java - 空指针异常：找不到解决方案

Question

我的代码中出现无法理解的错误。请帮我找出问题所在。

我有数据库类和主要活动..它显示在日志中，但是当它出现在我的模拟器屏幕上时，它给了我错误。

我的数据库类：

package com.example.nearby_places;

import java.util.ArrayList;
import java.util.List;


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;

public class Database extends SQLiteOpenHelper {

    //database name & version number
    private static final String db_name = "nearby_places";
    private static final int db_version = 1;

    //tables
    private static final String table_placetypes = "placetypes";
    private static final String table_places = "table_places";

    //column names
    private static final String type_id = "type_id";
    private static final String type_name = "type_name";
    private static final String place_id = "place_id";
    private static final String place_name = "place_name";
    private static final String place_address = "place_address";
    private static final String place_contact = "place_contact";


    public Database(Context context) {
        super(context, db_name, null, db_version);
        // TODO Auto-generated constructor stub
    }

    // create table queries
    String create_table_placetypes = "CREATE TABLE IF NOT EXISTS " + table_placetypes + "("
            + type_id + " INTEGER PRIMARY KEY NOT NULL," + type_name + " TEXT" + ")";



    String create_table_places = "CREATE TABLE IF NOT EXISTS table_places (place_id INTEGER PRIMARY KEY NOT NULL, place_name TEXT, place_address TEXT, place_contact TEXT, type_id INTEGER, FOREIGN KEY (type_id) REFERENCES table_placetypes(type_id))";



    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub

        db.execSQL(create_table_placetypes);

        Log.d("creating", "placetypes created");
        db.execSQL(create_table_places);
        Log.d("creating", "places created");


    }


    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
        db.execSQL("DROP TABLE IF EXISTS " + table_placetypes);
        db.execSQL("DROP TABLE IF EXISTS " + table_places);
        onCreate(db);

    }
    // add placetypes 
    void addplacetypes (placetypes pt) {

        SQLiteDatabase db = getWritableDatabase();
        ContentValues values = new ContentValues();

        values.put(type_name, pt.getTypename());

        db.insert(table_placetypes, null, values);
        db.close();

    }

     // Getting single placetypes
    placetypes getPlacetypes(int id) {
        SQLiteDatabase db = this.getReadableDatabase();

        Cursor cursor = db.query(table_placetypes, new String[] {type_id,
                type_name }, type_id + "=?",
                new String[] { String.valueOf(id) }, null, null, null, null);
        if (cursor != null)
            cursor.moveToFirst();

        placetypes pt = new placetypes(Integer.parseInt(cursor.getString(0)),
                cursor.getString(1));
        // return contact
        return pt;
    }

 // Getting All placetypes
    public List<placetypes> getAllPlacetypes() {
        List<placetypes> placetypesList = new ArrayList<placetypes>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + table_placetypes;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                placetypes pt = new placetypes();
                pt.setTypeid(Integer.parseInt(cursor.getString(0)));
                pt.setTypename(cursor.getString(1));


                //String name = cursor.getString(1);

                //MainActivity.ArrayofName.add(name);
                // Adding contact to list
                placetypesList.add(pt);
            } while (cursor.moveToNext());
        }

        // return placetype list
        return placetypesList;
}

    // Getting placetypes Count
    public int getPlacetypesCount() {
        String countQuery = "SELECT  * FROM " + table_placetypes;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(countQuery, null);
        cursor.close();

        // return count
        return cursor.getCount();
    }

    public void addplaces(places p) {
        SQLiteDatabase db = getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(place_name, p.getPlace_name());
        values.put(place_address, p.getPlace_address());
        values.put(place_contact, p.getPlace_contact());
        values.put(type_id, p.getT_id());

        Log.d("Type ID", String.valueOf(p.getT_id()));
        db.insert(table_places, null, values);
        db.close();

