I am learning the concept of Intents and the application I am working on crashes when I try to go from any other intent that’s not “Arabica” intent, the intent object is fired on onItemSelectedListener on a spinner. I want to make it so that when the user selects the other options on the spinner it will execute that intent and go to the other activity which displays a code of text for my coffee application.
Below is the code:
public class MainActivity extends AppCompatActivity {
private Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
spinner = findViewById(R.id.List);
List<String> categories = new ArrayList<>();
categories.add(0, "Choose Category");
categories.add("Arabica");
categories.add("Liberica");
categories.add("Robusta");
categories.add("Excelsa");
ArrayAdapter<String> dataAdapter;
dataAdapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, categories);
dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(dataAdapter);
spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
if (parent.getItemAtPosition(position).equals("Choose Category"))
{
}
else
{
String item = parent.getItemAtPosition(position).toString();
Toast.makeText(parent.getContext(), "Selected: " +item, Toast.LENGTH_SHORT).show();
if (parent.getItemAtPosition(position).equals("Arabica")) {
Intent intent = new Intent(MainActivity.this, Arabica.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Liberica")) {
Intent intent = new Intent(MainActivity.this, Liberica.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Robusta")) {
Intent intent = new Intent(MainActivity.this, Robusta.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Excelsa")) {
Intent intent = new Intent(MainActivity.this, Excelsa.class);
startActivity(intent);
}
}
}
@Override
public void onNothingSelected(AdapterView<?> parent) {
}
});
}
}
Here is the xml:
<TextView
android:id="@+id/MainTitle"
android:layout_width="199dp"
android:layout_height="143dp"
android:layout_marginStart="16dp"
android:text="Selecciona un nombre cientifico para ver su descripcion"
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.047"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.022" />
<Spinner
android:id="@+id/List"
android:layout_width="268dp"
android:layout_height="48dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.559"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.499" />
<Button
android:id="@+id/Button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="60dp"
android:text="Caracteristicas del Cafe"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/List" />
Here is also my manifest:
<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.CoffeeAssessor">
<activity
android:name=".Excelsa"
android:exported="false" />
<activity
android:name=".Liberica"
android:exported="false" />
<activity
android:name=".Robusta"
android:exported="false" />
<activity
android:name=".Arabica"
android:exported="false" />
<activity
android:name=".MainActivity"
android:exported="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
Finally here is the error log:
2021-11-01 12:52:48.840 10365-10365/com.revolution.coffeeassessor E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.revolution.coffeeassessor, PID: 10365
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.revolution.coffeeassessor/com.revolution.coffeeassessor.Liberica}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.revolution.coffeeassessor.Liberica.onCreate(Liberica.java:21)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
I didn’t implement a setOnCLickListener for the button but i dont think that’s the issue here is my onCreate though with all my Liberica code:
public class Liberica extends AppCompatActivity {
Button back2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
back2 = findViewById(R.id.back);
back2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(Liberica.this, MainActivity.class);
startActivity(intent);
}
});
}
}
and my xml:
<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="com.revolution.coffeeassessor.Liberica">
<TextView
android:id="@+id/LibericaTitle"
android:layout_width="219dp"
android:layout_height="74dp"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:text="Libérica (Coffea Libérica)"
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/LibericaDescription"
android:layout_width="321dp"
android:layout_height="533dp"
android:text=" Esta variedad es originaria de Monrovia (Liberia), de donde toma su nombre. El aroma y sabor de este grano de café es muy peculiar, por lo que su consumo está muy poco extendido, se centra principalmente en los países Escandinavos. "
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="@+id/back2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginBottom="16dp"
android:text="Back"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
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Answer
The reason is just a typo. In the Liberica Class change this line:
back2 = findViewById(R.id.back);
To this:
back2 = findViewById(R.id.back2);
Because the actual id of the button is back2 not back as you can see:
<Button
android:id="@+id/back2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginBottom="16dp"
android:text="Back"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
Also make sure that setContentView(R.layout.activity_main); In the Liberica Class is pointing to the Liberica XML layout file for example if it is called activity_liberica.xml set it to:
setContentView(R.layout.activity_liberica);