In this tutorial we will make a simple android timer application using a Handler.In the layout of the main activity we will set a TextView to Display the timer and to Buttons for Start and Pause. We set the string resource for the timer value.

Set the String Resource as this

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app&#95;name">"TimerSet "</string>
    <string name="action&#95;settings">Settings</string>
    <string name="hello&#95;world">Hello world!</string>

    <string name="timmerValue">00:00:00</string>

</resources>

The xml layout if the main activity will look like this :

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout&#95;width="match&#95;parent"
    android:layout&#95;height="match&#95;parent"
    android:paddingBottom="@dimen/activity&#95;vertical&#95;margin"
    android:paddingLeft="@dimen/activity&#95;horizontal&#95;margin"
    android:paddingRight="@dimen/activity&#95;horizontal&#95;margin"
    android:paddingTop="@dimen/activity&#95;vertical&#95;margin"
    tools:context=".MainActivity" >

    <TextView
        android:id="@+id/textView1"
        android:layout&#95;width="wrap&#95;content"
        android:layout&#95;height="wrap&#95;content"
        android:layout&#95;alignParentLeft="true"
        android:layout&#95;alignParentTop="true"
        android:layout&#95;marginLeft="70dp"
        android:layout&#95;marginTop="102dp"
        android:textSize="40sp"
        android:text="@string/timmerValue" />

    <Button
        android:id="@+id/button1"
        android:layout&#95;width="80dp"
        android:layout&#95;height="50dp"
        android:layout&#95;alignParentLeft="true"
        android:layout&#95;below="@+id/textView1"
        android:layout&#95;marginLeft="30dp"
        android:layout&#95;marginTop="84dp"
        android:text="Start" />

    <Button
        android:id="@+id/button2"
        android:layout&#95;width="80dp"
        android:layout&#95;height="50dp"
        android:layout&#95;marginTop="232dp"
        android:layout&#95;alignLeft="@+id/button1"
        android:layout&#95;marginLeft="150dp"
        android:text="Pause" />

</RelativeLayout>

in the MainActivity write the following code, the code is very simple and self explanatory

package com.example.timerset;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.SystemClock;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

    private Button startButton;
    private Button pauseButton;

    private TextView timerValue;

    private long startTime = 0L;

    private Handler customHandler = new Handler();

    long timeInMilliseconds = 0L;
    long timeSwapBuff = 0L;
    long updatedTime = 0L;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        timerValue = (TextView) findViewById(R.id.textView1);

        startButton = (Button) findViewById(R.id.button1);

        startButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {
                startTime = SystemClock.uptimeMillis();
                customHandler.postDelayed(updateTimerThread, 0);

            }
        });

        pauseButton = (Button) findViewById(R.id.button2);

        pauseButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {

                timeSwapBuff += timeInMilliseconds;
                customHandler.removeCallbacks(updateTimerThread);

            }
        });
    }

    private Runnable updateTimerThread = new Runnable() {

                    public void run() {

                        timeInMilliseconds = SystemClock.uptimeMillis() - startTime;

                        updatedTime = timeSwapBuff + timeInMilliseconds;

                        int secs = (int) (updatedTime / 1000);
                        int mins = secs / 60;
                        secs = secs % 60;
                        int milliseconds = (int) (updatedTime % 1000);
                        timerValue.setText("" + mins + ":"
                                + String.format("%02d", secs) + ":"
                                + String.format("%03d", milliseconds));
                        customHandler.postDelayed(this, 0);
                    }

                };
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}

This basic code can be use d in different kind of android applications like stopwatch,interval timer etc. Hope the code helped you.