Android 调用新活动时无法启动活动组件信息：NullPointerException

当我通过intent调用一个新活动并通过intent发送值时，我遇到了这个错误 在这里我给了类调用nw类和他们的代码分别请检查日志猫太我越来越接近我使用的是android版本4.0.3

调用btnf时调用另一个活动的活动 新命名的活动类 数据库助手类

---

您需要在

inter

活动的

onCreate（）

中实例化

myDbHelper

（您在

disp

活动中进行了实例化，但没有在

inter

活动中进行实例化）：

---

因为

FetchingData（）

@第28行调用了

myDbHelper.onCreateDataBase（）还未被实例化。
错误在<代码> DLP。Android .DigiCon Ro.I.OnCuto（In .java：28）< /C>（由“如果有一部分导致的最低值”，然后从上到下找到第一行，提到您的代码——即导致错误的原因），也可以考虑在复制粘贴之前对代码进行格式化；不好的缩进使它很难阅读。只是给出一个建议。以大写字母开头的类名。（Java的标准命名约定）。它应该是Inter.java而不是Inter.javasir谢谢sir，它很有用，但是我不能用dbhelper的updateevent方法中的query和变量ge更新我的数据库。有一个日期是从其他类中获取的，它在我的数据库中作为promary键，请帮助我sir是的，sir，前面的问题已经解决，并且应用程序正在正确执行，但数据库中的数据没有更新。请帮助我，先生。我急需它这是一个不同的问题，但是，你确定MainActivity.g有一个有效的值吗？是的，先生，我确定，因为我可以在System.out.println（）中看到它的值；在Logcati中，如果没有满足条件的行（KEY_DAT+“=”+dt），则不可能进行更新（在这种情况下，应使用insert）
package dlp.android.digichronicle;

import java.io.IOException;
import java.util.ArrayList;
import android.app.Activity;
import android.content.Intent;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.os.Bundle;
import android.text.method.ScrollingMovementMethod;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class displ extends Activity implements View.OnClickListener {
    Button btnf;// done
    TextView tvf;// ok
    EditText edtf, edt2;// db txt

    /** Called when the activity is first created. */

    DBHandandler myDbHelper = new DBHandandler(this);
    SQLiteDatabase Mydatabase;
    String stattionlist, s1;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.displ);
        tvf = (TextView) findViewById(R.id.tvf);
        edtf = (EditText) findViewById(R.id.edtf);
        edt2 = (EditText) findViewById(R.id.edt2);
        btnf = (Button) findViewById(R.id.btnf);
        btnf.setOnClickListener(this);
        first();
    }

    private void first() {
        String s1 = "", s2 = second.ge;
        this.myDbHelper = new DBHandandler(this);
        try {
            FetchingData();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        myDbHelper.openDataBase();
        Mydatabase = myDbHelper.getWritableDatabase();
        System.out.println("executed");
        Mydatabase = myDbHelper.getReadableDatabase();
        System.out.println("sure2");// debug info
        // calling open() function which will show the text in table in EditText
        // using SetText method
        // Function is defined in DBHanadanler.java file
        if (s2.equalsIgnoreCase("memo")) {
            s1 = myDbHelper.open("memo", Mydatabase);
            System.out.println("Called memo" + s1 + s2);
        }
        else if (s2.equalsIgnoreCase("reminder"))
            s1 = myDbHelper.open("reminder", Mydatabase);
        else if (s2.equalsIgnoreCase("tasks"))
            s1 = myDbHelper.open("tasks", Mydatabase);
        else if (s2.equalsIgnoreCase("events"))
            s1 = myDbHelper.open("events", Mydatabase);
        else if (s2.equalsIgnoreCase("accounts"))
            s1 = myDbHelper.open("accounts", Mydatabase);
        System.out.println("after open");

        edtf.setText(s1);
        System.out.println("string is : " + s1 + "2nd time");// debug info
        edtf.setMovementMethod(new ScrollingMovementMethod());
        tvf.setText(s2);
    }

    // call the method which is present in datbase
    private void FetchingData() throws SQLException {

        try {
            myDbHelper.onCreateDataBase();

        } catch (IOException ioe) {

            throw new Error("Unable to create database");

        }
    }

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        if (v == btnf)
        {
            s1 = edtf.getText().toString();

            Intent i3 = new Intent(displ.this, inter.class);
            i3.putExtra("value", s1);
            startActivity(i3);

        }

    }
}

package dlp.android.digichronicle;

import java.io.IOException;

import android.app.Activity;
import android.content.Intent;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;

public class inter extends Activity {
    DBHandandler myDbHelper;
    SQLiteDatabase Mydatabase;
    String s;
    String stattionlist, DBname = "widgets";

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Bundle b = getIntent().getExtras();
        System.out.println(b);
        if (b != null)
        {
            s = b.getString("value");
            System.out.println("INtermediate" + s);
            Log.e("xxxx2", s);
            try {
                FetchingData();
                myDbHelper.openDataBase();
                System.out.println("****** After open db");
                Mydatabase = myDbHelper.getWritableDatabase();
                System.out.println("###### after writable");

