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POAndroid: How to use classes in a different file
 

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  1. POAndroid: How to use classes in a different file
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    <p>I'm trying to make a handler class for many of my reusable methods. I'm having problems interfacing the separate class file.</p> <p>Can someone tell me what this example fails or what would have to be done to make it work?</p> <p>ClassExampleActivity.java: package com.apollo.classexample;</p> <pre><code>import android.app.Activity; import android.os.Bundle; import android.widget.TextView; public class ClassExampleActivity extends Activity { /** Called when the activity is first created. */ TextView infotext; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); String infostring = "Main Activity Class\n"; Myhandler mh = new Myhandler(); infostring += mh.testmethod(); // Trying to use the TextView widget from this MyHandler class // This works from the main class but fails in the MyHandler class TextView infotext = (TextView) findViewById(R.id.infotext); infotext.setText(infostring); } } </code></pre> <p>Myhandler.java: package com.apollo.classexample;</p> <pre><code>import android.widget.TextView; import android.app.Activity; public class Myhandler extends Activity { public String testmethod(){ String innermessage = "This is a message from the innerclass\n"; System.out.println(innermessage); /* This is an important component that I would like to use in this class Commenting these lines out and the application doesn't crash. I'm trying to learn how to use the next two lines in this MyHandler class which I'm trying to make portable and reusable for other projectes. */ TextView infotext = (TextView) findViewById(R.id.infotext); infotext.setText(innermessage); // Placed to show that the MyHandler class is actually accessed return innermessage; } } </code></pre> <p>AndroidManifest.xml:</p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.apollo.classexample" android:versionCode="1" android:versionName="1.0" &gt; &lt;uses-sdk android:minSdkVersion="7" /&gt; &lt;application android:icon="@drawable/ic_launcher" android:label="@string/app_name" &gt; &lt;activity android:name=".ClassExampleActivity" android:label="@string/app_name" &gt; &lt;intent-filter&gt; &lt;action android:name="android.intent.action.MAIN" /&gt; &lt;category android:name="android.intent.category.LAUNCHER" /&gt; &lt;/intent-filter&gt; &lt;/activity&gt; &lt;activity android:name="MyHandler" /&gt; &lt;activity android:name="MyCRUDSQL" /&gt; &lt;/application&gt; &lt;/manifest&gt; </code></pre> <p>When calling the testmethod() the android app will crash. If I comment it out it'll run and print the System.out.printlin() message to the LogCat.</p> <p>LogCat:</p> <pre><code>03-20 17:03:42.783: I/System.out(654): This is a message from the innerclass 03-20 17:03:42.783: D/AndroidRuntime(654): Shutting down VM 03-20 17:03:42.803: W/dalvikvm(654): threadid=1: thread exiting with uncaught exception (group=0x40014760) 03-20 17:03:42.853: E/AndroidRuntime(654): FATAL EXCEPTION: main 03-20 17:03:42.853: E/AndroidRuntime(654): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.apollo.classexample/com.apollo.classexample.ClassExampleActivity}: java.lang.NullPointerException 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1748) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1764) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.ActivityThread.access$1500(ActivityThread.java:122) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1002) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.os.Handler.dispatchMessage(Handler.java:99) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.os.Looper.loop(Looper.java:132) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.ActivityThread.main(ActivityThread.java:4025) 03-20 17:03:42.853: E/AndroidRuntime(654): at java.lang.reflect.Method.invokeNative(Native Method) 03-20 17:03:42.853: E/AndroidRuntime(654): at java.lang.reflect.Method.invoke(Method.java:491) 03-20 17:03:42.853: E/AndroidRuntime(654): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 03-20 17:03:42.853: E/AndroidRuntime(654): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 03-20 17:03:42.853: E/AndroidRuntime(654): at dalvik.system.NativeStart.main(Native Method) 03-20 17:03:42.853: E/AndroidRuntime(654): Caused by: java.lang.NullPointerException 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.Activity.findViewById(Activity.java:1744) 03-20 17:03:42.853: E/AndroidRuntime(654): at com.apollo.classexample.MyHandler.testmethod(MyHandler.java:16) 03-20 17:03:42.853: E/AndroidRuntime(654): at com.apollo.classexample.ClassExampleActivity.onCreate(ClassExampleActivity.java:23) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048) 03-20 17:03:42.853: E/AndroidRuntime(654): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1712) 03-20 17:03:42.853: E/AndroidRuntime(654): ... 11 more </code></pre>
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    1. USL. D. James
    1. USL. D. James
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      1. POAndroid: How to use classes in a different file
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    1. COdon't use myhandler mh = new myhandler(); for Activity.
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      1. POAndroid: How to use classes in a different file
      1. USVyacheslav Shylkin
    2. COBefore Initializing the the layout in OnCreate() method You can not make the intance of activity you probably have to start Activity override the oncreate method set its content by SetContentView() And then set properties of its views.
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      1. POAndroid: How to use classes in a different file
      1. USNitin
    3. CONitin I'm working on the project trying to implement what you suggested.
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      1. POAndroid: How to use classes in a different file
      1. USL. D. James
 

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