我目前正在使用Eclipse来创建一个简单的界面，允许用户插入他们的名字，然后转向其他页面。
然而，我现在因为一些奇怪的问题而陷入困境。虽然它显示“关闭虚拟机”，但我怀疑这是主要原因。这是我的代码
package com.ytl.test;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.TextView;
import com.ytl.test.MainActivity;
public class MainActivity extends Activity {
private TextView tvName;
private EditText Name;
private Button btnConfirm = new Button(this) ;
private String stringName = new String();
AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LinearLayout layout = new LinearLayout(this);
layout.setOrientation(LinearLayout.VERTICAL);
tvName = new TextView(this);
tvName.setText("Please enter your name: ");
Name = new EditText(this);
Name.setHint("Your Name");
btnConfirm = new Button(this);
btnConfirm.setText("Confirm");
btnConfirm.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
stringName = Name.getText().toString();
if(stringName.isEmpty()){
alertDialog.setTitle("Error");
alertDialog.setMessage("Name cannot be left empty. Please insert your name");
alertDialog.setCancelable(false);
alertDialog.setPositiveButton("Ok",
new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog,
int which) {
Name.setFocusable(true);
Name.setFocusableInTouchMode(true);
Name.requestFocus();
}
});
alertDialog.show();
Name.requestFocus();
}
else{
StringBuffer sbName = new StringBuffer();
sbName.append(stringName);
Intent intNext = new Intent(MainActivity.this,MainPage.class);
intNext.putExtra("Name",stringName.toString()); // i.putExtra only allow for String
startActivity(intNext);
}
}
});
layout.addView(tvName);
layout.addView(Name);
layout.addView(btnConfirm);
setContentView(layout);
}
@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;
}
}
这是我的logcat
03-15 14:37:38.143: D/AndroidRuntime(7217): Shutting down VM
03-15 14:37:38.143: W/dalvikvm(7217): threadid=1: thread exiting with uncaught exception (group=0x55e04b20)
03-15 14:37:38.143: D/AndroidRuntime(7217): procName from cmdline: com.ytl.test
03-15 14:37:38.143: E/AndroidRuntime(7217): in writeCrashedAppName, pkgName :com.ytl.test
03-15 14:37:38.143: D/AndroidRuntime(7217): file written successfully with content: com.ytl.test StringBuffer : ;com.ytl.test
03-15 14:37:38.153: E/AndroidRuntime(7217): FATAL EXCEPTION: main
03-15 14:37:38.153: E/AndroidRuntime(7217): Process: com.ytl.test, PID: 7217
03-15 14:37:38.153: E/AndroidRuntime(7217): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.ytl.test/com.ytl.test.MainActivity}: java.lang.NullPointerException
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.ActivityThread.access$800(ActivityThread.java:135)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.os.Handler.dispatchMessage(Handler.java:102)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.os.Looper.loop(Looper.java:136)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.ActivityThread.main(ActivityThread.java:5021)
03-15 14:37:38.153: E/AndroidRuntime(7217): at java.lang.reflect.Method.invokeNative(Native Method)
03-15 14:37:38.153: E/AndroidRuntime(7217): at java.lang.reflect.Method.invoke(Method.java:515)
03-15 14:37:38.153: E/AndroidRuntime(7217): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:827)
03-15 14:37:38.153: E/AndroidRuntime(7217): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
03-15 14:37:38.153: E/AndroidRuntime(7217): at dalvik.system.NativeStart.main(Native Method)
03-15 14:37:38.153: E/AndroidRuntime(7217): Caused by: java.lang.NullPointerException
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.content.ContextWrapper.getResources(ContextWrapper.java:89)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:78)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.view.View.<init>(View.java:3443)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.view.View.<init>(View.java:3510)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.widget.TextView.<init>(TextView.java:623)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.widget.Button.<init>(Button.java:107)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.widget.Button.<init>(Button.java:103)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.widget.Button.<init>(Button.java:99)
03-15 14:37:38.153: E/AndroidRuntime(7217): at com.ytl.test.MainActivity.<init>(MainActivity.java:22)
03-15 14:37:38.153: E/AndroidRuntime(7217): at java.lang.Class.newInstanceImpl(Native Method)
03-15 14:37:38.153: E/AndroidRuntime(7217): at java.lang.Class.newInstance(Class.java:1208)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.Instrumentation.newActivity(Instrumentation.java:1064)
03-15 14:37:38.153: E/AndroidRuntime(7217): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)
03-15 14:37:38.153: E/AndroidRuntime(7217): ... 11 more
03-15 14:37:38.213: D/AndroidRuntime(7230): Shutting down VM
03-15 14:37:38.213: W/dalvikvm(7230): threadid=1: thread exiting with uncaught exception (group=0x55e04b20)
提前感谢您的帮助
更新我的XML文件
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.ytl.test"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="19"
android:targetSdkVersion="21" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<service android:name=".BackgroundService" >
</service>
<activity
android:name=".MainPage"
android:label="@string/title_activity_main_page" >
</activity>
</application>
</manifest>
答案 0 :(得分：1)
据我所知，问题在于这两行中的第一行（尽管如果允许执行，第二行也会导致相同的异常）......
private Button btnConfirm = new Button(this) ;
...
AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);
...您无法在this的主体中使用Activity，因为Activity尚未在该阶段完全实例化this private Button btnConfirm;
...
AlertDialog.Builder alertDialog;
1}}将为空。
将它们更改为......
btnConfirm您正在使用onCreate(...)方法实例化alertDialog = new AlertDialog.Builder(this);
，所以您需要做的就是添加一行，例如......
onCreate(...)...也在{{1}}方法的某个地方。
答案 1 :(得分：0)
这是因为您在Button等于某事之前使用this - 关键字创建了新的this。
您不能使用this之外的onCreate() - 关键字或属于该类成员的方法（非静态）。
解决方案只是初始化Button和AlertDialog.Builder方法中的onCreate()。