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我有一个新问题，需要使用ArrayAdapter从Firebase数据库检索数据。我一直在我的代码中看到最后显示的错误。这里真正的问题是我怎样才能让它填充我的listview没有错误？Android在列表视图中查看Firebase数据错误
这里是我的activity_main.java：
package com.example.sean.firebasemessengerapp;
import android.provider.Settings;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import java.util.ArrayList;
import android.support.v7.widget.RecyclerView;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ValueEventListener;
import java.util.HashMap;
import java.util.Map;
public class MainActivity extends AppCompatActivity {
// [START declare_database_ref]
private DatabaseReference mDatabase;
//ADD ARRAY FOR MESSAGES TO CAPTURE
protected ArrayList<String> MessagesToPost = new ArrayList<>();
//final ListView listView = (ListView)findViewById(R.id.my_list_view);
@Override
protected void onCreate(Bundle savedInstanceState) {
mDatabase = FirebaseDatabase.getInstance().getReference();
mDatabase = mDatabase.child("Messages").child("Message");
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final ListView listView = (ListView)findViewById(R.id.my_list_view);
final Query myTopMessagesPostQuery = mDatabase.child("Messages").child("Message");
myTopMessagesPostQuery.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
String MessagesPost = (String) postSnapshot.getValue(); //Updated line
MessagesToPost.add(MessagesPost);
}
ArrayAdapter<String> adapter = new ArrayAdapter<>(
MainActivity.this,
android.R.layout.simple_list_item_multiple_choice,
MessagesToPost);
listView.setAdapter(adapter);
System.out.println("Array size is: " + MessagesToPost);
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
/*final Query myTopMessagesPostQuery = mDatabase.child("Messages").child("Message");
myTopMessagesPostQuery.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
String MessagesPost = (String) postSnapshot.child("Messages").child("Message").getValue();
MessagesToPost.add(MessagesPost);
}
ArrayAdapter<String> adapter = new ArrayAdapter<>(
MainActivity.this,
android.R.layout.simple_list_item_multiple_choice,
MessagesToPost);
listView.setAdapter(adapter);
System.out.println("Array size is: " + MessagesToPost.size());
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});*/
}
}
这里是我的XML文件：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.sean.firebasemessengerapp.MainActivity">
<ListView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/my_list_view"></ListView>
</RelativeLayout>这里是我的摇篮网络莱：
// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
repositories {
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle:2.2.3'
// NOTE: Do not place your application dependencies here; they belong
// in the individual module build.gradle files
classpath 'com.google.gms:google-services:3.0.0'
}
}
allprojects {
repositories {
jcenter()
}
}
task clean(type: Delete) {
delete rootProject.buildDir
}apply plugin: 'com.android.application'
android {
compileSdkVersion 24
buildToolsVersion "25.0.0"
defaultConfig {
applicationId "com.example.sean.firebasemessengerapp"
minSdkVersion 15
targetSdkVersion 24
versionCode 1
versionName "1.0"
testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
}
}
}
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
exclude group: 'com.android.support', module: 'support-annotations'
})
compile 'com.android.support:appcompat-v7:24.2.1'
compile 'com.google.firebase:firebase-database:10.0.1'
testCompile 'junit:junit:4.12'
compile 'com.android.support:recyclerview-v7:24.2.1'
}
apply plugin: 'com.google.gms.google-services'这里是我目前收到的错误：
12-27 16:41:39.542 18031-18031/? I/art: Not late-enabling -Xcheck:jni (already on)
12-27 16:41:39.542 18031-18031/? W/art: Unexpected CPU variant for X86 using defaults: x86
12-27 16:41:39.580 18031-18031/com.example.sean.firebasemessengerapp W/System: ClassLoader referenced unknown path: /data/app/com.example.sean.firebasemessengerapp-2/lib/x86
[ 12-27 16:41:39.610 1512: 1535 D/ ]
HostConnection::get() New Host Connection established 0x93c2ed80, tid 1535
12-27 16:41:39.620 18031-18031/com.example.sean.firebasemessengerapp I/InstantRun: Instant Run Runtime started. Android package is com.example.sean.firebasemessengerapp, real application class is null.
12-27 16:41:39.973 18031-18031/com.example.sean.firebasemessengerapp W/System: ClassLoader referenced unknown path: /data/app/com.example.sean.firebasemessengerapp-2/lib/x86
12-27 16:41:40.017 18031-18031/com.example.sean.firebasemessengerapp D/FirebaseApp: com.google.firebase.auth.FirebaseAuth is not linked. Skipping initialization.
12-27 16:41:40.032 18031-18031/com.example.sean.firebasemessengerapp D/FirebaseApp: com.google.firebase.crash.FirebaseCrash is not linked. Skipping initialization.
12-27 16:41:40.052 18031-18031/com.example.sean.firebasemessengerapp I/FA: App measurement is starting up, version: 10084
12-27 16:41:40.052 18031-18031/com.example.sean.firebasemessengerapp I/FA: To enable debug logging run: adb shell setprop log.tag.FA VERBOSE
12-27 16:41:40.052 18031-18031/com.example.sean.firebasemessengerapp D/FA: Debug-level message logging enabled
12-27 16:41:40.052 18031-18031/com.example.sean.firebasemessengerapp D/FA: AppMeasurement singleton hash: 23098228
12-27 16:41:40.064 18031-18031/com.example.sean.firebasemessengerapp V/FA: Collection enabled
12-27 16:41:40.064 18031-18031/com.example.sean.firebasemessengerapp V/FA: App package, google app id: com.example.sean.firebasemessengerapp, 1:79285920863:android:2946d5f3d63425bf
12-27 16:41:40.064 18031-18031/com.example.sean.firebasemessengerapp I/FA: To enable faster debug mode event logging run:
adb shell setprop debug.firebase.analytics.app com.example.sean.firebasemessengerapp
12-27 16:41:40.095 18031-18031/com.example.sean.firebasemessengerapp V/FA: Registered activity lifecycle callback
12-27 16:41:40.096 18031-18031/com.example.sean.firebasemessengerapp I/FirebaseInitProvider: FirebaseApp initialization successful
12-27 16:41:40.117 18031-18064/com.example.sean.firebasemessengerapp V/FA: Using measurement service
12-27 16:41:40.133 18031-18064/com.example.sean.firebasemessengerapp V/FA: Connecting to remote service
12-27 16:41:40.257 18031-18031/com.example.sean.firebasemessengerapp D/AndroidRuntime: Shutting down VM
--------- beginning of crash
12-27 16:41:40.257 18031-18031/com.example.sean.firebasemessengerapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.sean.firebasemessengerapp, PID: 18031
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.sean.firebasemessengerapp/com.example.sean.firebasemessengerapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:116)
at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:147)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:27)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:50)
at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:29)
at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:29)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:197)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:181)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:521)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
at com.example.sean.firebasemessengerapp.MainActivity.<init>(MainActivity.java:32)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 这里是我的数据库结构：
感谢您的帮助。明天我会试试这个代码。我觉得，因为我刚刚开始Android编程，我仍然犯了愚蠢的错误。 – seansanders
让我知道它是否有效 –
我做了适当的修改，但是现在没有数据正在显示。我不知道该怎么做，我哪里出错了？ – seansanders