java.lang.NullPointerException在emulator和手机中停止我的应用程序

我在网上搜索过，但找不到合适的答案。 该应用程序已安装在手机和emulator中，但无法打开。程序会立即停止并通知关闭它。但不使用Ringtone ring=RingtoneManager.getRingtonegetApplicationContext，RingtoneManager.getDefaultUriRingtoneManager.TYPE_Ringtone；和相关代码…应用程序运行完美。但是我不能用手机铃声。如果有任何解决方案或替代方案，请通知我

所以请帮忙。 我的代码如下

package com.example.analarm;

import androidx.appcompat.app.AppCompatActivity;
import android.media.Ringtone;
import android.media.RingtoneManager;
import android.os.Bundle;
import android.widget.TextClock;
import android.widget.TimePicker;
import java.util.Timer;
import java.util.TimerTask;


public class MainActivity extends AppCompatActivity {

    TimePicker alarmtime;
    TextClock time;

    Ringtone ring =RingtoneManager.getRingtone(getApplicationContext(),RingtoneManager.getDefaultUri(RingtoneManager.TYPE_RINGTONE));

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

        alarmtime=findViewById(R.id.timepicker);
        time=findViewById(R.id.textclock);
        Timer t= new Timer();
        t.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                if(time.getText().toString().equals(Alarmtime()))
                {
                    ring.play();
                }
                else
                {
                    ring.stop();
                }

            }
        },0,1000);
    }
    public String Alarmtime()
    {
        Integer alarmHours=alarmtime.getCurrentHour();
        Integer alarmMin=alarmtime.getCurrentMinute();
        String stringalarmmin;
        if(alarmMin<10)
        {
            stringalarmmin="0";
            stringalarmmin=stringalarmmin.concat(alarmMin.toString());
        }
        else{
            stringalarmmin=alarmMin.toString();
        }
        String stringalarmtime;
        if(alarmHours>12)
        {
            alarmHours= alarmHours-12;
            stringalarmtime=alarmHours.toString().concat(":").concat(stringalarmmin).concat(" PM");
        }
        else
        {
            stringalarmtime=alarmHours.toString().concat(":").concat(stringalarmmin).concat(" AM");
        }
        return stringalarmtime;
    }
}

我的错误日志在这里

2019-11-29 00:22:38.375 6352-6352/com.example.analarm E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.analarm, PID: 6352
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.analarm/com.example.analarm.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:2679)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     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:109)
        at com.example.analarm.MainActivity.<init>(MainActivity.java:17)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1174)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2669)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

---

应在调用onCreate方法后调用getApplicationContext

试一试

最有可能的是，ring实例为null，您尝试使用它

Ringtone ring;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ring =RingtoneManager.getRingtone(getApplicationContext(),RingtoneManager.getDefaultUri(RingtoneManager.TYPE_RINGTONE));
    alarmtime=findViewById(R.id.timepicker);
    time=findViewById(R.id.textclock);
    Timer t= new Timer();