Example project 1) (random number)

1) Computer generates a random number between 1 and 20

2) User enters a number and clicks Guess

3) Computer pops up a message if the guess is higher or lower or equal to random number

public class MainActivity extends AppCompatActivity {

    int randomNumer; //forward declaration. This variable will be set up onCreate method

    //refactor reused code into a method
    private void makeToast(String str){
        Toast.makeText(MainActivity.this,str,Toast.LENGTH_LONG).show();
    }

    public void clickFunction(View view){
        EditText guessText = (EditText) findViewById(R.id.editText);

        if (!guessText.getText().toString().isEmpty()){
            Integer guess_i = Integer.parseInt(guessText.getText().toString());

            if (guess_i < randomNumer)
                makeToast("Your guess is lower");
            else if (guess_i > randomNumer)
                makeToast("Your guess is higher");
            else
                makeToast("You are RIGHT!");

        }
    }

    //All code that must be executed on start up comes here
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Generate random number between 1 and 20
        Random rand = new Random();
        randomNumer = rand.nextInt(20) + 1;


    }
}

Example project 2) (class, try catch)

Check if the entered number is square, trianglular, both or neither

public class MainActivity extends AppCompatActivity {

    class Number {
        public int number=0;

        boolean isSquare(){
            double sqrtNumber = Math.sqrt(number);
            boolean ret = false;

            if(sqrtNumber == Math.floor(sqrtNumber))
                ret = true;
            return ret;
        }

        boolean isTriangular(){

            int trinum = 0;

            for(int i=1;trinum<number;i++)
                trinum += i;

            boolean ret=false;

            if(trinum==number)
                ret=true;

            return ret;

        }
    }

    public void clickFunction(View view){
        EditText numEditText = (EditText) findViewById(R.id.editText);

        String message = "";

        Number number = new Number();

        boolean success = true;
        try {
            number.number = Integer.parseInt(numEditText.getText().toString());
        }
        catch (Exception e){
            message = "Enter a number!";
            success = false;
        }

        if(success) {
            if (number.isSquare() && number.isTriangular())
                message = number.number + " is both square and triangular";
            else if (number.isSquare())
                message = number.number + " is square";
            else if (number.isTriangular())
                message = number.number + " is triangular";
            else
                message = number.number + " is neither square nor triangular";
        }

        Toast.makeText(MainActivity.this,message,Toast.LENGTH_LONG).show();

    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}