这是我写的主要活动，它不起作用！请帮忙
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;
public class MainActivity extends Activity {
Button button1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button1.findViewById(R.id.button1);
button1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Toast.makeText(getApplicationContext(),"onClick aaaayaaaa", Toast.LENGTH_LONG).show();
try {
MyDatabase db=new MyDatabase(MainActivity.this);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
}
}
这是我编写的MyDatabase java文件，用于访问使用不同应用程序预先填充的外部数据库
import android.database.sqlite.SQLiteException;
import android.widget.Toast;
import com.readystatesoftware.sqliteasset.SQLiteAssetHelper;
public class MyDatabase extends SQLiteAssetHelper {
private Context mycontext;
private String DB_PATH="/mnt/sdcard/Kitchen_Data.db";
private static String DB_NAME = "Kitchen_Data.db";
public SQLiteDatabase myDataBase;
public MyDatabase(Context context) throws IOException {
super(context,DB_NAME,null,1);
this.mycontext=context;
Toast.makeText(mycontext, "Constructor called", Toast.LENGTH_LONG).show();
boolean dbexist = checkdatabase();
if (dbexist) {
opendatabase();
} else {
System.out.println("Database doesn't exist");
createdatabase();
}
}
public void createdatabase() throws IOException {
boolean dbexist = checkdatabase();
if(!dbexist) {
this.getReadableDatabase();
try {
copydatabase();
} catch(IOException e) {
throw new Error("Error copying database");
}
}
}
private boolean checkdatabase() {
boolean checkdb = false;
try {
String myPath = DB_PATH;
File dbfile = new File(myPath);
Toast.makeText(mycontext, "Database exists", Toast.LENGTH_LONG).show();
checkdb = dbfile.exists();
} catch(SQLiteException e) {Toast.makeText(mycontext, "Error in database", Toast.LENGTH_LONG).show();
}
return checkdb;
}
private void copydatabase() throws IOException {
//Open your local db as the input stream
InputStream myinput = mycontext.getAssets().open(DB_NAME);
// Path to the just created empty db
String outfilename = DB_PATH;
//Open the empty db as the output stream
OutputStream myoutput = new FileOutputStream(outfilename);
// transfer byte to inputfile to 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();
}
public void opendatabase() throws SQLException {
//Open the database
String mypath = DB_PATH;
myDataBase = SQLiteDatabase.openDatabase(mypath, null, SQLiteDatabase.OPEN_READWRITE);
Toast.makeText(mycontext, "database opeened", Toast.LENGTH_LONG).show();
}
public synchronized void close() {
if(myDataBase != null) {
myDataBase.close();
}
super.close();
}
}
这里是logcat：
03-31 12:34:37.045: D/AndroidRuntime(15922): Shutting down VM
03-31 12:34:37.045: W/dalvikvm(15922): threadid=1: thread exiting with uncaught exception (group=0x40d79ae0)
03-31 12:34:37.055: E/AndroidRuntime(15922): FATAL EXCEPTION: main
03-31 12:34:37.055: E/AndroidRuntime(15922): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitchen_application/com.example.kitchen_application.MainActivity}: java.lang.NullPointerException
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2186)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.ActivityThread.access$600(ActivityThread.java:145)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1238)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.os.Handler.dispatchMessage(Handler.java:99)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.os.Looper.loop(Looper.java:137)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.ActivityThread.main(ActivityThread.java:5099)
03-31 12:34:37.055: E/AndroidRuntime(15922): at java.lang.reflect.Method.invokeNative(Native Method)
03-31 12:34:37.055: E/AndroidRuntime(15922): at java.lang.reflect.Method.invoke(Method.java:511)
03-31 12:34:37.055: E/AndroidRuntime(15922): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
03-31 12:34:37.055: E/AndroidRuntime(15922): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
03-31 12:34:37.055: E/AndroidRuntime(15922): at dalvik.system.NativeStart.main(Native Method)
03-31 12:34:37.055: E/AndroidRuntime(15922): Caused by: java.lang.NullPointerException
03-31 12:34:37.055: E/AndroidRuntime(15922): at com.example.kitchen_application.MainActivity.onCreate(MainActivity.java:30)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.Activity.performCreate(Activity.java:5117)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-31 12:34:37.055: E/AndroidRuntime(15922): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
03-31 12:34:37.055: E/AndroidRuntime(15922): ... 11 more
03-31 12:35:38.650: D/AndroidRuntime(16560): Shutting down VM
03-31 12:35:38.650: W/dalvikvm(16560): threadid=1: thread exiting with uncaught exception (group=0x40d79ae0)
03-31 12:35:38.660: E/AndroidRuntime(16560): FATAL EXCEPTION: main
03-31 12:35:38.660: E/AndroidRuntime(16560): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitchen_application/com.example.kitchen_application.MainActivity}: java.lang.NullPointerException
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2186)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.ActivityThread.access$600(ActivityThread.java:145)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1238)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.os.Handler.dispatchMessage(Handler.java:99)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.os.Looper.loop(Looper.java:137)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.ActivityThread.main(ActivityThread.java:5099)
