是什么让我的应用程序无错误地崩溃

Question

我正在写一个应用程序。您可以搜索有关名人的信息并将其保存到 SQlite 数据库中。我按照教程显示了信息，但是当我运行应用程序崩溃时没有错误...请协助

当我请求数据时，我只想在 Listview 中显示姓名，然后当用户单击 Celeb 的姓名时，它会打开一个包含所有已保存数据的页面。我不知道该去哪里甚至开始寻找问题..我可以打开应用程序，保存数据但是当我打开 activity 以显示 ListView 时它崩溃了没有错误...

my xml 用于显示列表视图

<?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="wrap_content"
    android:background="@drawable/mway"
    android:orientation="vertical"
    tools:context=".MainActivity">
        <TextView
            android:id="@+id/header"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:background="@color/colorPrimaryDark"
            android:text="@string/app_name"
            android:textAlignment="center"
            android:textColor="@color/colorPrimary"
            android:textSize="32sp"
            android:textStyle="bold"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />
    <ListView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/list_view"
        android:background="@drawable/mway">
    </ListView>
</LinearLayout>

这是我的 java 文件，用于显示列表视图

import android.database.Cursor;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.util.ArrayList;

public class viewceleb extends AppCompatActivity {

    DatabaseHelper db;
    ArrayList<String> listItems;
    ArrayAdapter adapter;
    ListView userList;

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

        userList = findViewById(R.id.list_view);
        listItems = new ArrayList<>();

        viewData();

        userList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int i, long id) {
                String text = userList.getItemAtPosition(i).toString();
                Toast.makeText(viewceleb.this, "Now something can happen", Toast.LENGTH_LONG).show();
            }
        });
    }

    private void viewData() {
        Cursor cursor = db.viewData();

        if (cursor.getCount()== 0){
            Toast.makeText(this,"No Data To Show", Toast.LENGTH_LONG).show();
        }else {
            while(cursor.moveToNext()){
                listItems.add(cursor.getString(1));//index 1 is name, 0 is ID
            }

            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItems);
            userList.setAdapter(adapter);

        }
    }
}

Answer 1

你只是在声明db，它需要被实例化。

那是你有 DatabaseHelper db; 并且你还需要 db = new DatabaseHelper(?); 在哪里？是 DatabaseHelper 构造函数所需的参数。通常只是上下文，所以可能 db = new DatabaseHelper(this);。这将在 activity 的 onCreate 方法 BEFORE 调用 viewData 方法中进行编码。

例如

public class viewceleb extends AppCompatActivity {

    DatabaseHelper db;
    ArrayList<String> listItems;
    ArrayAdapter adapter;
    ListView userList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_viewceleb);
        dn = new DatabaseHelper(this); //<<<<<<<<<< ADDED note just the context has been assumed, the parameters depend upon the constructor as per what is coded in the DatabaseHelper class.

        userList = findViewById(R.id.list_view);
        listItems = new ArrayList<>();

        viewData();

        userList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int i, long id) {
                String text = userList.getItemAtPosition(i).toString();
                Toast.makeText(viewceleb.this, "Now something can happen", Toast.LENGTH_LONG).show();
            }
        });
    }

    private void viewData() {
        Cursor cursor = db.viewData();

        if (cursor.getCount()== 0){
            Toast.makeText(this,"No Data To Show", Toast.LENGTH_LONG).show();
        }else {
            while(cursor.moveToNext()){
                listItems.add(cursor.getString(1));//index 1 is name, 0 is ID
            }

            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItems);
            userList.setAdapter(adapter);

        }
    }
}

是什么让我的应用程序无错误地崩溃

What is making my app crash without an error

crash

android-sqlite