我最近开始了一个新项目，目前我正在尝试从主活动中创建一个SQLite数据库，并从我的assests文件夹中的文本文件中读取。这是我的代码：
package com.example.kingmarkmcc.universe;
import android.content.Context;
import android.content.Intent;
import android.content.res.AssetManager;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.Toast;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
public class MainActivity extends AppCompatActivity {
private Button simulator;
private Button helper;
private Button settings;
private Button milkyway;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
View.OnClickListener listener = new View.OnClickListener() {
public void onClick(View v) {
if (v == simulator) {
Intent intent = new Intent(MainActivity.this, Simulator.class);
startActivity(intent);
} else if (v == helper) {
Intent intent = new Intent(MainActivity.this, HelpOptions.class);
startActivity(intent);
} else if (v == settings) {
Intent intent = new Intent(MainActivity.this, Settings.class);
startActivity(intent);
} else {
Intent intent = new Intent(MainActivity.this, MilkyWay.class);
startActivity(intent);
}
}
};
simulator = (Button) findViewById(R.id.buttonr1b1);
simulator.setOnClickListener(listener);
helper = (Button) findViewById(R.id.buttonr1b2);
helper.setOnClickListener(listener);
settings = (Button) findViewById(R.id.buttonr2b1);
settings.setOnClickListener(listener);
milkyway = (Button) findViewById(R.id.buttonr2b2);
milkyway.setOnClickListener(listener);
}
@Override
protected void onStart()
{
super.onStart();
MainActivity myMain = new MainActivity();
myMain.createDB();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.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();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
private void createDB(){
SQLiteDatabase db= null;
final String dbName = "starDB";
final String tableName = "starTable";
String[] rowRead;
String line;
try {
InputStream input;
input = getAssets().open("refinedData.txt");
InputStreamReader inputReader = new InputStreamReader(input);
BufferedReader bufferReader = new BufferedReader(inputReader);
//Retrieve or create DataBase
db = this.openOrCreateDatabase(dbName, MODE_PRIVATE, null);
//Creating table
db.execSQL("CREATE TABLE IF NOT EXISTS "
+ tableName
+ " (id INTEGER, proper TEXT, dist INTEGER , absmag INTEGER , spect TEXT , ci INTEGER , x INTEGER , y INTEGER , z INTEGER , vx INTEGER , vy INTEGER , vz INTEGER);");
while((line = bufferReader.readLine())!= null) {
rowRead = line.split(",");
// Put data into table
db.execSQL("INSERT INTO "
+ tableName
+ " (id, proper, dist, absmag, spect, ci, x, y, z, vx, vy, vz)"
+ " VALUES (" + rowRead[0] + "," + rowRead[1] + "," + rowRead[2] + "," + rowRead[3] + "," + rowRead[4] + "," + rowRead[5] + "," + rowRead[6] + "," + rowRead[7] + "," + rowRead[8] + "," + rowRead[9] + "," + rowRead[10] + "," + rowRead[11] + ");");
}
bufferReader.close();
Cursor c = db.rawQuery("SELECT * FROM " + tableName, null);
int Column1 = c.getColumnIndex("id");
int Column2 = c.getColumnIndex("x");
c.moveToFirst();
int id = c.getInt(Column1);
int x = c.getInt(Column2);
System.out.println(id);
System.out.println(x);
Toast.makeText(MainActivity.this, "End", Toast.LENGTH_LONG).show();
}
catch(Exception e) {
Log.e("Error", "Error", e);}
finally {
if (db != null)
db.close();
}
}
这是我收到的错误：
02-04 17:41:28.973 27362-27362/com.example.kingmarkmcc.universe D/AndroidRuntime: Shutting down VM
02-04 17:41:28.974 27362-27362/com.example.kingmarkmcc.universe E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.kingmarkmcc.universe, PID: 27362
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kingmarkmcc.universe/com.example.kingmarkmcc.universe.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2362)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2424)
at android.app.ActivityThread.access$900(ActivityThread.java:155)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1323)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:139)
at android.app.ActivityThread.main(ActivityThread.java:5298)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
at com.example.kingmarkmcc.universe.MainActivity.createDB(MainActivity.java:114)
at com.example.kingmarkmcc.universe.MainActivity.onStart(MainActivity.java:83)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1236)
at android.app.Activity.performStart(Activity.java:6088)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2424)
at android.app.ActivityThread.access$900(ActivityThread.java:155)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1323)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:139)
at android.app.ActivityThread.main(ActivityThread.java:5298)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)
02-04 17:41:28.977 27362-27362/com.example.kingmarkmcc.universe D/AppTracker: App Event: crash
我试图纠正这个错误，但似乎无法弄清楚问题！任何帮助将不胜感激
答案 0 :(得分：0)
你当前的问题是：
MainActivity myMain = new MainActivity();
永远不要以这种方式创建自己的活动实例。对于初学者来说，正如您所看到的那样，它们无法正常工作。此外，您 in 是MainActivity的一个实例。你不需要第二个。只需致电createDB()。
此外，您正在主应用程序线程上执行大量磁盘I / O.因此，您的UI将被冻结一段时间。不要这样做。请将重要的磁盘I / O移动到后台线程。
请注意，对于您的用户而言，use SQLiteAssetHelper在您的应用程序中打包准备好的数据库，而不是做一堆SQL语句来完成同样的事情，对您来说会更快。