how to fix android app crashing on startup?

Question

i'm a begginner developer and the app in question is a test app for an api, from jkutner's tutorial on github. it is crashing as soon as it opens, which is weird, because i've used this exact same code at least thrice and it used to work. i'll leave the code and the logcat below, thank you in advance for your help!

MainActivity

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import java.util.List;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class MainActivity extends AppCompatActivity
{

    final EditText isbnInput = (EditText) findViewById(R.id.isbnInput);
    final TextView textView = (TextView) findViewById(R.id.textView);
    final Button button = (Button) findViewById(R.id.button);

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

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("https://venusdove.herokuapp.com")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        final BookService service = retrofit.create(BookService.class);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Book book = new Book(isbnInput.getText().toString());
                Call<Book> createCall = service.create(book);
                createCall.enqueue(new Callback<Book>() {
                    @Override
                    public void onResponse(Call<Book> _, Response<Book> resp) {
                        Book newBook = resp.body();
                        textView.setText("Created Book with ISBN: " + newBook.isbn);
                    }

                    @Override
                    public void onFailure(Call<Book> _, Throwable t) {
                        t.printStackTrace();
                        textView.setText(t.getMessage());
                    }
                });
            }
        });
    }
}

Book class

package com.example.myapplication;

import com.google.gson.annotations.SerializedName;

public class Book
{
    @SerializedName("id")
    int id;

    @SerializedName("isbn")
    String isbn;

    public Book(int id, String isbn) {
        this.id = id;
        this.isbn = isbn;
    }

    public Book(String isbn) {
        this.isbn = isbn;
    }
}

BookService class

package com.example.myapplication;

import java.util.List;

import retrofit2.Call;
import retrofit2.http.Body;
import retrofit2.http.GET;
import retrofit2.http.POST;
import retrofit2.http.Path;

public interface BookService
{
    @GET("books")
    Call<List<Book>> all();

    @GET("books/{isbn}")
    Call<Book> get(@Path("isbn") String isbn);

    @POST("books/new")
    Call<Book> create(@Body Book book);
}

logcat

2022-10-19 08:56:03.651 8041-8041/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 8041
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2679)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:152)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
        at android.content.Context.obtainStyledAttributes(Context.java:655)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:852)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:819)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:640)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:261)
        at com.example.myapplication.MainActivity.<init>(MainActivity.java:22)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1174)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2669)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

score 0 · Answer 1 · answered Oct 19 '22 at 12:06

You need to "find" view after they are created.

EditText isbnInput;
TextView textView;
Button button;

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

    isbnInput = (EditText) findViewById(R.id.isbnInput);
    textView = (TextView) findViewById(R.id.textView);
    button = (Button) findViewById(R.id.button);
}

Vojin Purić · Answer 2 · 2022-10-19T12:11:34.450

It is because of findViewById. You should call it inside OnCreate. Because it needs View that it is being called from to be initialized.

private EditText isbnInput;
private TextView textView;
private Button button;

@Override
protected void onCreate(Bundle savedInstanceState)
{
    isbnInput = (EditText) findViewById(R.id.isbnInput);
    textView = (TextView) findViewById(R.id.textView);
    button = (Button) findViewById(R.id.button);

how to fix android app crashing on startup?

2 Answers2