創建SQLite數據庫時應用程序崩潰

Question

我是一名新開發人員，在嘗試創建數據庫時遇到問題。 當缺少特定的代碼行時，該應用程序運行良好。 我究竟做錯了什么？

另一個問題：我如何從片段訪問數據庫？

謝謝！

import android.app.SearchManager;
import android.content.Intent;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.Menu;
import android.view.MenuItem;


public class MainActivity extends ActionBarActivity {


    SQLiteDatabase db=openOrCreateDatabase("MyDB", MODE_PRIVATE,null);
    // it crashes when I add this line

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Intent intent = getIntent();

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

logcat的：

04-15 18:58:42.034: E/Trace(2497): error opening trace file: No such file or directory (2)
04-15 18:58:42.178: D/AndroidRuntime(2497): Shutting down VM
04-15 18:58:42.178: W/dalvikvm(2497): threadid=1: thread exiting with uncaught exception (group=0xa630a288)
04-15 18:58:42.194: E/AndroidRuntime(2497): FATAL EXCEPTION: main
04-15 18:58:42.194: E/AndroidRuntime(2497): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.craftyonhand.www/com.craftyonhand.www.MainActivity}: java.lang.NullPointerException
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.os.Looper.loop(Looper.java:137)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.ActivityThread.main(ActivityThread.java:4745)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at java.lang.reflect.Method.invokeNative(Native Method)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at java.lang.reflect.Method.invoke(Method.java:511)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at dalvik.system.NativeStart.main(Native Method)
04-15 18:58:42.194: E/AndroidRuntime(2497): Caused by: java.lang.NullPointerException
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:215)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at com.craftyonhand.www.MainActivity.<init>(MainActivity.java:14)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at java.lang.Class.newInstanceImpl(Native Method)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at java.lang.Class.newInstance(Class.java:1319)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
04-15 18:58:42.194: E/AndroidRuntime(2497):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
04-15 18:58:42.194: E/AndroidRuntime(2497):     ... 11 more
04-15 18:59:13.598: I/Process(2497): Sending signal. PID: 2497 SIG: 9

Answer 1

您太早初始化了。 將初始化代碼移動到onCreate() ，其中活動是有效的上下文。