错误：在 sqlite 数据库中检索详细信息时出现 NullPointerException

Question

我的数据库表有数据，可以在sqlite数据库浏览器中查看。在下面的类中出现错误...如果有异常，必须在catch循环中捕获，但仍然出现异常，不知道它是如何发生的，请帮我解决....如果需要任何信息，请发表评论提前谢谢您

DataBaseView.java

package com.vibrator;

import android.app.Activity;

import java.util.ArrayList;


//import android.app.ListActivity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;


public class DataBaseView extends Activity{
    DataBaseAdapter DataBaseAdapter;
    ArrayList<String> al;
    Cursor cursor;
    public  SQLiteDatabase db;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);

        setContentView(R.layout.database_view);
        System.out.println("entered in database");

        ListView list =(ListView) findViewById(R.id.list);
         DataBaseAdapter=new DataBaseAdapter(this);

         DataBaseAdapter=DataBaseAdapter.open();
             try {


             cursor=db.rawQuery("select * from DETAILS", null);
             if(cursor!=null){
                 if (cursor.moveToFirst()) {
                     do {
                         String id =cursor.getString(cursor.getColumnIndex("ID"));
                         String name =cursor.getString(cursor.getColumnIndex("NAME"));
                         //String pattern =cursor.getString(cursor.getColumnIndex("PATTERN"));
                         //store the values in arraylist
                         al.add(id+". "+name);

                    } while (cursor.moveToNext());

                }
             }
             DataBaseAdapter.close();
         }
             catch (Exception e) {
                Toast.makeText(this, "exception occurred", Toast.LENGTH_LONG).show();
            }

             ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_expandable_list_item_1, al);

            list.setAdapter(adapter);


    }


    }

日志猫：

08-05 20:25:10.988: E/AndroidRuntime(9500): FATAL EXCEPTION: main
08-05 20:25:10.988: E/AndroidRuntime(9500): java.lang.NullPointerException
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:291)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.ListView.onMeasure(ListView.java:1081)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.View.measure(View.java:8171)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1012)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:381)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:304)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.View.measure(View.java:8171)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.View.measure(View.java:8171)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.View.measure(View.java:8171)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.ViewRoot.performTraversals(ViewRoot.java:801)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.os.Looper.loop(Looper.java:123)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at android.app.ActivityThread.main(ActivityThread.java:4627)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at java.lang.reflect.Method.invokeNative(Native Method)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at java.lang.reflect.Method.invoke(Method.java:521)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
08-05 20:25:10.988: E/AndroidRuntime(9500):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

当您声明但忘记初始化对象/变量时，通常会得到 NullPointerException。

al = new ArrayList<String>();

您忘记初始化 al 对象

Answer 2

你永远不会初始化al。您在上面定义它，但从不创建实际对象。将此包含在您的 onCreate() 方法中，然后在您的数据库查询中向 al 添加任何内容：

al = new ArrayList<String>();