Android 应用程序在模拟器中崩溃

Question

我的 Android 应用程序在模拟器中崩溃，并在日志中输出以下内容：

09-26 14:04:02.123: D/dalvikvm(716): GC_FOR_ALLOC freed 66K, 6% free 2675K/2844K, paused 29ms, total 30ms
09-26 14:04:02.123: I/dalvikvm-heap(716): Grow heap (frag case) to 3.455MB for 782224-byte allocation
09-26 14:04:02.173: D/dalvikvm(716): GC_FOR_ALLOC freed 2K, 5% free 3437K/3608K, paused 42ms, total 42ms
09-26 14:04:02.303: D/dalvikvm(716): GC_FOR_ALLOC freed <1K, 5% free 3436K/3608K, paused 38ms, total 39ms
09-26 14:04:02.314: I/dalvikvm-heap(716): Grow heap (frag case) to 4.778MB for 1389712-byte allocation
09-26 14:04:02.363: D/dalvikvm(716): GC_FOR_ALLOC freed <1K, 4% free 4793K/4968K, paused 52ms, total 52ms
09-26 14:04:02.423: D/AndroidRuntime(716): Shutting down VM
09-26 14:04:02.423: W/dalvikvm(716): threadid=1: thread exiting with uncaught exception (group=0x414c4700)
09-26 14:04:02.433: E/AndroidRuntime(716): FATAL EXCEPTION: main
09-26 14:04:02.433: E/AndroidRuntime(716): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sample_proj/com.example.sample_proj.MainActivity}: java.lang.NullPointerException
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.os.Handler.dispatchMessage(Handler.java:99)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.os.Looper.loop(Looper.java:137)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.ActivityThread.main(ActivityThread.java:5103)
09-26 14:04:02.433: E/AndroidRuntime(716):  at java.lang.reflect.Method.invokeNative(Native Method)
09-26 14:04:02.433: E/AndroidRuntime(716):  at java.lang.reflect.Method.invoke(Method.java:525)
09-26 14:04:02.433: E/AndroidRuntime(716):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
09-26 14:04:02.433: E/AndroidRuntime(716):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-26 14:04:02.433: E/AndroidRuntime(716):  at dalvik.system.NativeStart.main(Native Method)
09-26 14:04:02.433: E/AndroidRuntime(716): Caused by: java.lang.NullPointerException
09-26 14:04:02.433: E/AndroidRuntime(716):  at com.example.sample_proj.MainActivity.onCreate(MainActivity.java:21)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.Activity.performCreate(Activity.java:5133)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
09-26 14:04:02.433: E/AndroidRuntime(716):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
09-26 14:04:02.433: E/AndroidRuntime(716):  ... 11 more

有人可以帮我理解为什么会抛出NullPointerException 以及如何解决它吗？

Answer 1

在你的 MainActivity 中检查这一行

MainActivity.java:21

Answer 2

您可能使用过 findviewbyid 并且发生以下情况

1. 你忘了把“setContentView”行或者放在findviewbyid之后
2. 或者你把 contentView 放到了错误的布局中
3. 1 和 2 不是，而且您确实 findviewbyid 到了给定布局中不存在的 id - 并试图向后使用该对象，因为 id 不正确，对象返回 null，您正在尝试使用它