你好我正在android studio中构建我的第一个应用程序，经过一天试图让我的应用程序正常工作后，我遇到了以下错误，因为我是android工作室和编程的新手，总的来说，我想要一些帮助诊断问题 提前感谢您提供的任何帮助
10-18 09:21:45.827 3677-3677/com.example.joelg.clapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.joelg.clapp, PID: 3677
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.joelg.clapp/com.example.joelg.clapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2718)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:118)
at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:152)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:29)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:53)
at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:31)
at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:31)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:200)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:184)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:518)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:189)
at com.example.joelg.clapp.MainActivity.<init>(MainActivity.java:54)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1173)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2708)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
这是我的主要活动类
package com.example.joelg.clapp;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import java.security.Timestamp;
import java.security.cert.CertPath;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity {
DBHandler dbHandler;
ArrayAdapter<String> mAdapter;
ListView lstJob;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
dbHandler = new DBHandler(this);
lstJob = (ListView)findViewById(R.id.lstJob);
LoadTaskList();
}
private void LoadTaskList() {
ArrayList<String> tasklist = dbHandler.getJobList();
if(mAdapter==null){
mAdapter = new ArrayAdapter<String>
(this,R.layout.row,R.id.Job_name,tasklist);
lstJob.setAdapter(mAdapter);
}else {
mAdapter.clear();
mAdapter.addAll(tasklist);
mAdapter.notifyDataSetChanged();
}
}
public void JobComplete(View view){
View parent = (View)view.getParent();
}
TextView taskTextView = (TextView)findViewById(R.id.Job_name);
}
DBHandler
package com.example.joelg.clapp;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import java.util.ArrayList;
import java.util.List;
/**
* Created by joelg on 6/10/2017.
*/
public class DBHandler extends SQLiteOpenHelper{
//database Version
private static final int DATABASE_VERSION = 1;
//database Name
private static final String DATABASE_NAME = "List";
//Contacts name table
private static final String DB_SHOPS="Shops";
//JobDB table columns names
private static final String DB_TABLE = "Job";
private static final String DB_COLUM ="JobNM";
private static final String DB_MOTEL_ADDR ="Motel_Address";
private static final String DB_PHOTO="Photo";
public DBHandler(Context context) {
super(context,DATABASE_NAME,null,DATABASE_VERSION);
}
// creating tables
@Override
public void onCreate(SQLiteDatabase db){
String query = String.format ("CREATE DB %s (ID INTEGER PRIMARY KEY
AUTOINCREMENT, %s TEXT NOT NULL);",DB_COLUM,DB_TABLE );
db.execSQL(query);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
// drop older table if existed
db.execSQL("DROP DB IF EXISTS" + DB_SHOPS);
// create tables again
onCreate(db);
}
// ADDING NEW Motel address
public void addclean (CleanList cleanList){
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(DB_TABLE, cleanList.getRoomNum()); // Room number
values.put(DB_COLUM, cleanList.getAddress()); // Motel address
//inserting row
db.insert(DB_SHOPS,null,values);
db.close(); //close db connection
}
public CleanList cleanList (int id) {
SQLiteDatabase db = this.getReadableDatabase();
Cursor cursor = db.query(DB_SHOPS, new String[]{DB_TABLE,
DB_COLUM,DB_MOTEL_ADDR,}, DB_TABLE + "=?"
, new String[]{String.valueOf(id)}, null, null, null, null);
if (cursor != null)
cursor.moveToFirst();
CleanList contact = new CleanList(Integer.parseInt(cursor.getString(0)),
cursor.getString(1), cursor.getString(2),cursor.getString(3));
//return contact
return contact;
}
// Getting All Job lists
public List<CleanList> getAllJobList() {
List<CleanList> CleanList = new ArrayList<CleanList>();
// Select All Query
String selectQuery = "SELECT * FROM " + DB_SHOPS;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
CleanList List = new CleanList();
List.setId(Integer.parseInt(cursor.getString(0)));
List.setRnum(cursor.getString(1));
List.setAddress(cursor.getString(2));
// Adding contact to list
CleanList.add(List);
} while (cursor.moveToNext());
}
// return count list
return CleanList;
}
// Updating a Job
public int updateJob(CleanList cleanList) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(DB_COLUM, cleanList.getRoomNum());
values.put(DB_TABLE, cleanList.getAddress());
// updating row
return db.update(DB_SHOPS, values, DB_TABLE + "= ?",
new String[]{String.valueOf(cleanList.getId())});
}
// Deleting a JOB
public void deleteJob (CleanList cleanList) {
SQLiteDatabase db = this.getWritableDatabase();
db.delete(DB_SHOPS, DB_TABLE + " = ?",new String[] {
String.valueOf(cleanList.getId()) });
}
// Getting shops Count
public ArrayList<String> getJobList() {
ArrayList<String> taskList = new ArrayList<>();
SQLiteDatabase db = this.getReadableDatabase();
Cursor cursor = db.query(DB_TABLE,new String[]
{DB_COLUM},null,null,null,null,null);
while(cursor.moveToNext()){
int index = cursor.getColumnIndex(DB_COLUM);
taskList.add(cursor.getString(index));
}
cursor.close();
db.close();
return getJobList();
}
// return count
}