你好，慢慢地爬进这个Android世界并遵循android bootcamp练习2012版。我只能使用一个布局和一个类在AVm上测试我的应用程序但是在引入第二个类，第二个布局和编码按钮时，应用程序无法打开并且日志cat错误看起来像这样
06-18 17:22:29.461: D/AndroidRuntime(542): Shutting down VM
06-18 17:22:29.461: W/dalvikvm(542): threadid=1: thread exiting with uncaught exception (group=0x409961f8)
06-18 17:22:29.471: E/AndroidRuntime(542): FATAL EXCEPTION: main
06-18 17:22:29.471: E/AndroidRuntime(542): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.helloworld/com.example.helloworld.Main}: java.lang.NullPointerException
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.ActivityThread.access$600(ActivityThread.java:122)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.os.Handler.dispatchMessage(Handler.java:99)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.os.Looper.loop(Looper.java:137)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.ActivityThread.main(ActivityThread.java:4340)
06-18 17:22:29.471: E/AndroidRuntime(542): at java.lang.reflect.Method.invokeNative(Native Method)
06-18 17:22:29.471: E/AndroidRuntime(542): at java.lang.reflect.Method.invoke(Method.java:511)
06-18 17:22:29.471: E/AndroidRuntime(542): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
06-18 17:22:29.471: E/AndroidRuntime(542): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
06-18 17:22:29.471: E/AndroidRuntime(542): at dalvik.system.NativeStart.main(Native Method)
06-18 17:22:29.471: E/AndroidRuntime(542): Caused by: java.lang.NullPointerException
06-18 17:22:29.471: E/AndroidRuntime(542): at com.example.helloworld.Main.onCreate(Main.java:24)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.Activity.performCreate(Activity.java:4465)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
06-18 17:22:29.471: E/AndroidRuntime(542): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
06-18 17:22:29.471: E/AndroidRuntime(542): ... 11 more
这是main.java
public class Main extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button b = (Button)findViewById(R.id.btnPlay);
b.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(Main.this, Recipe.class));
// TODO Auto-generated method stub
}
});
if (savedInstanceState == null) {
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
}
}
这是新课程
package com.example.helloworld;
import android.app.Activity;
import android.os.Bundle;
public class Recipe extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.recipe);
}
}
这是清单
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.helloworld"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="14"
android:targetSdkVersion="19" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.helloworld.Main"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name="Recipe"></activity>
</application>
</manifest>
activity_main
<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.example.helloworld.Main"
tools:ignore="MergeRootFrame" />
fragment_main
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
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.example.helloworld.Main$PlaceholderFragment" >
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="28dp"
android:text="@string/mduduzi_games"
android:textSize="30sp" />
<Button
android:id="@+id/btnPlay"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/imageView1"
android:layout_centerHorizontal="true"
android:layout_marginTop="28dp"
android:text="@string/play" />
<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/textView1"
android:layout_centerHorizontal="true"
android:layout_marginTop="27dp"
android:src="@drawable/anim4" />
</RelativeLayout>
答案 0 :(得分：1)
在堆栈跟踪中，Main.java的第24行有一个NullPointerException：
at com.example.helloworld.Main.onCreate(Main.java:24)
这是你的问题：
Button b = (Button)findViewById(R.id.btnPlay);
//b is null, so the next line throws a NullPointerException:
b.setOnClickListener(...
b为null，表示activity_main.xml
检查以确保在activity_main.xml：
<Button
android:layout_width="wrap_content"
android:text="Some text"
android:id="@+id/btnPlay"
android:layout_height="wrap_content" />
您正在findViewById致电Activity，Views正在Activity activity_main.xml定义Fragment。由于您已在Fragment布局中定义了按钮，因此您必须在Button代码中获得对按钮的引用。从Activity课程中删除Fragment代码，并将其粘贴到public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
//find your Button view in rootView
Button b = (Button)rootView.findViewById(R.id.btnPlay);
b.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(Main.this, Recipe.class));
// TODO Auto-generated method stub
}
});
return rootView;
}
}
：
{{1}}
答案 1 :(得分：0)
当您走在正确的道路上时，您需要在清单中略微更改一条线。尝试：
<activity android:name="com.example.helloworld.Recipe"></activity>
而不是
<activity android:name="Recipe"></activity>
编辑：实际上，这似乎不是您的当前问题的根源，但是一旦您当前{{1}将成为未来问题的根源是固定的。如其他答案中所述，它会处理您设置为导航到新活动的按钮。
答案 2 :(得分：0)
问题是你的片段布局中定义了btnPlay，但是在片段添加到活动之前，你试图在你的Activity中找到它。
当您致电findViewById(R.id.btnPlay);时，您的活动布局中唯一添加的内容是activity_main，其中只包含一个FrameLayout。在您调用
btnPlay的片段不存在
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
一种解决方案是将上述三行移至Button b = (Button)findViewById(R.id.btnPlay);之前。
但是，从活动中引用片段中定义的视图通常是一种不好的做法，因为它是一个非常差的关注点分离。您的Fragment应该处理用户与属于该Fragment的Views的交互，并且您的Activity应该只关注Activity的布局中定义的Views。