Android Studio Activity doesn't start

Question

When starting my activity there is always an error message and the app does not open.

Error:

2022-05-11 23:18:12.470 17222-17222/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 17222
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.myapplication/com.example.feuerwerkzndanlage.Biometric_Authentication}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3455)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3699)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2135)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:236)
        at android.app.ActivityThread.main(ActivityThread.java:8056)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:656)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:967)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
        at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:133)
        at com.example.feuerwerkzndanlage.Biometric_Authentication.<init>(Biometric_Authentication.java:23)
        at java.lang.Class.newInstance(Native Method)
        at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
        at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1254)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3443)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3699) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2135) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:236) 
        at android.app.ActivityThread.main(ActivityThread.java:8056) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:656) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:967) 

Activity:

package com.example.feuerwerkzndanlage;

import android.annotation.SuppressLint;
import android.content.Context;
import android.os.Bundle;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.biometric.BiometricPrompt;
import androidx.core.content.ContextCompat;

import java.util.concurrent.Executor;

public class Biometric_Authentication extends AppCompatActivity {

    private TextView authStatusTv;
    private BiometricPrompt biometricPrompt;
    private BiometricPrompt.PromptInfo promptInfo;

    Context context = getApplicationContext();
    CharSequence error_txt = "Indentifizierung Fehlgeschlagen: ";
    CharSequence suceed_txt = "Indentifizierung Erfolgreich";
    CharSequence failed_txt = "Indentifizierung Fehlgeschlagen";
    int duration = Toast.LENGTH_SHORT;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_biometric_authentication);

        authStatusTv = findViewById(R.id.authStatusTv);
        Button btn_auth = findViewById(R.id.btn_auth);

        Executor executor = ContextCompat.getMainExecutor(this);
        biometricPrompt = new BiometricPrompt(Biometric_Authentication.this, executor, new BiometricPrompt.AuthenticationCallback() {
            @SuppressLint("SetTextI18n")
            @Override
            public void onAuthenticationError(int errorCode, @NonNull CharSequence errString) {
                super.onAuthenticationError(errorCode, errString);
                authStatusTv.setText("Indentifizierung Fehlgeschlagen: " + errString);
                Toast.makeText(context, error_txt, duration).show();
            }

            @SuppressLint("SetTextI18n")
            @Override
            public void onAuthenticationSucceeded(@NonNull BiometricPrompt.AuthenticationResult result) {
                super.onAuthenticationSucceeded(result);
                authStatusTv.setText("Indentifizierung Erfolgreich");
                Toast.makeText(context, suceed_txt, duration).show();
            }

            @SuppressLint("SetTextI18n")
            @Override
            public void onAuthenticationFailed() {
                super.onAuthenticationFailed();
                authStatusTv.setText("Indentifizierung Fehlgeschlagen");
                Toast.makeText(context, failed_txt, duration).show();
            }
        });

        promptInfo = new BiometricPrompt.PromptInfo.Builder()
                .setTitle("Biometrische Indentifizierung")
                .setSubtitle("Login")
                .setNegativeButtonText("Benutzer Passwort")
                .build();

        btn_auth.setOnClickListener(view -> biometricPrompt.authenticate(promptInfo));

    }
}

Answer 1

Context context = getApplicationContext();

You can't do that at init time. You can't call any Context related functions, including getApplicationContect() until onCreate is called. This is because the framework hasn't fully initialized the activity before then. To make this work, you'd need to move this line into onCreate, and move any code that depends on this variable being set into onCreate or later as well.