Android menu_exit + back button

package com.yalnizlarmektebi.ytech;

import com.google.ads.AdRequest;
import com.google.ads.AdSize;
import com.google.ads.AdView;

import android.os.Bundle;
import android.app.Activity;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.MenuItem;
import android.webkit.*;
import android.webkit.WebSettings.ZoomDensity;
import android.widget.LinearLayout;

public class MainActivity extends Activity {

    private WebView myWebView;
   
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
       
        myWebView = (WebView) findViewById(R.id.webView1);
        myWebView.getSettings().setJavaScriptEnabled(true);
       
   
        myWebView.getSettings().setBuiltInZoomControls(true);
        myWebView.getSettings().setSupportZoom(true);
        myWebView.getSettings().setUseWideViewPort(true);
        myWebView.getSettings().setLoadWithOverviewMode(true);
           
       
        //myWebView.getSettings().setDomStorageEnabled(true);
        //myWebView.setWebChromeClient(new WebChromeClient());
        myWebView.setWebViewClient(new MyWebViewClient());
       
        myWebView.loadUrl("http://www.selcuk.edu.tr");
       
       
        LinearLayout layout = (LinearLayout)findViewById(R.id.reklam);
        AdView adView = new AdView(this, AdSize.BANNER, "xxxxxxxxxxxxxxxxxxxxx");
        layout.addView(adView);
        AdRequest request = new AdRequest();
        //request.setTesting(true);
        adView.loadAd(request);
       
       
    }

    class MyWebViewClient extends WebViewClient {
        @Override
        public boolean shouldOverrideUrlLoading(WebView view, String url) {
           
                view.loadUrl(url); // Stay within this webview and load url
                return true;
            }
        }
   
   
    @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;
    }
   
   
   
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event){
        if((keyCode == KeyEvent.KEYCODE_BACK) && (myWebView.canGoBack()))
        {
            myWebView.goBack();
            return true;
        }else if ( (keyCode == KeyEvent.KEYCODE_BACK) && (myWebView.canGoBack()==false) )
            {              
                finish();
                return true;
            } else
                return false;
       
    }
   
   
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case R.id.cikis:
            finish();
            break;
        }
        return true;
    }
   

}