Android Java：通過子類的全局變量;無法啓動應用程序？

Question

我一直在閱讀如何做到這一點的例子，試圖從本網站和其他人的帖子拼湊出一個有效的例子。我確信我錯過了一些小東西，但作爲Java的新手，我可以使用一些幫助。

在Eclipse中創建一個新項目時，我只是想創建一個小例子，從自動化的hello world導出生成。我想要做的就是能夠在子類中存儲一些全局變量，然後在主要活動中引用這些值。不幸的是，每次我嘗試運行該應用程序時，它都會崩潰「不幸的是，GeneralTest1已停止」，並且日誌貓錯誤不是很有幫助。

快速概述：

• GlobalVars類擴展應用
• 在清單中，機器人：已添加名字引用其他GlobalVars類
• 在我的主要活動，我初始化的全局vars class with getApplicationContext（）

這裏是我所得到的一切;任何幫助將非常感激！

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清單

<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> 

登錄貓

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 

Answer 1

請查看這篇文章，瞭解活動和它們的生命週期： http://developer.android.com/reference/android/app/Activity.html#ActivityLifecycle

您的活動應該是這樣的：

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; 
    } 
} 

Answer 2

要調用的類定義，這是在活動太早getApplicationContext()終身並導致nullpo例外。將作業移動到您的onCreate()函數，它應該按預期工作。

Answer 3

在OnCreate中添加這些行（）.. GlobalVars myVars = ((GlobalVars)getApplicationContext());
TextView myText = (TextView) findViewById(R.id.myText);