android.app.Application cannot be cast to android.app.Activity

Try add to AndroidManifest.xml configChanges for your Activity
android:configChanges="keyboardHidden|orientation|screenSize"

 <activity android:name=".MainActivity"
                  android:label="@string/app_name"
                  android:configChanges="keyboardHidden|orientation|screenSize"
                  >

Špičkový smartphone od Samsungu.

Technické parametry Samsung i9300 Galaxy S III
Rozměry a hmotnost: 137 x 71 x 9 mm, 133 gramů
Datové funkce: GPS modul, WiFi, Bluetooth, NFC, GPRS, EDGE, HSCSD, Hardwarový modem, Infraport
Sítě: pásma GSM 850, 900, 1 800, 1 900 MHz pásma WCDMA (3G) 850, 900, 1 900, 2 100 MHz
Displej: HD Super AMOLED diplej o velikosti 1280x720 4,8 palců
Procesor: čtyřjádrový procesor 1.4 GHz
Uživatelská paměť: 16 MB
paměť RAM 1 024 MB
Operační systém: Android 4.0 ICS
Hudba: MP3
Video: HD rozlišení
Fotoaparát: 8 Mpix
Navigace:
Baterie: Li-Pol, pohotovostní doba 390 hodin
Cena: cca 14.500 korun / léto 2012

Samsung i9300 Galaxy S III obrázek

Cycle for in Java example

public class MainClass {
	public static void main(String[] arg) {
		String[] arrayOfString = { "Hello", "people", "hello", "world!" };

		for (int i = 0; // start position , first cycle i == 0
                      i < arrayOfString.length; // if i < 4 (length of array) do loop
                       i++ // incrementation after cycle
                       ){
			System.out.println(arrayOfString[i]); 
                       // i++ incrementation
		}
	}
}

You have to put your font fonts/samplefont.ttf into assets/fonts folder in your project!

// set own custom font from assets
TextView txt = (TextView) findViewById(R.id.custom_font);
Typeface mFace = Typeface.createFromAsset(getContext().getAssets(),
                                             "fonts/samplefont.ttf");
    txt.setTypeface(mFace);

Spinner, ArrayAdapter, setOnItemSelectedListener, onItemSelected, setDropDownViewResource

MainClass.java

public class Vypocet extends Activity {
 Spinner   hSpinnerMetry;
// .......
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

	       hSpinnerMetry = (Spinner) findViewById(R.id.idSpinnerMetry);
	        ArrayAdapter<CharSequence> adapterMetry = ArrayAdapter.createFromResource(
	        		this, R.array.metry_array, android.R.layout.simple_spinner_item);
	        adapterMetry.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
	        hSpinnerMetry.setAdapter(adapterMetry);
			
	           hSpinnerMetry.setOnItemSelectedListener(
	                    new OnItemSelectedListener() {
	                        public void onItemSelected(
	                                AdapterView<?> parent, View view, int position, long id) {
	                        int nPos = position;
	                        }

	                        public void onNothingSelected(AdapterView<?> parent) {
	                        
	                        }
	                    });	        

      }
}

strings.xml

<string-array name="metry_array">
<item>m</item>
<item>dm</item>
<item>cm</item>
<item>mm</item>
</string-array>