ForceClose error in android TabWidget application

Force close error in TAB widget application I am using the tutorial at http://developer.android.com/resources/tuto开发者_运维问答rials/views/hello-tabwidget.html to create TAB view. I want to display different list in each tab. I have created each list in separate activity and calling it by creating new instance of intent and then pass it to setcontent(); Following is the code for TabWidget.java

package com.TabWidget;

import android.app.TabActivity;
import android.content.Intent;
import android.content.res.Resources;
import android.os.Bundle;
import android.widget.TabHost;

public class TabWidget extends TabActivity {

    static final String[] COUNTRIES = new String[] {
        "Afghanistan", "Albania", "Algeria", "American Samoa", "Andorra",
        "Angola", "Anguilla", "Antarctica", "Antigua and Barbuda", "Argentina",
        "Armenia", "Aruba", "Australia", "Austria", "Azerbaijan",
        "Bahrain", "Bangladesh", "Barbados", "Belarus", "Belgium"
      };

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        TabHost mTabHost = getTabHost();
        Resources res = getResources(); 

        Intent aIntent = new Intent(this, a.class);
        Intent bIntent = new Intent(this, b.class);
        Intent cIntent = new Intent(this, c.class);

        mTabHost.addTab(mTabHost.newTabSpec("A").setIndicator("A", res.getDrawable(R.drawable.icon)).setContent(aIntent));
        mTabHost.addTab(mTabHost.newTabSpec("B").setIndicator("B", res.getDrawable(R.drawable.icon)).setContent(bIntent));
        mTabHost.addTab(mTabHost.newTabSpec("C").setIndicator("C", res.getDrawable(R.drawable.icon)).setContent(cIntent));

        mTabHost.setCurrentTab(1);
    }

}

A.java

package com.TabWidget;

import android.app.Activity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class A extends Activity {

    static final String[] COUNTRIES = new String[] {
        "Afghanistan", "Albania", "Algeria", "American Samoa", "Andorra",
        "Angola", "Anguilla", "Antarctica", "Antigua and Barbuda", "Argentina",
        "Armenia", "Aruba", "Australia", "Austria", "Azerbaijan",
        "Bahrain", "Bangladesh", "Barbados", "Belarus", "Belgium"
      };

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ListView list=(ListView)findViewById(R.id.countries);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1, COUNTRIES);
        list.setAdapter(adapter);
        setContentView(R.layout.tab1);
    }

}

Main .xml

<?xml version="1.0" encoding="utf-8"?>
<TabHost xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/tabhost"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <LinearLayout
        android:orientation="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">
        <TabWidget
            android:id="@android:id/tabs"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content" />
        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent">
            <include layout="@layout/tab1"/>      
            <include layout="@layout/tab2"/> 
            <include layout="@layout/tab3"/>
        </FrameLayout>
    </LinearLayout>
</TabHost>

I am novice to android development and interested in UI development. Thanks in advance.

In A.onCreate() you're trying to find a view before setting any content with setContentView(). So, list is null and you're getting a NullPointerException (NPE) at line

list.setAdapter(adapter);

Check this out in the LogCat, to learn how to find problems. LogCat is a priceless tool.