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

import android.content.Context;
import android.media.AudioManager;
import android.media.SoundPool;

public class SoundManager
{
  private SoundPool soundPool;
  private int[] sm;
  Context context;

  public SoundManager(Context context) {
    this.context = context;
    soundPool = new SoundPool(4, AudioManager.STREAM_MUSIC, 0);
    sm = new int[2];
    // fill your sounds
    sm[0] = soundPool.load(context, R.raw.my_sound1, 1);
    sm[1] = soundPool.load(context, R.raw.my_sound2, 1);
  }

  public final void playSound(int sound) {
      AudioManager mgr = (AudioManager)context.getSystemService(
          Context.AUDIO_SERVICE);  
      float streamVolumeCurrent =
          mgr.getStreamVolume(AudioManager.STREAM_MUSIC);
      float streamVolumeMax = mgr.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
      float volume = streamVolumeCurrent / streamVolumeMax;
      soundPool.play(sm[sound], volume, volume, 1, 0, 1f);
  }

  public final void cleanUpIfEnd() {
    sm = null;
    context = null;
    soundPool.release();
    soundPool = null;
  }
}

		Random rand = new Random();
		int i = rand.nextInt() % 256; // range -255  +255
		System.out.print(i + "

"); // -184
		i = Math.abs(rand.nextInt() % 12); // range 0  +11
		System.out.print(i); // 7

// Math.random() start with 0.  e.g. 0.35981234 
int nRan = (int) (Math.random()*10); // 0 - 10

// nextDouble(), nextFloat(), nextInt(), nextLong() returns 0 - 10
import java.util.Random;´

Random r = new Random();
int nRan = r.nextInt(); // 0 - 10
double dRan = r.nextDouble() * 10; // e.g. 7.496285271597397

nextDouble – return 0 - 1
nextFloat – same as double
nextInt –  -2147483648  +2147483647
nextLong –  -922337203685775808  +9223372036854775807
nextGaussian –  0.0 aberation 1.0.

EditText hEditText;

// TextChanged in onCreate

hEditText = (EditText)findViewById(R.id.idEditText);

hEditText.addTextChangedListener(new TextWatcher() {

	public void afterTextChanged(Editable str) {

	}



	public void beforeTextChanged(CharSequence s, int start, int count,

			int after) {

	}



	public void onTextChanged(CharSequence sq, int start, int before,

			int count) {

	}

});

<LinearLayout
    android:baselineAligned="false"
    android:background="@android:color/transparent"
    android:id="@+id/linearLayout1"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
   >
<LinearLayout>
////////////////////////////
LinearLayout  mLinearLayout1 = (LinearLayout)findViewById(R.id.linearLayout1);
mLinearLayout1.setBackgroundColor(Color.TRANSPARENT);

// file name MyFirstClass.java

import java.util.*;
import java.lang.Math;
import java.io.*;
import javax.swing.*;

public class MyFirstClass{ // start of program
  public static void main(String[] args) { // basic  function main

// variables and calculation
int a=2;
int b=3;
int c=Integer.parseInt(JOptionPane.showInputDialog(" Put number: ", "1"));
System.out.println("Number is: "+c);
System.out.println(a+" * "+b+" = "+(a*b));
System.out.println("a^3 "+Math.pow(a,b));

//array
int[] array_my=new int[10];
array_my[0]=3;
array_my[1]=5;
System.out.println("Number of elements "+array_my.length+" 1 + 2 element of array "+(array_my[0]+array_my[1]));

//strings
String txt="Quick red fox";
String txt2=JOptionPane.showInputDialog("Write text: ", "word");
System.out.println("Text is: "+txt2);
String[] count_of_word=txt.split(" ");
System.out.println("Length: "+txt.length()+" Count of words "+count_of_word.length);
System.out.println(txt +" -> "+txt.replace("red","brown"));
System.out.println("First 5 chars of string is: "+ txt.substring(0,5));

//for a if
for(int i=0; i<10;i++){
	if(i==3)System.out.println("i equal "+i);
	}

// file
try {
  File soubor=new File("some_file.txt");
  if(!soubor.exists()){
	System.out.println("File dont exist");
  }
  else { // utf-8 encoding
    BufferedReader in1 = new BufferedReader(new InputStreamReader(new FileInputStream(soubor),"UTF-8"));
    BufferedWriter out1 = new BufferedWriter(new OutputStreamWriter(new FileOutputStream("zapis.txt"),"UTF-8"));
    String str;
      while ((str = in1.readLine())!=null){
        System.out.println(str);
        out1.write(str+"

");
      }
    in1.close();
    out1.close();
   }
 } catch(IOException e){System.out.println("Error " + e);}

// dir
 File pathName = new File("some_dir");
 String[] fileNames = pathName.list();
 if(pathName.exists())
 System.out.println("Name of first file in  \"some_dir\": "+fileNames[0]);
 else System.out.println("dir not exist");

//function
int nResult = calculateMyFc(3,5);
System.out.println("Result of function: "+nResult);

} // end of function main

// new function , you can add to end MyFc = my function
public static int calculateMyFc(int a, int b){
return (a+b);
}

} // end of class of program