android studio 用usb連接手機出現已停止

不好意思我是   android studio新手，想請問為甚麼我連線到手機他會一直出現已停止，我演就好幾天了還是看不太懂，下面這是我的locat，希望大家能幫幫我



   FATAL EXCEPTION: main
    Process: com.example.ble3, PID: 5617
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ble3/com.example.ble3.MainActivity}: java.lang.ClassCastException: com.example.ble3.MainActivity cannot be cast to tw.com.flag.api.OnFlagMsgListener
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6944)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
     Caused by: java.lang.ClassCastException: com.example.ble3.MainActivity cannot be cast to  tw.com.flag.api.OnFlagMsgListener
        at tw.com.flag.api.FlagBt.<init>(SourceFile:149)
        at com.example.ble3.MainActivity.onCreate(MainActivity.java:26)
        at android.app.Activity.performCreate(Activity.java:7183)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6944)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)




我判斷可能是因為這裡 at com.example.ble3.MainActivity.onCreate(MainActivity.java:26) 出了問題，不太清楚為什麼

這是我java那段的程式碼

25行     txv = findViewById(R.id.txv);
26行     bt = new FlagBt(this);
27行     }