Android Studio 应用开发

Question

我正在 Android 上开发一个应用程序，我试图通过模拟器运行该应用程序，但每次它告诉我它已停止工作，我得到这些行：

02-22 23:22:21.977    2170-2170/com.example.sarvagyagupta1.myapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.sarvagyagupta1.myapp, PID: 2170
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.sarvagyagupta1.myapp/com.example.sarvagyagupta1.myapp.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:2209)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     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:105)
            at com.example.sarvagyagupta1.myapp.MainActivity.<init>(MainActivity.java:61)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1572)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
device not found

请有人帮我解决这个问题。

Answer 1

在调用 onCreate 之前，您不能调用 getApplicationContext()。直到那时 Activity 才完全初始化。