Apps Problems with Timers/changing a TextView's text

[Mekuso]

Hi! I'm new to Android development, and although I have used Java for some school projects in the past, I consider Lua my "native language". Still, I wanted to learn to make Android apps, and started with something I thought would be quite simple, a simple night clock to my dad. Right now it can show the time, and I tried to make it update the time by using a Timer, which is activated after 5 seconds, and at that time
1. update the TextView's text to match the current time
2. start a new, identical, timer

The code looks like this at this point:

package com.tboneproductions.nightclock;

import java.util.Calendar;
import java.util.Timer;
import java.util.TimerTask;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;

public class NightClockformyfatherActivity extends Activity {
	TextView tv;
	Timer t;
	
	class AwesomeTask extends TimerTask {
		private NightClockformyfatherActivity a;
		
		public AwesomeTask(NightClockformyfatherActivity a) {
			
			this.a = a;
			
		}
		
		@Override
		public void run() {
			Log.w("NightClock","timer works");
			a.setClockText(a.tv,Calendar.getInstance());
			a.t.schedule(new AwesomeTask(this.a),5000);
		}
		
		
	}
	
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
		tv = new TextView(this);
        tv.setTextSize(120);
        setClockText(tv,Calendar.getInstance());
        //tv.setText("no timer yet");
        t = new Timer();
        t.schedule(new AwesomeTask(this),5000);
        setContentView(tv);
    }
    
    public void setClockText(TextView tv,Calendar cal){
    	
    	
    	int h = cal.get(Calendar.HOUR_OF_DAY);
    	int m = cal.get(Calendar.MINUTE);
    	String hs;
    	String ms;
    	if (h < 10) {
    		hs = "0" + h;
    	} else {
    		hs = "" + h;
    	}
    	
    	if (m < 10) {
    		ms = "0" + m;
    		
    	} else {
    		ms = "" + m;
    	}
    	
        String s = hs + ":" + ms;
        
        tv.setText(s); //this line here crashes everything for some reason
    }
}

At the marked line near the bottom, the app crashes. The LogCat complains that "Only the original thread that a view hierarcy can touch its views". What does this mean, and how do I work around this? It sounds like the Timer creates a separate thread, which is unable to "touch" the TextView (even though it is given a reference to it). I guess I need to restructure my app somehow? Any ideas or explanations are very welcome

 

[jonbonazza]

You cannot modify UI elements from any thread but the UI thread. Because TimerTask is run on a separate thread, you need to create a Handler object and post back to the UI thread, telling it to change the text.

 

[Mekuso]

You cannot modify UI elements from any thread but the UI thread. Because TimerTask is run on a separate thread, you need to create a Handler object and post back to the UI thread, telling it to change the text.

I see. I've never worked with threads before so I really don't know how to do that. Will experiment a little, but any instructions would be appreciated.

EDIT: Solved it! For those who might be interested, here's how I did it:

package com.tboneproductions.nightclock;

import java.util.Calendar;
import java.util.Timer;
import java.util.TimerTask;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.widget.TextView;

public class NightClockformyfatherActivity extends Activity {
	TextView tv;
	Timer t;
	Handler h;
	
	class AwesomeTask extends TimerTask {
		private NightClockformyfatherActivity a;
		
		public AwesomeTask(NightClockformyfatherActivity a) {
			this.a = a;
		}
		
		class RunnableLol implements Runnable {
			private NightClockformyfatherActivity a;
			
			public RunnableLol(NightClockformyfatherActivity a) {
				this.a = a;
			}
			
			@Override
			public void run() {
				this.a.setClockText(this.a.tv,Calendar.getInstance());
				this.a.t.schedule(new AwesomeTask(this.a),5000);
				
			}
			
			
		}
		
		@Override
		public void run() {
			//Log.w("NightClock","timer works");
			RunnableLol r = new RunnableLol(this.a);
			this.a.h.post(r);
			
		}
		
		
	}
	
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
		tv = new TextView(this);
        tv.setTextSize(120);
        setClockText(tv,Calendar.getInstance());
        //tv.setText("no timer yet");
        t = new Timer();
        t.schedule(new AwesomeTask(this),5000);
        setContentView(tv);
        h = new Handler();
    }
    
    public void setClockText(TextView tv,Calendar cal){
    	
    	
    	int h = cal.get(Calendar.HOUR_OF_DAY);
    	int m = cal.get(Calendar.MINUTE);
    	String hs;
    	String ms;
    	if (h < 10) {
    		hs = "0" + h;
    	} else {
    		hs = "" + h;
    	}
    	
    	if (m < 10) {
    		ms = "0" + m;
    		
    	} else {
    		ms = "" + m;
    	}
    	
        String s = hs + ":" + ms;
        
        tv.setText(s); //this line here crashes everything for some reason
    }
}