android java sqlite中的空指针异常

g0czyy6m  于 2021-08-25  发布在  Java
关注(0)|答案(1)|浏览(315)

此问题已在此处找到答案：

什么是nullpointerexception，如何修复它(（12个答案）
三天前关门。
现在我正在尝试使用sqlite创建一个图书应用程序，这是我第一次使用它。但是，当我从光标获取数据时，出现了一个问题。

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.ar.team.company.app.ar_app_08, PID: 18302
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ar.team.company.app.ar_app_08/com.ar.team.company.app.ar_app_08.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor com.ar.team.company.app.ar_app_08.controller.ARDatabaseHelper.getBooks()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    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$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor com.ar.team.company.app.ar_app_08.controller.ARDatabaseHelper.getBooks()' on a null object reference
    at com.ar.team.company.app.ar_app_08.HomeActivity.storeDataInArray(HomeActivity.java:58)
    at com.ar.team.company.app.ar_app_08.HomeActivity.onCreate(HomeActivity.java:39)
    at android.app.Activity.performCreate(Activity.java:8000)
    at android.app.Activity.performCreate(Activity.java:7984)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
    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$H.handleMessage(ActivityThread.java:2066) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:223) 
    at android.app.ActivityThread.main(ActivityThread.java:7656) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

家庭活动：

package com.ar.team.company.app.ar_app_08;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.app.AppCompatDelegate;
import androidx.recyclerview.widget.LinearLayoutManager;

import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.util.Log;
import android.view.View;

import com.ar.team.company.app.ar_app_08.adapters.BooksAdapter;
import com.ar.team.company.app.ar_app_08.controller.ARDatabaseHelper;
import com.ar.team.company.app.ar_app_08.databinding.ActivityHomeBinding;
import com.ar.team.company.app.ar_app_08.model.Book;
import com.google.android.material.snackbar.Snackbar;

import java.util.ArrayList;

@SuppressWarnings("FieldCanBeLocal")
public class HomeActivity extends AppCompatActivity {

    // This For Control The XML-Main Views:
    private ActivityHomeBinding binding;
    private ARDatabaseHelper helper;
    private BooksAdapter adapter;
    private ArrayList<Book> books;
    private ArrayList<String> id;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = ActivityHomeBinding.inflate(getLayoutInflater()); // INFLATE THE LAYOUT.
        View view = binding.getRoot(); // GET ROOT [BY DEF(CONSTRAINT LAYOUT)].
        AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
        setContentView(view); // SET THE VIEW CONTENT TO THE (VIEW).
        // Store the data:
        storeDataInArray();
        // Initialize:
        helper = new ARDatabaseHelper(this);
        books = new ArrayList<>();
        id = new ArrayList<>();
        adapter = new BooksAdapter(this, books, id, binding.parentHomeLayout);
        // Developing the new activity fab:
        binding.newBookFloatingActionButton.setOnClickListener(this::launchNewBookActivity);
        // Developing the recycler view:
        binding.booksRecyclerView.setAdapter(adapter);
        binding.booksRecyclerView.setLayoutManager(new LinearLayoutManager(this));
    }

    private void launchNewBookActivity(View view) {
        Intent newBookActivity = new Intent(this, NewBookActivity.class);
        startActivity(newBookActivity);
    }

    private void storeDataInArray() {
        Cursor cursor = helper.getBooks();
        if (cursor.getCount() == 0) {
            Snackbar.make(binding.parentHomeLayout, "No data.", Snackbar.LENGTH_LONG)
                    .setAction("Close", v -> Log.i("SnackBar_Click", "Closed")).show();
        }else {
            cursor.moveToFirst();
            while (cursor.moveToNext()) {
                String bookID = cursor.getString(0);
                String bookTitle = cursor.getString(1);
                String bookAuthor = cursor.getString(2);
                int bookPages = Integer.parseInt(cursor.getString(3));
                books.add(new Book(bookTitle, bookAuthor, bookPages));
                id.add(bookID);
            }
        }
    }

