How to use Instance Save State in Android?

package com.example.preferenceactivity;
 
import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
 
public class MainActivity extends Activity {
 
    TextView text;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button but1 = (Button) findViewById(R.id.Prefsbutton);
        Button but2 = (Button) findViewById(R.id.GetPreferencesbutton);
        text = (TextView) findViewById(R.id.Prefstext);
        but1.setOnClickListener(new View.OnClickListener() {
 
            Intent in = new Intent("android.intent.action.Prefs");
 
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                startActivity(in);
            }
        });
 
        but2.setOnClickListener(new View.OnClickListener() {
 
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                displaySharedPreferences();
            }
        });
    }
 
    @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;
    }
 
    private void displaySharedPreferences() {
        SharedPreferences prefs = PreferenceManager
                .getDefaultSharedPreferences(this);
 
        String username = prefs.getString("username", null);
        String passw = prefs.getString("password", null);
        boolean checkBox = prefs.getBoolean("checkBox", false);
        String listPrefs = prefs.getString("listpref", "Default list prefs");
 
        StringBuilder builder = new StringBuilder();
        builder.append("Username: " + username + "\n");
        builder.append("Password: " + passw + "\n");
        builder.append("Keep me logged in: " + String.valueOf(checkBox) + "\n");
        builder.append("List preference: " + listPrefs);
 
        text.setText(builder.toString());
    }
}

package com.example.preferenceactivity;
 
import android.os.Bundle;
import android.preference.PreferenceActivity;
 
public class Prefs extends PreferenceActivity {
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
 
        //addPreferencesFromResource(R.xml.pref);
        getFragmentManager().beginTransaction().replace(android.R.id.content, new PrefFragement()).commit();
    }
 
}

package com.example.preferenceactivity;
 
import android.os.Bundle;
import android.preference.PreferenceFragment;
 
public class PrefFragement extends PreferenceFragment{
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        // Load the preferences from an XML resource
        addPreferencesFromResource(R.xml.pref);
    }
}