app.ActivityThread.access$900(ActivityThread.java:154) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:135) 
at android.app.ActivityThread.main(ActivityThread.java:5291) 
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:904) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699) 
All help is appreciated and if you need any more code to see the error just tell me.
Thanks
/Rivera

You are calling setContentView(R.layout.activity_main);, which loads a layout file that contains a WebView with android:id="#+id/wv. You are then trying calling findViewById(R.id.webView);. Your widget IDs do not match, and they need to match.