Java com.example.appname.MainActivity.onCreate处的AndroidRuntime错误

我试图编写Java代码，创建一个按钮点击事件，用我的名字填充编辑文本框。不幸的是，我得到运行时错误。我检查了几乎所有与stackoverflow相关的主题和问题，但无法找出我的问题所在。 谢谢你的帮助

MainActivity.java

package com.example.question5;

import android.app.Activity;
import android.os.Bundle;
import android.view.*;
import android.widget.*;

public class MainActivity extends Activity {
    Button b;
    EditText et;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        et=(EditText)findViewById(R.id.editText1);
        b=(Button)findViewById(R.id.button1);
        b.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                et.setText("My Name");          
            }
        });     
    }
}

fragment_main.xml

<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.question5.MainActivity$PlaceholderFragment" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="178dp"
        android:text="Populate" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/button1"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="54dp"
        android:ems="10"
        android:inputType="textPersonName" >

        <requestFocus />
    </EditText>

</RelativeLayout>

---

对于MainActivity布局，请使用

fragment\u main.xml

而不是

activity\u main.xml

，因为按钮和编辑文本位于

fragment\u main

中，您试图通过id在MainActivity中访问：

setContentView(R.layout.fragment_main);

是真的吗：）：）我花了7-8个小时来查找错误，还没有检查xml文件名：）非常感谢。现在它正在工作。

03-21 23:38:26.227: W/dalvikvm(28343): threadid=1: thread exiting with uncaught exception (group=0x40bfd1f8)
03-21 23:38:26.227: E/AndroidRuntime(28343): FATAL EXCEPTION: main
03-21 23:38:26.227: E/AndroidRuntime(28343): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.question5/com.example.question5.MainActivity}: java.lang.NullPointerException
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1970)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1995)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.ActivityThread.access$600(ActivityThread.java:128)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1161)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.os.Looper.loop(Looper.java:137)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.ActivityThread.main(ActivityThread.java:4514)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at java.lang.reflect.Method.invokeNative(Native Method)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at java.lang.reflect.Method.invoke(Method.java:511)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at dalvik.system.NativeStart.main(Native Method)
03-21 23:38:26.227: E/AndroidRuntime(28343): Caused by: java.lang.NullPointerException
03-21 23:38:26.227: E/AndroidRuntime(28343):    at com.example.question5.MainActivity.onCreate(MainActivity.java:19)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.Activity.performCreate(Activity.java:4465)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1053)
03-21 23:38:26.227: E/AndroidRuntime(28343):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1934)
03-21 23:38:26.227: E/AndroidRuntime(28343):    ... 11 more

setContentView(R.layout.fragment_main);