非常基本的代码。我在这里使用db1对象以便db1.rawQuery工作，但它似乎抛出了异常。我知道如果我不使用Sqlitedatabse对象，那么rawquery（）就不会起作用。任何人都可以建议我更好的编码想法吗？我只是Android的初学者。所以原谅我找不到看似显而易见的错误。我只是不想谷歌nullpointerexception，因为我想我明白它是什么。但我只是不明白它是如何发生的。
package com.example.cp.profiletoggler;
import android.app.Dialog;
import android.content.ContentValues;
import android.content.Context;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
SQLiteDatabase db1;
EditText passcode, lockcode, username, password,enterpassword;
Button setcode, signin,enter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
username=(EditText)findViewById(R.id.usernameid);
password=(EditText)findViewById(R.id.passwordid);
signin=(Button)findViewById(R.id.signinbutton);
passcode=(EditText)findViewById(R.id.editpasscode);
lockcode=(EditText)findViewById(R.id.editlockcode);
setcode=(Button)findViewById(R.id.btncode);
enterpassword=(EditText)findViewById(R.id.idpassword);
enter=(Button)findViewById(R.id.btnenter);
final DBAdapter db = new DBAdapter(getApplicationContext());
String query = "SELECT * FROM Profile";
Cursor cursor = db1.rawQuery(query, null);
if (cursor.getCount() == 0)
{
Dialog dialogbox=new Dialog(MainActivity.this);
dialogbox.setContentView(R.layout.profiledetails);
dialogbox.setCancelable(false);
signin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String name=username.getText().toString();
String password1=password.getText().toString();
if(name.trim().length()>0 || password1.trim().length()>0) {
db.addprofile(name, password1);
Toast.makeText(getApplicationContext(),"Added",Toast.LENGTH_LONG).show();
}
else {
Toast.makeText(getApplicationContext(),"please enter something",Toast.LENGTH_LONG).show();
}
}
});
}
if(cursor.getCount()!=0)
{
final Dialog dialogbox2=new Dialog(MainActivity.this);
dialogbox2.setContentView(R.layout.enterpassword);
dialogbox2.setCancelable(false);
enter.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String permpassword=enterpassword.getText().toString();
if(permpassword.trim().length()>0)
{
Cursor c=db1.rawQuery("SELECT * FROM Profile WHERE _PASSCODE='" + permpassword + "'",null);
if (c.moveToFirst()) {
Toast.makeText(MainActivity.this,"Login Success",Toast.LENGTH_LONG).show();
dialogbox2.dismiss();
} else {
Toast.makeText(MainActivity.this, "Invalid attempt",Toast.LENGTH_LONG).show();
finish();
}
}
}
});
}
setcode.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String passcode1=passcode.getText().toString();
String lockcode1=lockcode.getText().toString();
if(passcode1.trim().length()>0 || lockcode1.trim().length()>0) {
db.insert(passcode1, lockcode1);
Toast.makeText(getApplicationContext(),"Added",Toast.LENGTH_LONG).show();
}
else {
Toast.makeText(getApplicationContext(),"please enter something",Toast.LENGTH_LONG).show();
}
}
});
}
}
这是继承SQliteOpenhelper类的DBAdapter.java文件
package com.example.cp.profiletoggler;
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.SQLiteOpenHelper;
import android.widget.Toast;
public class DBAdapter extends SQLiteOpenHelper{
Context context1;
private static final int Database_Version=1;
private static final String Database_Name="MyDatabase";
private static final String Table_Passcode="Passcode";
public static final String COLUMN_Passcode = "_PASSCODE ";
private static final String Table_Lockcode="Lockcode";
public static final String COLUMN_Lockcode = "_LOCKCODE ";
private static final String Table_Profile="Profile";
public static final String COLUMN_Username = "_USERNAME ";
public static final String COLUMN_Password = "_PASSWORD ";
public DBAdapter(Context context) {
super(context, Database_Name,null,Database_Version);
this.context1=context;
}
@Override
public void onCreate(SQLiteDatabase db) {
db=getWritableDatabase();
String query="CREATE TABLE " + Table_Passcode + "(" + COLUMN_Passcode + " INTEGER NOT NULL )";
db.execSQL(query);
String query2="CREATE TABLE "+ Table_Lockcode + "(" + COLUMN_Lockcode + " INTEGER NOT NULL )";
db.execSQL(query2);
String query3="CREATE TABLE "+ Table_Profile+ "(" +COLUMN_Username + " VARCHAR NOT NULL, " + COLUMN_Password + " VARCHAR NOT NULL)" ;
db.execSQL(query3);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+Table_Passcode);
db.execSQL("DROP TABLE IF EXISTS "+Table_Lockcode);
db.execSQL("DROP TABLE IF EXISTS "+Table_Profile);
onCreate(db);
}
public void insert(String pass,String lock) {
SQLiteDatabase db=getWritableDatabase();
db.execSQL("INSERT INTO " + Table_Passcode + "VALUES" + "(" + pass + ");");
db.execSQL("INSERT INTO " + Table_Lockcode + "VALUES" + "(" + lock + ");");
}
public void addprofile(String name, String pass2) {
// ContentValues values=new ContentValues();
SQLiteDatabase db=getWritableDatabase();
db.execSQL("INSERT INTO Profile(_USERNAME,_PASSWORD) VALUES('"+ name + "' ,'"+ pass2+ "');");
