我一整天都在努力解决这个问题，阅读许多帖子和教程等等，而且我已经接近解决这个问题了。我认为有经验的编码人员很有可能很容易破解它，所以发布它希望有人会纠正它，其他的新手会看到它。
我有一个简单的按钮 bagButton 按下时应该从 MainActivity 过渡到 BagActivity
在启动应用程序时，它会停止，我再也没有机会看到它运行，更不用说按下按钮了。
当我拿出这段代码时，app运行正常（但显然没有转换）：
Button b = (Button)findViewById(R.id.buttonBag);
b.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent i = new Intent(MainActivity.this, BagActivity.class);
startActivity(i);
}
});
更大的 MainActivity.java 代码是：
package nz.co.gamekeeper;
import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button b = (Button)findViewById(R.id.buttonBag);
b.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent i = new Intent(MainActivity.this, BagActivity.class);
startActivity(i);
}
});
if (savedInstanceState == null) {
getSupportFragmentManager().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;
}
}
}
logcat输出如下：
05-11 16:45:02.494: I/Process(1437): Sending signal. PID: 1437 SIG: 9
05-11 16:46:12.599: D/AndroidRuntime(2028): Shutting down VM
05-11 16:46:12.599: W/dalvikvm(2028): threadid=1: thread exiting with uncaught exception (group=0x4182b2a0)
05-11 16:46:12.599: E/AndroidRuntime(2028): FATAL EXCEPTION: main
05-11 16:46:12.599: E/AndroidRuntime(2028): java.lang.RuntimeException: Unable to start activity ComponentInfo{nz.co.gamekeeper/nz.co.gamekeeper.MainActivity}: java.lang.NullPointerException
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.ActivityThread.access$700(ActivityThread.java:140)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1237)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.os.Handler.dispatchMessage(Handler.java:99)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.os.Looper.loop(Looper.java:137)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.ActivityThread.main(ActivityThread.java:4921)
05-11 16:46:12.599: E/AndroidRuntime(2028): at java.lang.reflect.Method.invokeNative(Native Method)
05-11 16:46:12.599: E/AndroidRuntime(2028): at java.lang.reflect.Method.invoke(Method.java:511)
05-11 16:46:12.599: E/AndroidRuntime(2028): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
05-11 16:46:12.599: E/AndroidRuntime(2028): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)
05-11 16:46:12.599: E/AndroidRuntime(2028): at dalvik.system.NativeStart.main(Native Method)
05-11 16:46:12.599: E/AndroidRuntime(2028): Caused by: java.lang.NullPointerException
05-11 16:46:12.599: E/AndroidRuntime(2028): at nz.co.gamekeeper.MainActivity.onCreate(MainActivity.java:35)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.Activity.performCreate(Activity.java:5206)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)
05-11 16:46:12.599: E/AndroidRuntime(2028): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)
05-11 16:46:12.599: E/AndroidRuntime(2028): ... 11 more
XML有两部分 activity_main.xml ：
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="nz.co.gamekeeper.MainActivity"
tools:ignore="MergeRootFrame" />
及其 fragment_main.xml ：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:baselineAligned="false"
android:orientation="vertical"
tools:context="nz.co.gamekeeper.MainActivity$PlaceholderFragment" >
<TableRow
android:id="@+id/tableRow1"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="1" >
<Button
android:id="@+id/buttonBag"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_bag" />
<Button
android:id="@+id/buttonSeason"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_season" />
</TableRow>
<TableRow
android:id="@+id/tableRow2"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="1" >
<Button
android:id="@+id/buttonMates"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_mates" />
<Button
android:id="@+id/buttonBirds"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_birds" />
</TableRow>
<TableRow
android:id="@+id/tableRow3"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="1" >
<Button
android:id="@+id/buttonSetup"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_setup" />
<Button
android:id="@+id/buttonSync"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_sync" />
</TableRow>
<TableRow
android:id="@+id/tableRow4"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="1" >
<Button
android:id="@+id/buttonSpare1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_spare1" />
<Button
android:id="@+id/buttonSpare2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="@string/button_text_spare2" />
</TableRow>
</LinearLayout>
我已经尝试过清理它，重新启动Eclipse，通过添加日志，尝试不同的Intent args，确保清单正常，再加载更多内容来更好地了解错误......
有人能说清楚我会尝试什么吗？
答案 0 :(得分：0)
问题在于Button视图。您只有Button不在Fragment layout布局中的MainActivity视图，但您正尝试从MainActivity访问该按钮。
因此，您只能在Fragment。
将代码从MainActivity移至PlaceholderFragment，如下所示
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
// you can find the button here from the fragment layout
Button b = (Button) rootView .findViewById(R.id.buttonBag);
b.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent i = new Intent(getActivity(), BagActivity.class);
startActivity(i);
}
});
return rootView;
}
答案 1 :(得分：0)
activity_main.xml不包含ID为@id/buttonBag的视图，该视图位于fragment_main.xml。当您尝试在MainActivity中查找视图时，您会获得一个NPE，因为该视图在您刚刚充气的布局中不存在！您应该在OnClickListener的{{1}}中添加onCreateView(...)，如下所示：
PlaceholderFragment