我是 Android 开发和 Java 脚本新手。我正在尝试创建一个应用程序,允许我通过开关打开和关闭内部麦克风。我发现了两个脚本(一个开关脚本和一个麦克风控制脚本),并且我已将它们放在一起,纠正任何调试问题,目前脚本显示干净。但是,当它在手机或模拟器上运行时,它会立即崩溃并显示以下错误,
android.content.Context android.content.Context.getApplicationContext()' on a null object reference"focused around my use of import android.content.Context;.
该错误似乎是由于 Context.getApplicationContext 请求未能找到结果并因此发布 NULL。
我查看了一长串遭受类似领域困扰的其他人,但所有解决方案似乎都集中于对其代码的不同领域的更改,而我无法将这些更改与我自己的脚本联系起来。
package com.example.myfirstapp;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Context;
import android.media.AudioManager;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Switch;
public class MainActivity extends AppCompatActivity {
private Switch sw1;
Button btnGet;
Context context = getApplicationContext();
AudioManager audioManager = ((AudioManager)context.getSystemService(Context.AUDIO_SERVICE));
@Override
protected void onCreate(Bundle savedInstanceState) {
audioManager.setStreamMute(AudioManager.STREAM_SYSTEM, true);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
sw1 = findViewById(R.id.switch1);
btnGet = findViewById(R.id.getBtn);
btnGet.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (sw1.isChecked()) {
audioManager.setMicrophoneMute(false);
}
else {
audioManager.setMicrophoneMute(true);
}
}
});
}
}
即使脚本不起作用,预期的结果是我能够运行该应用程序,目前该应用程序在启动时崩溃并发布以下错误消息...
[Logcat]
2019-08-20 13:23:01.710 5559-5559/com.example.myfirstapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myfirstapp, PID: 5559
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' 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:866)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
at com.example.myfirstapp.MainActivity.<init>(MainActivity.java:16)
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:866)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
最佳答案
您的问题是您正在尝试在初始化时检索上下文,即。
Context context = getApplicationContext();
在创建 Activity 之前,不保证 getApplicationContext();
将返回有效值。在这种情况下,它是空的,当您尝试在下一行访问它时,您会收到空指针异常。您需要在 onCreate()
中与音频管理器一起分配变量 context
。
例如,像这样:
Context context;
AudioManager audioManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
context = getApplicationContext();
audioManager = ((AudioManager)context.getSystemService(Context.AUDIO_SERVICE));
...
}
另一件事需要注意的是,Activity
类是 Context
的子类,因此您不需要检索您可以编写的应用程序上下文
AudioManager audioManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
audioManager = ((AudioManager)getSystemService(Context.AUDIO_SERVICE));
...
}
关于java - 在空对象引用上接收 'android.content.Context android.content.Context.getApplicationContext()',我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/57579209/