}
}
这是profiledetails.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:background="@color/colorRed" android:layout_gravity="center"
android:orientation="vertical" android:layout_width="wrap_content"
android:layout_height="wrap_content"
>
<LinearLayout
android:layout_width="250dp"
android:layout_height="match_parent">
<EditText
android:layout_marginTop="50dp"
android:layout_width="500dp"
android:layout_height="wrap_content"
android:hint="Username"
android:background="@drawable/rounded_edittext"
android:layout_weight="1"
android:id="@+id/usernameid"/>
<EditText
android:layout_marginLeft="-240dp"
android:layout_marginTop="100dp"
android:layout_width="500dp"
android:layout_height="wrap_content"
android:hint="Password"
android:layout_weight="1"
android:background="@drawable/rounded_edittext"
android:id="@+id/passwordid"/>
</LinearLayout>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/signinbutton"
android:text="Sign In"
android:textColor="@color/colorWhite"
android:background="@color/colorBlack"/>
</LinearLayout>
activity_main.xml中
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:background="@color/colorRed" android:layout_width="match_parent" android:layout_height="match_parent" >
<TextView
android:layout_width="150dp"
android:layout_height="wrap_content"
android:id="@+id/textpasscode"
android:text="SET YOUR PASSCODE"/>
<EditText
android:layout_width="300dp"
android:layout_height="wrap_content"
android:layout_toRightOf="@id/textpasscode"
android:id="@+id/editpasscode"
android:background="@drawable/rounded_edittext"/>
<TextView
android:layout_marginTop="30dp"
android:layout_below="@id/textpasscode"
android:layout_width="150dp"
android:layout_height="wrap_content"
android:id="@+id/textlockcode"
android:text="SET YOUR LOCKCODE"/>
<EditText android:layout_marginTop="20dp"
android:layout_below="@id/editpasscode"
android:layout_width="300dp"
android:layout_height="wrap_content"
android:layout_toRightOf="@id/textlockcode"
android:id="@+id/editlockcode"
android:background="@drawable/rounded_edittext"/>
<Button android:background="@color/colorBlack"
android:textColor="@color/colorWhite"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Set Code"
android:layout_marginTop="100dp"
android:id="@+id/btncode"/>
<Button android:background="@color/colorBlack" android:layout_marginLeft="20dp"
android:layout_toRightOf="@id/btncode"
android:textColor="@color/colorWhite"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Current Mode"
android:layout_marginTop="100dp"
android:id="@+id/btnmode"/>
<Button
android:layout_width="50dp"
android:layout_height="20dp"
android:background="@drawable/keyimage"
android:layout_below="@+id/btnmode"
android:layout_alignParentStart="true"
android:layout_marginStart="37dp"
android:layout_marginTop="27dp"
android:id="@+id/keybutton" />
</RelativeLayout>
日志
02-25 00:24:48.927 12403-12403/com.example.cp.profiletoggler
E/MultiWindowProxy: getServiceInstance failed!
02-25 00:24:49.224 12403-12553/com.example.cp.profiletoggler E/GED:
Failed to get GED Log Buf, err(0)
02-25 00:24:54.208 12403-12403/com.example.cp.profiletoggler
E/MultiWindowProxy: getServiceInstance failed!
02-25 00:24:54.260 12403-12403/com.example.cp.profiletoggler E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.cp.profiletoggler, PID: 12403
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.cp.profiletoggler/com.example.cp.profiletoggler.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2584)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2666)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1493)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:207)
at android.app.ActivityThread.main(ActivityThread.java:5769)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.cp.profiletoggler.MainActivity.onCreate(MainActivity.java:71)
at android.app.Activity.performCreate(Activity.java:6583)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1114)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2531)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2666)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1493)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:207)
at android.app.ActivityThread.main(ActivityThread.java:5769)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)
答案 0 :(得分：1)
试试这个：
在DBAdapter中添加以下方法。
public Cursor getCursorForQuery(String query) {
SQLiteDatabase db = this.getWritableDatabase();
return db.rawQuery(query, null);
}
从您的活动中这样打电话。
final DBAdapter db = new DBAdapter(getApplicationContext());
String query = "SELECT * FROM Profile";
Cursor cursor = db.getCursorForQuery(query);
答案 1 :(得分：-1)
在MainActivity上试试这个
SQLiteDatabase db1 = this.getWritableDatabase();