                System.out.println("###### updated string :" + s);
                String s2 = second.ge;

                if (s2.equalsIgnoreCase("memo")) {
                    myDbHelper.updateevent("memo", Mydatabase, s);
                    System.out.println("Called memo" + s2 + s);
                }
                else if (s2.equalsIgnoreCase("reminder"))
                    myDbHelper.updateevent("reminder", Mydatabase, s);
                else if (s2.equalsIgnoreCase("tasks"))
                    myDbHelper.updateevent("tasks", Mydatabase, s);
                else if (s2.equalsIgnoreCase("events"))
                    myDbHelper.updateevent("events", Mydatabase, s);
                else if (s2.equalsIgnoreCase("accounts"))
                    myDbHelper.updateevent("accounts", Mydatabase, s);
                Toast.makeText(getApplicationContext(), "updated", Toast.LENGTH_LONG).show();
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            myDbHelper.close();
            Mydatabase.close();
            Intent i3 = new Intent(inter.this, second.class);
            startActivity(i3);
            System.out.println("button end btnf");
        }
        // stattionlist=this.myDbHelper.updateevent(Mydatabase,s1);
        // Toast.makeText(getApplicationContext()," "+s1,100).show();

    }

    private void FetchingData() {
        // TODO Auto-generated method stub
        try {
            myDbHelper.onCreateDataBase();
        } catch (IOException ioe) {
            throw new Error("Unable to create database");
        }
        try {
            myDbHelper.openDataBase();
            Mydatabase = myDbHelper.getWritableDatabase();
            System.out.println("executed");
        } catch (SQLException sqle) {
            throw sqle;
        }
    }

    // Intent i2=new Intent(intermediate.this,second.class);
    // startActivity(i2);
}

package dlp.android.digichronicle; // ur package name

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.widget.Toast;

public class DBHandandler extends SQLiteOpenHelper {
    private static String DB_PATH = "/data/data/dlp.android.digichronicle/databases/";
    private static String DB_NAME = "widget.sqlite";
    private static String TABLENAME = "widgets";
    private SQLiteDatabase myDataBase;
    private final Context myContext;
    public static final String KEY_MEM = "memo";
    public static final String KEY_TAS = "tasks";
    public static final String KEY_REM = "reminder";
    public static final String KEY_EVE = "events";
    public static final String KEY_DAT = "dated";
    public static final String KEY_INC = "income";
    public static final String KEY_EXP = "exp";
    ArrayList<String> sw;
    String s = "";

    public DBHandandler(Context context) {
        super(context, DB_NAME, null, 1);
        // TODO Auto-generated constructor stub
        this.myContext = context;
    }

    public int onCreateDataBase() throws IOException
    {
        boolean dbExist = checkDatabase();
        if (dbExist) {
            return 0;
        }
        else {
            System.out.println("onCreateDataBase method execution starts");
            this.getReadableDatabase();
            copyDataBase();
            return 1;

        }
    }

    private boolean checkDatabase() {
        // TODO Auto-generated method stub
        SQLiteDatabase checkDB = null;
        try
        {
            String myPath = DB_PATH + DB_NAME;
            checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
        } catch (Exception e) {
            // TODO: handle exception
        }
        if (checkDB != null) {
            checkDB.close();
        }
        return checkDB != null ? true : false;

    }

    private void copyDataBase() throws IOException {

        InputStream myInput = myContext.getAssets().open(DB_NAME);

        // Path to the just created empty db
        String outFileName = DB_PATH + DB_NAME;

        // Open the empty db as the output stream
        OutputStream myOutput = new FileOutputStream(outFileName);

        // transfer bytes from the inputfile to the outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer)) > 0) {
            myOutput.write(buffer, 0, length);

        }

        // Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();

    }

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

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub

    }

    public void openDataBase() throws SQLException {

        // Open the database
        String myPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);

    }

    public String open(String s3, SQLiteDatabase Mydatabase)
    {
        ArrayList<String> list = new ArrayList<String>();
        System.out.println("b4 string is : " + list);
        String s = "";
        try {
            System.out.println("string got is " + s3);
            String dt = MainActivity.g;
            System.out.println("Date obtained is " + dt);
            Cursor c = Mydatabase.rawQuery("select " + " " + s3 + " "
                    + "from widgets where dated='" + dt + "'", null);
            System.out.println("after cursor");
            if (c.moveToFirst())
            {

                System.out.println("cursor" + c.moveToFirst());
                do {
                    System.out.println("before columnindex");
                    list.add(c.getString(c.getColumnIndex(s3)));
                    System.out.println("after columnindex");
                    System.out.println(" string is :  " + list);

                } while (c.moveToNext());
            }
            c.close();
        } catch (SQLiteException se) {
            Mydatabase.close();
            System.out.println("exception");
        }

        s = list.toString();
        int len = s.length();
        s = s.substring(1, len - 1);
        System.out.println("string is : " + list);
        System.out.println("string is : " + s);
        return s;

    }

    public String eventdia(SQLiteDatabase mydatabase1) {

        ArrayList<String> list = new ArrayList<String>();
        System.out.println("Event dialogue start " + list);
        String s = "";
        try {
            System.out.println("geting event");
            String dt = MainActivity.g;
            System.out.println("Date obtained in events " + dt);
            Cursor c = mydatabase1.rawQuery("select events from widgets where dated='" + dt + "'",
                    null);

            if (c.moveToFirst())
            {
                System.out.println("cursor" + c.moveToFirst());
                do {
                    list.add(c.getString(c.getColumnIndex("events")));
                    System.out.println(" string is :  " + list);

                } while (c.moveToNext());
            }
            c.close();
        } catch (SQLiteException se) {
            mydatabase1.close();
            System.out.println("exception");
        }

        s = list.toString();
        int len = s.length();
        s = s.substring(1, len - 1);
        System.out.println("string is : " + list);
        System.out.println("string is : " + s);
        return s;
    }

    public String checkin(SQLiteDatabase mydatabase1) {

        ArrayList<String> list = new ArrayList<String>();
        System.out.println("b4 string is : " + list);
        String s = "";
        try {
            System.out.println("geting event");
            String dt = MainActivity.g;
            System.out.println("Date obtained is" + dt);
            Cursor c = mydatabase1.rawQuery("select events from widgets where dated='" + dt + "'",
                    null);

            if (c.moveToFirst())
            {
                System.out.println("cursor" + c.moveToFirst());
                do {
                    list.add(c.getString(c.getColumnIndex("events")));
                    System.out.println(" string is :  " + list);

                } while (c.moveToNext());
            }
            c.close();
        } catch (SQLiteException se) {
            mydatabase1.close();
            System.out.println("exception");
        }

        s = list.toString();
        int len = s.length();
        s = s.substring(1, len - 1);
        System.out.println("string is : " + list);
        System.out.println("string is : " + s);
        return s;
    }

    public void updateevent(String ge, SQLiteDatabase mydatabase2, String s1) {
        // TODO Auto-generated method stub
        String dt = MainActivity.g;
        System.out.println("###### before query");
        ContentValues cvupdate = new ContentValues();
        cvupdate.put(ge, s1);

        mydatabase2.update(TABLENAME, cvupdate, KEY_DAT + "=" + dt, null);

        // mydatabase2.rawQuery("update widgets set memo="+" "+s1+" "+" where dated='"+dt+"'",null);
        // mydatabase2.rawQuery("update widgets set "+" "+ge+" = "+s1+" "+"where where dated='"+dt+"'",null);
        System.out.println("###### after query");
    }

    // Function inserting new record
    public void Insertinto(SQLiteDatabase mydatabase2, String date, String memo, String tasks,
            String reminder, float income, String events, float exp)
    {
        ContentValues data = createContentValues(date, memo, tasks, reminder, income, events, exp);
        System.out.println("data " + data + "  is ");
        mydatabase2.insert(TABLENAME, null, data);
        System.out.println("record is done ");
    }

    // return a content of the database
    private ContentValues createContentValues(String date, String memo, String tasks,
            String reminder, float income, String events, float exp)
    {
        ContentValues values = new ContentValues();
        values.put(KEY_DAT, date);
        values.put(KEY_MEM, memo);
        values.put(KEY_TAS, tasks);
        values.put(KEY_REM, reminder);
        values.put(KEY_INC, income);
        values.put(KEY_EVE, events);
        values.put(KEY_EXP, exp);
        return values;
    }

    // write ur retreival methods here
}

D/AndroidRuntime(553): Shutting down VM
W/dalvikvm(553): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
E/AndroidRuntime(553): FATAL EXCEPTION: main
E/AndroidRuntime(553): java.lang.RuntimeException: Unable to start activity ComponentInfo{dlp.android.digichronicle/dlp.android.digichronicle.inter}: java.lang.NullPointerException
E/AndroidRuntime(553):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
E/AndroidRuntime(553):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
E/AndroidRuntime(553):  at android.app.ActivityThread.access$600(ActivityThread.java:123)
E/AndroidRuntime(553):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
E/AndroidRuntime(553):  at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(553):  at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime(553):  at android.app.ActivityThread.main(ActivityThread.java:4424)
E/AndroidRuntime(553):  at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(553):  at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime(553):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
E/AndroidRuntime(553):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
E/AndroidRuntime(553):  at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(553): Caused by: java.lang.NullPointerException
E/AndroidRuntime(553):  at dlp.android.digichronicle.inter.FetchingData(inter.java:84)
E/AndroidRuntime(553):  at dlp.android.digichronicle.inter.onCreate(inter.java:28)
E/AndroidRuntime(553):  at android.app.Activity.performCreate(Activity.java:4465)
E/AndroidRuntime(553):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
E/AndroidRuntime(553):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
E/AndroidRuntime(553):    ... 11 more
I/dalvikvm(553): threadid=3: reacting to signal 3
I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
I/dalvikvm(553): threadid=3: reacting to signal 3
I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
I/dalvikvm(553): threadid=3: reacting to signal 3
I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'

DBHandandler myDbHelper = new DBHandandler(this);