java - Android 启动新 Activity 时出错

Question

我是 Android 开发的新手，在创建新的 Activity 时遇到了一些问题。我想在单击 Button 但应用程序崩溃时启动一个新的 Activity。以下是控制台和 logcat 中的内容:

LogCat

03-11 00:21:08.639: D/AndroidRuntime(336): Shutting down VM03-11 00:21:08.649: W/dalvikvm(336): threadid=1: thread exiting with uncaught exception (group=0x40015560)03-11 00:21:08.659: E/AndroidRuntime(336): FATAL EXCEPTION: main03-11 00:21:08.659: E/AndroidRuntime(336): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.src.vicnote/com.src.vicnote.MainActivity}: java.lang.NullPointerException03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.os.Handler.dispatchMessage(Handler.java:99)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.os.Looper.loop(Looper.java:123)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.ActivityThread.main(ActivityThread.java:3683)03-11 00:21:08.659: E/AndroidRuntime(336):  at java.lang.reflect.Method.invokeNative(Native Method)03-11 00:21:08.659: E/AndroidRuntime(336):  at java.lang.reflect.Method.invoke(Method.java:507)03-11 00:21:08.659: E/AndroidRuntime(336):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)03-11 00:21:08.659: E/AndroidRuntime(336):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)03-11 00:21:08.659: E/AndroidRuntime(336):  at dalvik.system.NativeStart.main(Native Method)03-11 00:21:08.659: E/AndroidRuntime(336): Caused by: java.lang.NullPointerException03-11 00:21:08.659: E/AndroidRuntime(336):  at com.src.vicnote.MainActivity.onCreate(MainActivity.java:26)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)03-11 00:21:08.659: E/AndroidRuntime(336):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)03-11 00:21:08.659: E/AndroidRuntime(336):  ... 11 more03-11 00:21:38.579: D/AndroidRuntime(365): Shutting down VM03-11 00:21:38.579: W/dalvikvm(365): threadid=1: thread exiting with uncaught exception (group=0x40015560)03-11 00:21:38.599: E/AndroidRuntime(365): FATAL EXCEPTION: main03-11 00:21:38.599: E/AndroidRuntime(365): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.src.vicnote/com.src.vicnote.MainActivity}: java.lang.NullPointerException03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.os.Handler.dispatchMessage(Handler.java:99)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.os.Looper.loop(Looper.java:123)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.ActivityThread.main(ActivityThread.java:3683)03-11 00:21:38.599: E/AndroidRuntime(365):  at java.lang.reflect.Method.invokeNative(Native Method)03-11 00:21:38.599: E/AndroidRuntime(365):  at java.lang.reflect.Method.invoke(Method.java:507)03-11 00:21:38.599: E/AndroidRuntime(365):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)03-11 00:21:38.599: E/AndroidRuntime(365):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)03-11 00:21:38.599: E/AndroidRuntime(365):  at dalvik.system.NativeStart.main(Native Method)03-11 00:21:38.599: E/AndroidRuntime(365): Caused by: java.lang.NullPointerException03-11 00:21:38.599: E/AndroidRuntime(365):  at com.src.vicnote.MainActivity.onCreate(MainActivity.java:26)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)03-11 00:21:38.599: E/AndroidRuntime(365):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)03-11 00:21:38.599: E/AndroidRuntime(365):  ... 11 more<p></p>

<p><strong>activity_main</strong></p>

<pre><code><pre><FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.src.vicnote.MainActivity"
tools:ignore="MergeRootFrame" /><code>
</code></pre>

<p><strong>fragment_main</strong></p>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/MainScreen"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.src.vicnote.MainActivity$PlaceholderFragment" >

<Button
android:id="@+id/buttonNew"
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentRight="true"
android:layout_alignParentTop="true"
android:text="New" />

</RelativeLayout>

Java代码

http://pastebin.com/K8eAWD2L

Answer 1

从它看起来像你设置的内容 View

setContentView(R.layout.activity_main);

但是根据您提供的内容，activity_main.xml 中没有按钮，它显示空指针异常，因为

newButton = (Button) findViewById(R.id.buttonNew);

为空，因为您提供的信息中的实际按钮位于

fragment _main.xml

所以你可以试试

setContentView(R.layout.fragment_main);