    }

    places getPlaces(int pid) {

        SQLiteDatabase db = getReadableDatabase();
        Cursor cursor = db.query(table_places, new String[] {place_id, place_name, place_address, place_contact,type_id}, place_id + "=?", new String[] { String.valueOf(pid) } , null, null, null, null);

        if(cursor != null)
             cursor.moveToFirst();

        places p = new places(Integer.parseInt(cursor.getString(0)),
                Integer.parseInt(cursor.getString(1)),
                cursor.getString(2),
                cursor.getString(3),
                cursor.getString(4)
                );

        cursor.close();

        return p;

    }

    public List<places> getAllPlaces(String typeName) {

        List<places> placeList = new ArrayList<places>();
        //String selectQuery = "SELECT * FROM table_places INNER JOIN placetypes ON placetypes.type_id=table_places.type_id "; 
        //String selectQuery = "SELECT * FROM table_places WHERE table_places.type_id="+Integer.toString(typeid);
        String selectQuery ="SELECT * FROM table_places WHERE placetypes.place_name="+typeName+" INNER JOIN placetypes ON placetypes.type_id=table_places.type_id";
        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);


        if(cursor.moveToFirst() )
        {
            do{
            places p = new places();
            /*p.setT_id(cursor.getColumnIndex(type_id));
            p.setPlace_id(cursor.getColumnIndex(place_id));
            p.setPlace_name(cursor.getColumnIndex(place_name));
            */
            p.setT_id(cursor.getInt(0));
            p.setPlace_id(cursor.getInt(1));
            p.setPlace_name(cursor.getString(2));
            p.setPlace_address(cursor.getString(3));
            p.setPlace_contact(cursor.getString(4));

            /*String t_id = cursor.getString(4);
            String p_name = cursor.getString(2);
            String p_address = cursor.getString(3);
            String p_contact = cursor.getString(1);*/

            placeList.add(p);
            }while(cursor.moveToNext());
        }

        cursor.close();
        return placeList;
    }

    public int getPlaceCount () {

        String selectQuery = "SELECT * FROM " +table_places;
        SQLiteDatabase db = getReadableDatabase();
        Cursor cursor = db.rawQuery(create_table_places, null);
        cursor.close();

        return cursor.getCount();
    }

}

主要活动

    package com.example.nearby_places;

import java.util.ArrayList;
import java.util.List;


import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import android.widget.AdapterView.OnItemClickListener;

public class MainActivity extends Activity {

    private ListView listView;
    public static ArrayList<String> ArrayofName = new ArrayList<String>();
    public static final String PLACETYPE = "com.example";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
         Database db = new Database(this);
         if(db.getAllPlacetypes().isEmpty())
         {
         /**
             * CRUD Operations
             * */
            // Inserting Places
            Log.d("Insert: ", "Inserting ..");
            db.addplacetypes(new placetypes("RESTURAUNTS"));
            db.addplacetypes(new placetypes("MALLS"));
            db.addplacetypes(new placetypes("GAS STATIONS"));
            db.addplacetypes(new placetypes("HOTELS"));
            db.addplacetypes(new placetypes("MOTELS")); 
         }
            // Reading all Places
            Log.d("Reading: ", "Reading all placetypes..");

            if(ArrayofName.isEmpty())
            {
                List<placetypes> placetypes = db.getAllPlacetypes();
                for (placetypes pt : placetypes) 
                {
                    String log = "Id: "+pt.getTypeid()+" ,Name: " + pt.getTypename();
                    // Writing Places to log
                    Log.d("Name: ", log);
                    System.out.println(log);
                    ArrayofName.add(pt.getTypename());
                }
            }   

            listView = (ListView) findViewById(R.id.listView1);

            ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                    android.R.layout.simple_list_item_1, ArrayofName);
            int pos = listView.getAdapter().getCount() -1;
            listView.getAdapter().getItemId(pos);
            listView.setAdapter(adapter);

            listView.setOnItemClickListener(new OnItemClickListener() 
                {
                    public void onItemClick(AdapterView<?> parent, View v, int position, long id) 
                    {
                        String type = ((TextView) v).getText().toString();
                        Toast.makeText(getApplicationContext(), type, Toast.LENGTH_SHORT).show();
                        Intent i = new Intent(getApplicationContext(),MainActivity2.class);
                        i.putExtra(PLACETYPE, type);
                        startActivity(i);
                        /*Cursor cursor = (Cursor) parent.getItemAtPosition(position);
                        Toast.makeText(getApplicationContext(), "id: " +id+ "position: " +position+ "row id: " +(cursor.getColumnIndex("" +
                            "")), Toast.LENGTH_LONG).show();
                        */





