安卓新手上路，求问各路大神ava.lang.ClassNotFoundException与java.lang.RuntimeException是什么原因

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

    package="com.example.stopwatch"

    android:versionCode="1"

    android:versionName="1.0" >

    <uses-sdk

        android:minSdkVersion="14"

        android:targetSdkVersion="18" />

    <application

        android:allowBackup="true"

        android:icon="@drawable/ic_launcher"

        android:label="@string/app_name"

        android:theme="@style/AppTheme" >

        <activity 

            android:name=".StopWatch"

            android:label="StopWatch by XGW">

            <intent-filter>

                <action android:name="android.intent.action.MAIN" />

				<category android:name="android.intent.category.LAUNCHER" />

            </intent-filter>

        </activity>

        <activity android:name=".SecondActivity" 

            		android:label="Stopped">

        </activity>

    </application>

</manifest>

import com.example.stopwatch.R;

import android.app.Activity;

import android.content.Intent;

import android.os.Bundle;

import android.util.Log;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.TextView;

import android.widget.Toast;



import java.util.*;





public class StopWatch extends Activity{

	final Calendar cal = Calendar.getInstance();

	int seconds;

	long startTime, t2;

	boolean started = false;

	int second;

	int minute;

	ArrayList<Integer> ListOfSecond = new ArrayList<Integer>();

	

	public String showTimeCount(long time) {

		String timeCount = "";

	    long hourc = time/3600000;

	    String hour = "0" + hourc;

	    hour = hour.substring(hour.length()-2, hour.length());

	        

	    long minuec = (time-hourc*3600000)/(60000);

	    String minue = "0" + minuec;

	    minue = minue.substring(minue.length()-2, minue.length());

	        

	    long secc = (time-hourc*3600000-minuec*60000)/1000;

	    String sec = "0" + secc;

	    sec = sec.substring(sec.length()-2, sec.length());

	    timeCount = hour + ":" + minue + ":" + sec;

	    return timeCount;

	}

	

	class Timer extends Thread{

		String temp;

		long startTime;

		Timer(long time){

			startTime = time;

		}

		public void run(){

			try{

				while(true){

					cal.setTime(new Date());

					long currentTime = cal.getTimeInMillis();

					temp = showTimeCount(currentTime - startTime);

					text.setText(temp);

					Thread.sleep(1000);

				}

			}

			catch(Exception e){

				e.printStackTrace();

			}

		}

	}

	 

	Button button1;

	Button button2;

	Button button3;

	TextView text;

	

	protected void onCreat(Bundle savedInstanceState){

		super.onCreate(savedInstanceState);

		setContentView(R.layout.first_layout);

		

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

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

		button3 = (Button) findViewById(R.id.button3);

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

		button1.setOnClickListener(new OnClickListener(){

			@Override

			public void onClick(View v) {

				if(started == false){

					cal.setTime(new Date());

					startTime = cal.getTimeInMillis();

					started = true;

					Timer timer = new Timer(startTime);

					timer.start();

					

				}

				else {

					Toast.makeText(getBaseContext(), "you have already started it!", Toast.LENGTH_LONG).show();

				}

			}		

		});

		Log.e("1","1");

		button2.setOnClickListener(new OnClickListener(){

			@Override

			public void onClick(View v) {

			// TODO Auto-generated method stub

			cal.setTime(new Date());

		    long t2= cal.getTimeInMillis();

		    seconds = (int) ((t2 - startTime) % 1000);

		    ListOfSecond.add(seconds);

		    Toast.makeText(getBaseContext(), "time recorded.", Toast.LENGTH_SHORT).show();

			}

		});

		

		button3.setOnClickListener(new OnClickListener(){

			@Override

			public void onClick(View v) {

				Toast.makeText(getBaseContext(), "stopped.", Toast.LENGTH_LONG).show();

				Bundle data = new Bundle();

				Intent intent = new Intent(StopWatch.this, SecondActivity.class);

				data.putIntegerArrayList("second",ListOfSecond);

				intent.putExtras(data);

				startActivity(intent);

				}

					

		});

		

		

	}

}

import java.util.ArrayList;

import java.util.Date;

import android.util.Log;

import com.example.stopwatch.R;



import android.app.Activity;

import android.content.Intent;

import android.os.Bundle;

import android.util.Log;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.TextView;

import android.widget.Toast;



public class SecondActivity extends Activity{

	public String showTimeCount(long time) {

		

		String timeCount = "";

	    long hourc = time/3600;

	    String hour = "0" + hourc;

	    hour = hour.substring(hour.length()-2, hour.length());

	        

	    long minuec = (time-hourc*3600);

	    String minue = "0" + minuec;

	    minue = minue.substring(minue.length()-2, minue.length());

	        

	    long secc = (time-hourc*3600-minuec*60);

	    String sec = "0" + secc;

	    sec = sec.substring(sec.length()-2, sec.length());

	    timeCount = hour + ":" + minue + ":" + sec;

	    return timeCount;

	}

	

	TextView text;

	Button button1;

	

	protected void onCreate(Bundle saveInstanceState){

		super.onCreate(saveInstanceState);

		setContentView(R.layout.second_layout);

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

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

		Bundle bundle = this.getIntent().getExtras();   

		ArrayList<Integer> ListOfSecond  = bundle.getIntegerArrayList("second");

		String display = "";

		for (int i= 0; i < ListOfSecond.size(); ++ i){

			display += showTimeCount(ListOfSecond.get(i));

			display += '\n';

		}

		

		text.setText(display);

	

		button1.setOnClickListener(new OnClickListener(){

		@Override

		public void onClick(View v) {

			Intent intent = new Intent(SecondActivity.this, StopWatch.class);

			startActivity(intent);

		}		

		});

	}

}