java - Android 在 ListView 中查看 Firebase 数据错误

Question

我遇到了一个新问题，我需要使用 ArrayAdapter 从 firebase 数据库检索数据。我的代码中不断出现错误，这些错误显示在最后。这里真正的问题是如何让它填充我的 ListView 而不会出现错误？

这是我的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>

这是我的 Gradle 文件:

// 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'

以下是我当前收到的错误:

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) 

这是我的数据库结构:

Answer 1

这是因为您甚至在创建布局之前就初始化了列表。添加这一行

listView = (ListView)findViewById(R.id.my_list_view);

之后

setContentView(R.layout.activity_main);

更新对于获取问题，您以错误的方式获取数据。应该是这样的:

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);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });