Hello everyone! I recently started learning to make android apps, and
I decided that I needed to learn how to go about working with multiple
threads (I eventually want to make fairly simple, 2D games). So I
figured I would design a super-simple app that would use two threads
and send messages from one thread to the other.
Here's the app I came up with: It should simply display one text view
on the screen. A handler should be set up in the main thread that will
respond to messages - if the message sent contains the value 1, then
it should set the text of this text view to "Start." If it receives a
2, then it should change that text to "It worked!"
That message should come from the other, secondary thread, which
should perform like this: It contains one variable, an int named
counter that starts off at 0. Then, if the counter is 0, the counter
is set to 1, and the value of counter is sent to the main thread
handler in the what field of a message called m (where the main thread
handler should receive it and change the text view text to "Start.").
Then, the thread should sleep for 5 seconds. When it resumes and runs
a second time, it should change the value of counter to 2, and send
that to the main thread handler, which would then change the text view
text to "It worked!"
Here is my code:
---------- threadTest.java ----------------------------------
package com.thread.test;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;
public class threadTest extends Activity {
//Fields
TextView myTextView = null;
Thread refreshThread = null;
//Handler
Handler updateHandler = new Handler()
{
//Handle message routine
//This gets called on every message that is recieved
//@Override
public void handleMessage(Message msg)
{
if (msg.what == 1)
{
myTextView.setText("Start");
}
if (msg.what == 2)
{
myTextView.setText("It Worked!");
}
super.handleMessage(msg);
}
};
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
//View referencing
this.myTextView = (TextView)findViewById(R.id.myTextView);
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//Thread creation
this.refreshThread = new Thread(new updateThread());
this.refreshThread.start();
}
class updateThread implements Runnable{
private int counter = 0;
//@Override
public void run()
{
while(!Thread.currentThread().isInterrupted())
{
Message m = new Message();
if (this.counter == 1)
{
this.counter = 2;
m.what = this.counter;
threadTest.this.updateHandler.sendMessage(m);
}
if (this.counter == 0)
{
this.counter = 1;
m.what = this.counter;
threadTest.this.updateHandler.sendMessage(m);
}
try{
Thread.sleep(5000);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
}
}//End class updateThread
}
-----------------------------------------------------------
Unfortunately, it does not work. As soon as I try to run it, I get a
force close error. i checked my logcat out, and this is what I found:
----------- My logcat --------------------------------
09-30 00:24:47.883: INFO/ActivityManager(71): Starting activity:
Intent { act=android.intent.action.MAIN
cat=[android.intent.category.LAUNCHER] flg=0x10200000
cmp=com.thread.test/.threadTest }
09-30 00:24:48.543: WARN/ActivityManager(71): Activity pause timeout
for HistoryRecord{43f9cf00 com.android.launcher/
com.android.launcher2.Launcher}
09-30 00:24:48.783: INFO/ActivityManager(71): Start proc
com.thread.test for activity com.thread.test/.threadTest: pid=250
uid=10039 gids={1015}
09-30 00:24:50.453: DEBUG/dalvikvm(71): GREF has increased to 301
09-30 00:24:50.483: DEBUG/AndroidRuntime(250): Shutting down VM
09-30 00:24:50.483: WARN/dalvikvm(250): threadid=1: thread exiting
with uncaught exception (group=0x4001d800)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): FATAL EXCEPTION: main
09-30 00:24:50.552: ERROR/AndroidRuntime(250):
java.lang.NullPointerException
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
com.thread.test.threadTest$1.handleMessage(threadTest.java:33)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
android.os.Handler.dispatchMessage(Handler.java:99)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
android.os.Looper.loop(Looper.java:123)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
android.app.ActivityThread.main(ActivityThread.java:4627)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
java.lang.reflect.Method.invokeNative(Native Method)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
java.lang.reflect.Method.invoke(Method.java:521)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
com.android.internal.os.ZygoteInit
$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-30 00:24:50.552: ERROR/AndroidRuntime(250): at
dalvik.system.NativeStart.main(Native Method)
09-30 00:24:50.683: WARN/ActivityManager(71): Force finishing
activity com.thread.test/.threadTest
-------------------------------------------------------
Unfortunately, I don't exactly what this means, but I gather that the
problem comes from an uncaught exception occuring in "threadid=1",
that it was a java.lang.NullPointerException, and that it had
something to do with handleMessage. Am i correct in this
interpretation? And can anyone give me pointers on what I'm doing
wrong here?
Thank you very much for your time!
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