android - 将微调器添加到应用程序，然后崩溃

Question

我在我的应用程序中添加了一个微调器，但应用程序崩溃了，无法构建。微调器显示存储中的 .mp3 文件名项目。有人能说出崩溃的原因吗？我的代码有什么问题？如果您需要更多代码，请告诉我。

这是MainActivity的一部分

import java.io.File;
import java.io.FilenameFilter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

//import android.view.Window;



   public class MainActivity extends AppCompatActivity implements 
 AdapterView.OnItemSelectedListener {

    private Integer images[]={R.drawable.pic1, R.drawable.pic21, 
R.drawable.pic3, R.drawable.pic2, R.drawable.pic4,
            R.drawable.pic5, R.drawable.pic6, R.drawable.pic30, 
R.drawable.pic31, R.drawable.pic26, R.drawable.pic7, R.drawable.pic25, 
R.drawable.pic8, R.drawable.pic9,
            R.drawable.pic10, R.drawable.pic27, R.drawable.pic28, 
R.drawable.pic29, R.drawable.pic11, R.drawable.pic22, R.drawable.pic23, 
R.drawable.pic13, R.drawable.pic14,
            R.drawable.pic15, R.drawable.pic16, R.drawable.pic24, 
R.drawable.pic17, R.drawable.pic18, R.drawable.pic19
            , R.drawable.pic20, R.drawable.pic12};
.........
.........
 // for spinner
class Mp3Filter implements FilenameFilter {
    public boolean accept (File dir, String name){
        return (name.endsWith(".mp3"));
    }
}
 // spinner
public final String SD_PATH =new String("/sdcard/music/");
public List<String> songs = new ArrayList<String>();
Spinner mySpinner = (Spinner)findViewById(R.id.spinner1);

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
.....
.....
  mp = MediaPlayer.create(this, R.raw.shlomoimagine);
    mp.start();
    // currImage = 0;
    setContentView(R.layout.activity_main);


    //initializing the loop at the beginning of the app run
    initializeImageSwitcher();
    setImageRotateListener();
   // handler.post(runnable);
    updatePlayList();

...
...
private void updatePlayList() {
    File home=new File(SD_PATH);

    File[] files = home.listFiles(new Mp3Filter());
    if(files.length>0){
        for(File file:files){
            songs.add(file.getName());
        }
        ArrayAdapter<String> songList = new ArrayAdapter<String>
(this,R.layout.song_item,songs);
        mySpinner.setAdapter(songList);
        mySpinner.setOnItemSelectedListener(this);

    }
}

这些是 list 中的部分

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.nati_1.ImageGallerySwitching_v8_2">

<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />


<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/Theme.AppCompat.NoActionBar">

歌曲项目.xml

<?xml version="1.0" encoding="utf-8"?>
<TextView android:id="@+id/text1" 
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:textSize="20dp"
android:textStyle="bold"
/>

来自 logcat

 12-20 00:22:48.014 12887-12887/com.nati_1.ImageGallerySwitching_v6 
E/AndroidRuntime: FATAL EXCEPTION: main

Process: com.nati_1.ImageGallerySwitching_v6, PID: 12887



  java.lang.RuntimeException: Unable to instantiate activity      
ComponentInfo{com.nati_1.ImageGallerySwitching_v6/com.nati_1.ImageGallerySwitching_v8_2.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:2614)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2787)

at android.app.ActivityThread.-wrap12(ActivityThread.java)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1504)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:154)

at android.app.ActivityThread.main(ActivityThread.java:6247)

at java.lang.reflect.Method.invoke(Native Method)

at 


 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)

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.nati_1.ImageGallerySwitching_v8_2.MainActivity.<init>(MainActivity.java:225)
                                                                                     at java.lang.Class.newInstance(Native Method)
                                                                                     at android.app.Instrumentation.newActivity(Instrumentation.java:1079)
                                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2604)
                                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2787) 
                                                                                     at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1504) 
                                                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                     at android.os.Looper.loop(Looper.java:154) 
                                                                                     at android.app.ActivityThread.main(ActivityThread.java:6247) 
                                                                                     at java.lang.reflect.Method.invoke(Native Method) 

Answer 1

问题定义为:

java.lang.NullPointerException: Attempt to invoke virtual method 
'android.view.Window$Callback android.view.Window.getCallback()' on a null  object reference

问题是您正在为 Spinner 创建一个变量并同时获取它们的引用:

Spinner mySpinner = (Spinner)findViewById(R.id.spinner1);

但这是不正确的，您必须在 onCreate() 方法中获取引用，有些像这样:

Spinner mySpinner;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);

    mySpinner = (Spinner)findViewById(R.id.spinner1);
    ...
    ...
}