Question

我从android开发开始，当我实例化SQLiteOpenHelper的子类时遇到问题，因为当我这样做时，我的应用程序简单并没有启动。

这是我的活动。

package com.almostGames.sera;

import android.os.Bundle;
import android.app.Activity;
import android.support.v4.view.ViewPager;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;
import android.widget.Button;

public class SeraActivity extends Activity {

Button btJogar, btPlacares, btDesafios, btHP;

ImagensDbHelper mDbHelper = new ImagensDbHelper(getApplicationContext());


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
        WindowManager.LayoutParams.FLAG_FULLSCREEN);

    chamarTela1(); //metodo que chama a tela1


}

public void chamarTela1() {
    setContentView(R.layout.home); //chama a tela
    btJogar = (Button) findViewById(R.id.btnJogar);
    btJogar.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            chamarTela2(); // Chama a tela2 ao clicar no botao
        }
    });
}

public void chamarTela2() {
    setContentView(R.layout.seleciona_livros); //chama a tela
    ViewPager galeria = (ViewPager) findViewById(R.id.galeria);
    GaleriaImagensAdapter adapter = new GaleriaImagensAdapter(this);
    galeria.setAdapter(adapter);
}   
}

这是我的合同类

package com.almostGames.sera;

import android.database.sqlite.*;
import android.provider.BaseColumns;

public final class ImagensContrato {


// Construtor vazio
public ImagensContrato() {}


/* Definição do conteúdo da tabela*/
public static abstract class ImagensEntry implements BaseColumns {
    public static final String TABLE_NAME = "imagens";
}

private  String SQL_CREATE_ENTRIES =
    "CREATE TABLE sera " +
         " (_id INTEGER(11) NOT NULL," +
         " livro VARCHAR(25) NOT NULL," +
         " resposta VARCHAR(30) NOT NULL," +
         " acertou INTEGER(11) NOT NULL," +
         " localizacao BLOB," +
         " PRIMARY KEY (_id)," +
         " UNIQUE (_id))";

private static final String SQL_DELETE_ENTRIES =
    "DROP TABLE IF EXISTS " + ImagensEntry.TABLE_NAME;

public String getSqlCreateEntries() {
    return SQL_CREATE_ENTRIES;
}

public String getSqlDeleteEntries() {
    return SQL_DELETE_ENTRIES;
}

}

这是我的数据库助手

package com.almostGames.sera;


import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;


public class ImagensDbHelper extends SQLiteOpenHelper {
// If you change the database schema, you must increment the database version.
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "sera.db";
ImagensContrato contrato = new ImagensContrato();

public ImagensDbHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

public void onCreate(SQLiteDatabase db) {
    db.execSQL(contrato.getSqlCreateEntries());
}

public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // This database is only a cache for online data, so its upgrade policy is
    // to simply to discard the data and start over
    db.execSQL(contrato.getSqlDeleteEntries());
    onCreate(db);
}

public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    onUpgrade(db, oldVersion, newVersion);
}

}

这是例外

08-27 22:26:06.784: E/AndroidRuntime(25370): FATAL EXCEPTION: main
08-27 22:26:06.784: E/AndroidRuntime(25370): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.almostGames.sera/com.almostGames.sera.SeraActivity}: java.lang.NullPointerException
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2064)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2178)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.ActivityThread.access$700(ActivityThread.java:141)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1271)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.os.Handler.dispatchMessage(Handler.java:99)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.os.Looper.loop(Looper.java:137)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.ActivityThread.main(ActivityThread.java:5118)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at java.lang.reflect.Method.invokeNative(Native Method)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at java.lang.reflect.Method.invoke(Method.java:511)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at dalvik.system.NativeStart.main(Native Method)
08-27 22:26:06.784: E/AndroidRuntime(25370): Caused by: java.lang.NullPointerException
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at com.almostGames.sera.SeraActivity.<init>(SeraActivity.java:15)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at java.lang.Class.newInstanceImpl(Native Method)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at java.lang.Class.newInstance(Class.java:1319)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
08-27 22:26:06.784: E/AndroidRuntime(25370):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2055)
08-27 22:26:06.784: E/AndroidRuntime(25370):    ... 11 more

我正在做这个教程（http://developer.android.com/training/basics/data-storage/databases.html），但是当我实例化我的DbHelper时，应用程序无法启动。

PS：我收到消息：＆＃34;遗憾的是已停止＆＃34;。

Answer 1

ImagensDbHelper mDbHelper = new ImagensDbHelper(getApplicationContext());

此行崩溃且getApplicationContext（）返回null，因为它在onCreate（）方法之前被调用，而活动尚未创建。因此，尝试将此初始化移动到onCreate（）方法中，如下所示：

ImagensDbHelper mDbHelper;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
        WindowManager.LayoutParams.FLAG_FULLSCREEN);

    mDbHelper = new ImagensDbHelper(getApplicationContext());
    chamarTela1(); //metodo que chama a tela1
}

当我实例化我的SQLiteOpenHelper的子类时，应用程序无法启动

1 个答案: