我一直在阅读有关如何执行此操作的示例，尝试将此网站上的帖子和其他人的功能示例拼凑在一起。我确定我错过了一些小东西，但作为Java的新手，我可以使用一些帮助。
我只是想创建一个小例子，它源自在Eclipse中创建新项目时生成的自动化hello世界。我想要做的就是能够在子类中存储一些全局变量，然后在我的主活动中引用这些值。不幸的是，每当我尝试运行应用程序时，它都会崩溃“不幸的是，GeneralTest1已停止”，并且日志cat错误不是很有帮助。
快速概述：
这是我所拥有的一切;任何帮助将不胜感激！
MainActivity.java
package com.example.generaltest1;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.TextView;
public class MainActivity extends Activity {
GlobalVars myVars = ((GlobalVars)getApplicationContext());
TextView myText = (TextView) findViewById(R.id.myText);
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
myText.setText(myVars.getMyString());
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
}
GlobalVars.java
package com.example.generaltest1;
import android.app.Application;
public class GlobalVars extends Application {
String myString = "Some Text";
public String getMyString() {
return myString;
}
public String setMyString(String string) {
this.myString = string;
return myString;
}
}
activity_main.xml中
<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" >
<TextView
android:id="@+id/myText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:text="@string/hello_world"
tools:context=".MainActivity" />
</RelativeLayout>
的strings.xml
<resources>
<string name="app_name">GeneralTest1</string>
<string name="hello_world">Hello world!</string>
<string name="menu_settings">Settings</string>
<string name="title_activity_main">MainActivity</string>
</resources>
GeneralTest1 Manifest
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.generaltest1"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="15" />
<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" android:name="GlobalVars">
<activity
android:name=".MainActivity"
android:label="@string/title_activity_main" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
Log Cat
08-14 09:53:05.546: E/Trace(620): error opening trace file: No such file or directory (2)
08-14 09:53:05.656: D/AndroidRuntime(620): Shutting down VM
08-14 09:53:05.656: W/dalvikvm(620): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
08-14 09:53:05.676: E/AndroidRuntime(620): FATAL EXCEPTION: main
08-14 09:53:05.676: E/AndroidRuntime(620): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.generaltest1/com.example.generaltest1.MainActivity}: java.lang.NullPointerException
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.ActivityThread.access$600(ActivityThread.java:130)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.os.Handler.dispatchMessage(Handler.java:99)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.os.Looper.loop(Looper.java:137)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.ActivityThread.main(ActivityThread.java:4745)
08-14 09:53:05.676: E/AndroidRuntime(620): at java.lang.reflect.Method.invokeNative(Native Method)
08-14 09:53:05.676: E/AndroidRuntime(620): at java.lang.reflect.Method.invoke(Method.java:511)
08-14 09:53:05.676: E/AndroidRuntime(620): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-14 09:53:05.676: E/AndroidRuntime(620): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-14 09:53:05.676: E/AndroidRuntime(620): at dalvik.system.NativeStart.main(Native Method)
08-14 09:53:05.676: E/AndroidRuntime(620): Caused by: java.lang.NullPointerException
08-14 09:53:05.676: E/AndroidRuntime(620): at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101)
08-14 09:53:05.676: E/AndroidRuntime(620): at com.example.generaltest1.MainActivity.<init>(MainActivity.java:10)
08-14 09:53:05.676: E/AndroidRuntime(620): at java.lang.Class.newInstanceImpl(Native Method)
08-14 09:53:05.676: E/AndroidRuntime(620): at java.lang.Class.newInstance(Class.java:1319)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
08-14 09:53:05.676: E/AndroidRuntime(620): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
08-14 09:53:05.676: E/AndroidRuntime(620): ... 11 more
答案 0 :(得分：2)
您在类定义中调用getApplicationContext()，这在活动生命周期中过早，导致nullpointer异常。将作业移至onCreate()功能，它应按预期工作。
答案 1 :(得分：1)
请查看此文章有关活动及其生命周期： http://developer.android.com/reference/android/app/Activity.html#ActivityLifecycle
您的活动应如下所示：
public class MainActivity extends Activity {
GlobalVars myVars;
TextView myText;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
myVars = ((GlobalVars)getApplication());
myText = (TextView) findViewById(R.id.myText);
myText.setText(myVars.getMyString());
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
}
答案 2 :(得分：0)
在oncreate（）中添加这些行。
GlobalVars myVars = ((GlobalVars)getApplicationContext());
TextView myText = (TextView) findViewById(R.id.myText);