java - 非常奇怪的Android NullPointerException

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What is a NullPointerException, and how do I fix it?
                                
                                    （12个答案）
                                
                        
                                去年关闭。
            
                    
我一直在用从我的sqlite数据库中检索到的数据填充ListView，
我的数据库文件DBMainAdapter中的代码可以完美地工作（片段以节省空间），我知道这一点，因为当我在一个正在使用的单独活动中调用我的getAll()时（不包括在内），我得到了我的ArrayList并且可以使用可以很好地打印出内部内容，但是我注意到，每当我尝试在ProfileListActivity内部执行相同的操作时，都会不断抛出NullPointerException。我究竟做错了什么？！

错误日志：

04-21 06:42:31.669: W/dalvikvm(6107): threadid=1: thread exiting with uncaught exception (group=0xb3a8db90)
04-21 06:42:31.749: E/AndroidRuntime(6107): FATAL EXCEPTION: main
04-21 06:42:31.749: E/AndroidRuntime(6107): Process: com.test.profilekeeper, PID: 6107
04-21 06:42:31.749: E/AndroidRuntime(6107): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.profilekeeper/com.test.profilekeeper.ProfileListActivity}: java.lang.NullPointerException
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.ActivityThread.access$700(ActivityThread.java:135)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.os.Looper.loop(Looper.java:137)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.ActivityThread.main(ActivityThread.java:4998)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at java.lang.reflect.Method.invokeNative(Native Method)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at java.lang.reflect.Method.invoke(Method.java:515)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at dalvik.system.NativeStart.main(Native Method)
04-21 06:42:31.749: E/AndroidRuntime(6107): Caused by: java.lang.NullPointerException
04-21 06:42:31.749: E/AndroidRuntime(6107):     at com.test.profilekeeper.LoadedData.<init>(LoadedData.java:17)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at com.test.profilekeeper.ProfileListActivity.onCreate(ProfileListActivity.java:23)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.Activity.performCreate(Activity.java:5243)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-21 06:42:31.749: E/AndroidRuntime(6107):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
04-21 06:42:31.749: E/AndroidRuntime(6107):     ... 11 more

在com.test.profilekeeper.LoadedData。（LoadedData.java:17）的位置：ArrayList listdata = dbhelper.getAll（）; //构造函数

在com.test.profilekeeper.ProfileListActivity.onCreate（ProfileListActivity.java:23）处：LoadedData mydata = new LoadedData（）;

public class DBMainAdapter{
    DBMain dbhelper;

    public DBMainAdapter(Context context){
        dbhelper = new DBMain(context);
    }

    public long insertprofiledata(String fname, String lname, String age, String gender, String username, String password){
        SQLiteDatabase db = dbhelper.getWritableDatabase();
        ContentValues cv = new ContentValues();

        cv.put(DBMain.FIRSTNAME, fname);
        cv.put(DBMain.LASTNAME, lname);
        cv.put(DBMain.AGE, age);
        cv.put(DBMain.GENDER, gender);
        cv.put(DBMain.USERNAME, username);
        cv.put(DBMain.PASSWORD, password);

        long id = db.insert(DBMain.TABLE_NAME, null, cv);
        return id;
    }

    public ArrayList<String[]> getAll(){
        SQLiteDatabase db = dbhelper.getWritableDatabase();
        String[] columns = {DBMain.UID, DBMain.FIRSTNAME, DBMain.LASTNAME, DBMain.AGE, DBMain.GENDER};
        Cursor curs = db.query(DBMain.TABLE_NAME, columns, null, null, null, null, null);

        int index1 = curs.getColumnIndex(DBMain.UID);
        int index2 = curs.getColumnIndex(DBMain.FIRSTNAME);
        int index3 = curs.getColumnIndex(DBMain.LASTNAME);
        int index4 = curs.getColumnIndex(DBMain.AGE);
        int index5 = curs.getColumnIndex(DBMain.GENDER);

        StringBuffer buffer = new StringBuffer();

        while(curs.moveToNext()){
            String uid = curs.getString(index1);
            String fname = curs.getString(index2);
            String lname = curs.getString(index3);
            String age = curs.getString(index4);
            String gender = curs.getString(index5);

            buffer.append(uid+","+fname+","+lname+","+age+","+gender+" ");
        }

        String[] row = buffer.toString().split(" ");
        String profilebuilder[][] = new String[row.length][row[0].length()];

        for(int i = 0; i < row.length; i++){
            String[] profile = row[i].split(",");
            for(int j = 0; j < profile.length; j++){
                profilebuilder[i][j] = profile[j];
            }
        }

        String[] ids = new String[profilebuilder.length];
        String[] firstname = new String[profilebuilder.length];
        String[] ages = new String[profilebuilder.length];
        String[] genders = new String[profilebuilder.length];

        for(int k = 0; k < profilebuilder.length; k++){
            for(int l = 0; l < profilebuilder[0].length; l++){
                ids[k] = profilebuilder[k][0];
                firstname[k] = profilebuilder[k][1];
                ages[k] = profilebuilder[k][3];
                genders[k] = profilebuilder[k][4];
            }
        }

        ArrayList<String[]> alldata = new ArrayList<String[]>();
        alldata.add(ids);
        alldata.add(firstname);
        alldata.add(ages);
        alldata.add(genders);

        return alldata;
    }
}

    public class LoadedData {
        DBMainAdapter dbhelper;

        String[] ids;
        String[] firstnames;
        String[] ages;
        String[] genders;

        public LoadedData(){
            ArrayList<String[]> listdata = dbhelper.getAll();

            ids =  new String[listdata.get(0).length];
            firstnames = new String[listdata.get(0).length];
            ages = new String[listdata.get(0).length];
            genders  = new String[listdata.get(0).length];

            ids = listdata.get(0);
            firstnames = listdata.get(1);
            ages = listdata.get(2);
            genders = listdata.get(3);
        }

        public String[] getIds() {
            return ids;
        }

        public String[] getFirstnames() {
            return firstnames;
        }

        public String[] getAges() {
            return ages;
        }

        public String[] getGenders() {
            return genders;
        }
    }


public class ProfileListActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        LoadedData mydata = new LoadedData();
        populateListView(mydata);
    }

    private void populateListView(LoadedData mydata) {
        ArrayAdapter<LoadedData> adapter = new MyListAdapter(mydata);
        ListView list = (ListView) findViewById(R.id.listView);
        list.setAdapter(adapter);
    }

    private class MyListAdapter extends ArrayAdapter<LoadedData>{
        LoadedData mdata;
        public MyListAdapter(LoadedData data){
            super(ProfileListActivity.this, R.layout.row);
            this.mdata = data;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View itemView = convertView;
            if(itemView == null){
                itemView = getLayoutInflater().inflate(R.layout.row, parent, false);
            }

            TextView first_name = (TextView) itemView.findViewById(R.id.nameListTextView);
            TextView id = (TextView) itemView.findViewById(R.id.idListTextView);
            TextView age = (TextView) itemView.findViewById(R.id.ageListTextView);
            ImageView img = (ImageView) itemView.findViewById(R.id.imageViewList);

            id.setText(mdata.getIds()[position]);
            first_name.setText(mdata.getFirstnames()[position]);
            age.setText(mdata.getAges()[position]);

            if(mdata.getGenders()[position].equalsIgnoreCase("Male")){
                img.setImageResource(R.raw.man_thumb);
            }else{
                img.setImageResource(R.raw.woman_thumb);
            }

            return itemView;
        }
    }
}

最佳答案

您忘记了初始化LoadedData类中的dbhelper对象。在下面的类中创建一个公共构造函数，

public DBMainAdapter(Context context)
{
    dbhelper = new DBMain(context);
}

同样在DBMainAdapter类的getAll（）方法中，在while循环之前编写以下语句，

curs.moveToFirst();