03-31 12:35:38.660: E/AndroidRuntime(16560): at java.lang.reflect.Method.invokeNative(Native Method)
03-31 12:35:38.660: E/AndroidRuntime(16560): at java.lang.reflect.Method.invoke(Method.java:511)
03-31 12:35:38.660: E/AndroidRuntime(16560): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
03-31 12:35:38.660: E/AndroidRuntime(16560): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
03-31 12:35:38.660: E/AndroidRuntime(16560): at dalvik.system.NativeStart.main(Native Method)
03-31 12:35:38.660: E/AndroidRuntime(16560): Caused by: java.lang.NullPointerException
03-31 12:35:38.660: E/AndroidRuntime(16560): at com.example.kitchen_application.MainActivity.onCreate(MainActivity.java:30)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.Activity.performCreate(Activity.java:5117)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-31 12:35:38.660: E/AndroidRuntime(16560): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
03-31 12:35:38.660: E/AndroidRuntime(16560): ... 11 more
03-31 12:35:39.982: I/Process(16560): Sending signal. PID: 16560 SIG: 9
03-31 12:36:19.714: D/AndroidRuntime(17185): Shutting down VM
03-31 12:36:19.714: W/dalvikvm(17185): threadid=1: thread exiting with uncaught exception (group=0x40d79ae0)
03-31 12:36:19.794: E/AndroidRuntime(17185): FATAL EXCEPTION: main
03-31 12:36:19.794: E/AndroidRuntime(17185): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitchen_application/com.example.kitchen_application.MainActivity}: java.lang.NullPointerException
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2186)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.ActivityThread.access$600(ActivityThread.java:145)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1238)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.os.Handler.dispatchMessage(Handler.java:99)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.os.Looper.loop(Looper.java:137)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.ActivityThread.main(ActivityThread.java:5099)
03-31 12:36:19.794: E/AndroidRuntime(17185): at java.lang.reflect.Method.invokeNative(Native Method)
03-31 12:36:19.794: E/AndroidRuntime(17185): at java.lang.reflect.Method.invoke(Method.java:511)
03-31 12:36:19.794: E/AndroidRuntime(17185): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
03-31 12:36:19.794: E/AndroidRuntime(17185): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
03-31 12:36:19.794: E/AndroidRuntime(17185): at dalvik.system.NativeStart.main(Native Method)
03-31 12:36:19.794: E/AndroidRuntime(17185): Caused by: java.lang.NullPointerException
03-31 12:36:19.794: E/AndroidRuntime(17185): at com.example.kitchen_application.MainActivity.onCreate(MainActivity.java:28)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.Activity.performCreate(Activity.java:5117)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-31 12:36:19.794: E/AndroidRuntime(17185): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
03-31 12:36:19.794: E/AndroidRuntime(17185): ... 11 more
03-31 12:36:21.236: I/Process(17185): Sending signal. PID: 17185 SIG: 9
答案 0 :(得分：0)
在logcat中，由于您的External database，没有任何错误说明。只是错误在onCreate()方法上。在您刚刚声明button1的方法上，您需要初始化按钮。所以你需要从
button1.findViewById(R.id.button1);
到
button1 = (Button)findViewById(R.id.button1);
答案 1 :(得分：0)
尝试这样：
private static final int DATABASE_VERSION=1;
private SQLiteDatabase OurDb;
private final Context myContext;
private static String DB_PATH = "/data/data/com.example.classroom/databases/";
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
public DbHelper(Context context) {
super(context, DATABASE_NAME, null, 1);
this.myContext = context;
}
/**
* Creates a empty database on the system and rewrites it with your own database.
* */
/**
* Check if the database already exist to avoid re-copying the file each time you open the application.
* @return true if it exists, false if it doesn't
*/
private boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DATABASE_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
return checkDB != null ? true : false;
}
/**
* Copies your database from your local assets-folder to the just created empty database in the
* system folder, from where it can be accessed and handled.
* This is done by transfering bytestream.
* */
private void copyDataBase() throws IOException{
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DATABASE_NAME);
// Path to the just created empty db
String outFileName = DB_PATH + DATABASE_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);
}
Log.v("log", "copy finish");
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
@Override
public synchronized void close() {
if(OurDb != null)
OurDb.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public void openDataBase() throws SQLException{
// TODO Auto-generated method stub
//Open the database
String myPath = DB_PATH + DATABASE_NAME;
OurDb = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}
public void createDataBase() throws IOException{
// TODO Auto-generated method stub
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
Toast.makeText(myContext, "exist", Toast.LENGTH_SHORT).show();
}else{
//By calling this method and empty database will be created into the default system path
//of your application so we are gonna be able to overwrite that database with our database.
Toast.makeText(myContext, "creating", Toast.LENGTH_SHORT).show();
this.getReadableDatabase();
try {
copyDataBase();
} catch (IOException e) {
Log.v("log",e.toString());
throw new Error("Error copying database");
}
}
}