java - 运行 Android 应用程序时出错

Question

我制作了一个简单的 android 应用程序，用户可以通过单击按钮在静音模式和静音模式之前的状态之间切换，但是当在模拟器或 android 设备上运行该应用程序时，消息“应用程序已意外停止”正在显示。我的代码有什么问题。

P.S.:我从 Android for Dummies 复制了代码，并做了一些小修改，例如更改照片的名称。

这是我的代码:包 com.example.silentmodetoggle;

import android.widget.Button;
import android.media.AudioManager;
import android.view.*;
import android.os.Bundle;
import android.app.Activity;
import android.widget.ImageView;
import android.graphics.drawable.Drawable;


public class MainActivity extends Activity{
    private AudioManager mAudioManager;
    private boolean mPhoneIsSilent;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);



        setButtonClickListener();
        checkIfPhoneIsSilent();

        mAudioManager=(AudioManager)getSystemService(AUDIO_SERVICE);

    }
    private void setButtonClickListener(){
        Button toggleButton=(Button)findViewById(R.id.toggleButton);
        toggleButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
            if(mPhoneIsSilent){
                //change back to normal mode
                mAudioManager.setRingerMode(AudioManager.RINGER_MODE_NORMAL);
                mPhoneIsSilent=false;
            }
            else
            {
                //change to silent mode
                mAudioManager.setRingerMode(AudioManager.RINGER_MODE_SILENT);
                mPhoneIsSilent=true;
            }

            //now toggle the UI again
            toggleUi();




            }
        });
    }

    private void checkIfPhoneIsSilent(){
        int ringerMode=mAudioManager.getRingerMode();
        if(ringerMode==AudioManager.RINGER_MODE_SILENT){
            mPhoneIsSilent=true;}
        else
            mPhoneIsSilent=false;

    }

// Toggles the UI images from silent to normal and vice versa.

private void toggleUi()
{
    ImageView imageView=(ImageView)findViewById(R.id.phone_icon);
    Drawable newPhoneImage;

    if(mPhoneIsSilent){
        newPhoneImage=getResources().getDrawable(R.drawable.silent);
    }
    else{
        newPhoneImage=getResources().getDrawable(R.drawable.ringer);
    }
    imageView.setImageDrawable(newPhoneImage);

}


@Override
protected void onResume() {
super.onResume();
checkIfPhoneIsSilent();
toggleUi();
}









}

这是 AndroidManifest.xml:

<?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:background="#ffffff"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    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=".MainActivity" >

    <ImageView
        android:id="@+id/phone_icon"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_gravity="center_horizontal"
        android:contentDescription="@+id/desc"
        android:src="@drawable/ringer" />

    <Button
        android:id="@+id/toggleButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/phone_icon"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="40dp"
        android:text="Toggle_Silent_Mode" />

</RelativeLayout>

LogCat 中显示以下内容:

12-29 21:21:31.475: E/AndroidRuntime(821): FATAL EXCEPTION: main
    12-29 21:21:31.475: E/AndroidRuntime(821): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.silentmodetoggle/com.example.silentmodetoggle.MainActivity}: java.lang.NullPointerException
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.os.Handler.dispatchMessage(Handler.java:99)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.os.Looper.loop(Looper.java:123)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.ActivityThread.main(ActivityThread.java:4627)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at java.lang.reflect.Method.invokeNative(Native Method)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at java.lang.reflect.Method.invoke(Method.java:521)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at dalvik.system.NativeStart.main(Native Method)
    12-29 21:21:31.475: E/AndroidRuntime(821): Caused by: java.lang.NullPointerException
    12-29 21:21:31.475: E/AndroidRuntime(821):  at com.example.silentmodetoggle.MainActivity.checkIfPhoneIsSilent(MainActivity.java:58)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at com.example.silentmodetoggle.MainActivity.onCreate(MainActivity.java:23)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    12-29 21:21:31.475: E/AndroidRuntime(821):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)

Answer 1

在实例化 AudioManager 之前调用 checkIfPhoneIsSilent。这会导致 NullPointerException。更改您的 onCreate 方法以解决此问题:

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

    mAudioManager=(AudioManager)getSystemService(AUDIO_SERVICE);

    setButtonClickListener();
    checkIfPhoneIsSilent();
}