}

新书店活动：

package com.ar.team.company.app.ar_app_08;

import androidx.appcompat.app.AppCompatActivity;
import androidx.constraintlayout.widget.ConstraintLayout;

import android.database.Cursor;
import android.os.Bundle;
import android.util.Log;
import android.view.View;

import com.ar.team.company.app.ar_app_08.controller.ARDatabaseHelper;
import com.ar.team.company.app.ar_app_08.databinding.ActivityNewBookBinding;
import com.ar.team.company.app.ar_app_08.model.Book;
import com.google.android.material.snackbar.Snackbar;

import java.util.ArrayList;
import java.util.Objects;

@SuppressWarnings("FieldCanBeLocal")
public class NewBookActivity extends AppCompatActivity {

    // This For Control The XML-Main Views:
    private ActivityNewBookBinding binding;
    private ARDatabaseHelper helper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = ActivityNewBookBinding.inflate(getLayoutInflater()); // INFLATE THE LAYOUT.
        View view = binding.getRoot(); // GET ROOT [BY DEF(CONSTRAINT LAYOUT)].
        setContentView(view); // SET THE VIEW CONTENT TO THE (VIEW).
        // Initialize:
        helper = new ARDatabaseHelper(this);
        // Developing the new book button:
        binding.newBookButton.setOnClickListener(this::createNewBook);
    }

    private void createNewBook(View view) {
        String bookTitle = Objects.requireNonNull(binding.bookTitleEditText.getText()).toString();
        String bookAuthor = Objects.requireNonNull(binding.bookAuthorEditText.getText()).toString();
        int bookPages = Integer.parseInt(Objects.requireNonNull(binding.bookPagesEditText.getText()).toString());
        Book book = new Book(bookTitle, bookAuthor, bookPages);
        helper.newBook(book, binding.parentNewBookLayout);
    }

}

书本适配器：

package com.ar.team.company.app.ar_app_08.adapters;

import android.content.Context;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.constraintlayout.widget.ConstraintLayout;
import androidx.recyclerview.widget.RecyclerView;

import com.ar.team.company.app.ar_app_08.R;
import com.ar.team.company.app.ar_app_08.model.Book;
import com.google.android.material.card.MaterialCardView;
import com.google.android.material.snackbar.Snackbar;

import org.jetbrains.annotations.NotNull;

import java.util.ArrayList;

public class BooksAdapter extends RecyclerView.Adapter<BooksAdapter.ViewHolder> {

    private final Context context;
    private final ArrayList<Book> books;
    private final ArrayList<String> id;
    private final ConstraintLayout layout;

    public BooksAdapter(Context context, ArrayList<Book> books, ArrayList<String> id, ConstraintLayout layout) {
        this.context = context;
        this.books = books;
        this.id = id;
        this.layout = layout;
        notifyDataSetChanged();
    }

    @NotNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull @NotNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(context).inflate(R.layout.books_row, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull @NotNull BooksAdapter.ViewHolder holder, int position) {
        holder.id.setText(id.get(position));
        holder.title.setText(books.get(position).getBookTitle());
        holder.author.setText(books.get(position).getBookAuthor());
        holder.pages.setText(books.get(position).getBookPages());
        holder.cardView.setOnClickListener(v -> {
            Snackbar.make(layout, books.get(position).getBookTitle(), Snackbar.LENGTH_LONG)
                    .setAction("Close", v1 -> Log.i("SnackBar_Click", "Closed")).show();
        });
    }

    @Override
    public int getItemCount() {
        return books.size();
    }

    public static class ViewHolder extends RecyclerView.ViewHolder {
        private final MaterialCardView cardView;
        private final TextView id, title, author, pages;

        public ViewHolder(@NonNull @org.jetbrains.annotations.NotNull View itemView) {
            super(itemView);
            cardView = itemView.findViewById(R.id.cardView);
            id = itemView.findViewById(R.id.book_id_text_view);
            title = itemView.findViewById(R.id.book_title_text_view);
            author = itemView.findViewById(R.id.book_author_text_view);
            pages = itemView.findViewById(R.id.book_pages_text_view);
        }
    }
}

ardatabasehelper：

package com.ar.team.company.app.ar_app_08.controller;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

import androidx.annotation.Nullable;
import androidx.constraintlayout.widget.ConstraintLayout;

import com.ar.team.company.app.ar_app_08.model.Book;
import com.ar.team.company.app.ar_app_08.utils.ARUtils;
import com.google.android.material.snackbar.Snackbar;

    public class ARDatabaseHelper extends SQLiteOpenHelper {

        private final Context context;

        public ARDatabaseHelper(@Nullable Context context) {
            super(context, ARUtils.DATABASE_FILE_NAME, null, ARUtils.DATABASE_VERSION);
            this.context = context;
        }

        public Context getContext() {
            return context;
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            String tN = ARUtils.DATABASE_TABLE_NAME;
            String id = ARUtils.COLUMN_ID;
            String bT = ARUtils.COLUMN_TITLE;
            String bA = ARUtils.COLUMN_AUTHOR;
            String bP = ARUtils.COLUMN_PAGES;
            String query = "CREATE TABLE " + tN +
                    "(" + id + " INTEGER PRIMARY KEY AUTOINCREMENT, " + bT + " TEXT, " + bA + " TEXT, " + bP + " TEXT )";
            db.execSQL(query);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL("DROP TABLE IF EXISTS " + ARUtils.DATABASE_TABLE_NAME);
            onCreate(db);
        }

        public void newBook(Book book, ConstraintLayout layout) {
            // Initialize database:
            SQLiteDatabase db = this.getWritableDatabase();
            ContentValues values = new ContentValues();
            // Put the values:
            values.put(ARUtils.COLUMN_TITLE, book.getBookTitle());
            values.put(ARUtils.COLUMN_AUTHOR, book.getBookAuthor());
            values.put(ARUtils.COLUMN_PAGES, book.getBookPages());
            // Finishing:
            long res = db.insert(ARUtils.DATABASE_TABLE_NAME, null, values);
            if (res == -1) showSnackBar("Failed to create new book.", layout);
            else showSnackBar("Successfully created new book.", layout);
        }

        public Cursor getBooks() {
            // Initialize database:
            SQLiteDatabase db = this.getReadableDatabase();
            Cursor cursor = null;
            if (db != null) {
                cursor = db.rawQuery("SELECT * FROM " + ARUtils.DATABASE_TABLE_NAME, null);
            }
            return cursor;
        }

        public void showSnackBar(String mes, ConstraintLayout layout) {
            Snackbar.make(layout, mes, Snackbar.LENGTH_LONG)
                    .setAction("Close", v -> Log.i("SnackBar_Click", "Closed")).show();
        }

}
Javasqliteandroidandroid-studioandroid-sqlite

1条答案

按热度按时间
ss2ws0br

ss2ws0br1#

你在打电话给警察 storeDataInArray 方法，然后再获取（初始化） helper 例如 helper 是空的。
因此，当尝试执行该行时 Cursor cursor = helper.getBooks(); 你得到npe是因为 helper 是空的。日志告诉您的是：-

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor com.ar.team.company.app.ar_app_08.controller.ARDatabaseHelper.getBooks()' on a null object reference
at com.ar.team.company.app.ar_app_08.HomeActivity.storeDataInArray(HomeActivity.java:58)

而不是：-

// Store the data:
    storeDataInArray();
    // Initialize:
    helper = new ARDatabaseHelper(this);

使用：-

// Initialize:
    helper = new ARDatabaseHelper(this);
    // Store the data:
    storeDataInArray();
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