                       Intent intent = new Intent(MainActivity.this, MainActivity2.class);
                       startActivity(intent);


                    }
                }
            );
    }

    }

日志猫

 10-11 17:21:51.871: D/Reading:(4932): Reading all placetypes..
10-11 17:21:51.871: D/Name:(4932): Id: 1 ,Name: RESTURAUNTS
10-11 17:21:51.875: I/System.out(4932): Id: 1 ,Name: RESTURAUNTS
10-11 17:21:51.875: D/Name:(4932): Id: 2 ,Name: MALLS
10-11 17:21:51.875: I/System.out(4932): Id: 2 ,Name: MALLS
10-11 17:21:51.875: D/Name:(4932): Id: 3 ,Name: GAS STATIONS
10-11 17:21:51.875: I/System.out(4932): Id: 3 ,Name: GAS STATIONS
10-11 17:21:51.875: D/Name:(4932): Id: 4 ,Name: HOTELS
10-11 17:21:51.875: I/System.out(4932): Id: 4 ,Name: HOTELS
10-11 17:21:51.875: D/Name:(4932): Id: 5 ,Name: MOTELS
10-11 17:21:51.875: I/System.out(4932): Id: 5 ,Name: MOTELS
10-11 17:21:51.887: D/AndroidRuntime(4932): Shutting down VM
10-11 17:21:51.887: W/dalvikvm(4932): threadid=1: thread exiting with uncaught exception (group=0x41c77300)
10-11 17:21:51.894: E/AndroidRuntime(4932): FATAL EXCEPTION: main
10-11 17:21:51.894: E/AndroidRuntime(4932): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.nearby_places/com.example.nearby_places.MainActivity}: java.lang.NullPointerException
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.os.Looper.loop(Looper.java:137)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.ActivityThread.main(ActivityThread.java:4745)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at java.lang.reflect.Method.invokeNative(Native Method)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at java.lang.reflect.Method.invoke(Method.java:511)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at dalvik.system.NativeStart.main(Native Method)
10-11 17:21:51.894: E/AndroidRuntime(4932): Caused by: java.lang.NullPointerException
10-11 17:21:51.894: E/AndroidRuntime(4932):     at com.example.nearby_places.MainActivity.onCreate(MainActivity.java:62)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.Activity.performCreate(Activity.java:5008)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
10-11 17:21:51.894: E/AndroidRuntime(4932):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
10-11 17:21:51.894: E/AndroidRuntime(4932):     ... 11 more
10-11 17:21:53.695: I/Process(4932): Sending signal. PID: 4932 SIG: 9

score 3 · Accepted Answer

您对 getAdapter 的调用返回 null，因为您在 setAdapter 之前调用它，请尝试以下操作：

listView.setAdapter(adapter);
int pos = listView.getAdapter().getCount() -1;
listView.getAdapter().getItemId(pos);

score 0 · Accepted Answer

如果您有NULLPOINTER 异常，请深入了解您的 LogCat。特别是在它说的那一行Caused by。学习如何阅读和使用您的 LogCat，并尝试找到它提到您的类/包名称的行并分析该行。

Caused by: java.lang.NullPointerException
10-11 17:21:51.894: E/AndroidRuntime(4932): at com.example.nearby_places.MainActivity.onCreate(MainActivity.java:62)

score 0 · Accepted Answer

问题出在

List<placetypes> placetypes = db.getAllPlacetypes();

您使用的查询是错误的。它应该是

`SELECT * FROM table_places INNER JOIN placetypes ON placetypes.type_id=table_places.type_id WHERE placetypes.place_name="+typeName+`"

java - 空指针异常：找不到解决方案

3 回答 3

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