<Button
 android:id="@+id/buttonClickMe"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:layout_below="@+id/textView7"
 android:layout_centerHorizontal="true"
 android:layout_marginTop="18dp"
 android:onClick="sendMessage"
 android:text="@string/button_start" />

final Button buttonStart = (Button) findViewById(R.id.button1)
buttonStart.setOnClickListener(new OnClickListener() {

        public void onClick(View arg0) {

            int no = Integer.valueOf(numberField.getText().toString());

            switch (no)
            {
               case 1 : maxRand = 15;  // the random number will be between 0 and maxRand
               break;
               case 2 : maxRand = 30;
               break;
               case 3 : maxRand = 50;
               break;
               case 4 : maxRand = 100;
               break;
               default : textField.setText("Invalid Argument, Please try again.");
               break;
            }

            if(maxRand != 0){
                randNumber = (int) (Math.random() * maxRand);
                /*textField.setText("Now click Start button !");
                  buttonStart.setVisibility(View.VISIBLE);*/
                startActivity(intent);   //This starts a new activity
            }
        }
    });

public void sendMessage(View view){     //This method is called everytime when the button is clicked

    intent = new Intent(this,ShowResultsActivity.class);
}

public void onClick(View arg0) {

        int no = Integer.valueOf(numberField.getText().toString());

        switch (no)
        {
           case 1 : maxRand = 15;  // the random number will be between 0 and maxRand
           break;
           case 2 : maxRand = 30;
           break;
           case 3 : maxRand = 50;
           break;
           case 4 : maxRand = 100;
           break;
           default : textField.setText("Invalid Argument, Please try again.");
           break;
        }

        if(maxRand != 0){
            randNumber = (int) (Math.random() * maxRand);
            intent = new Intent(this,ShowResultsActivity.class);
            startActivity(intent);   //This starts a new activity
        }
    }
});

public void sendMessage(View view){
    //This method is called everytime when the button is clicked
    int no = Integer.valueOf(numberField.getText().toString());

            switch (no)
            {
               case 1 : maxRand = 15;  // the random number will be between 0 and maxRand
               break;
               case 2 : maxRand = 30;
               break;
               case 3 : maxRand = 50;
               break;
               case 4 : maxRand = 100;
               break;
               default :
                   textField.setText("Invalid Argument, Please try again.");
                   // add this to ensure that you won't start activity
                   maxRand = 0;
               break;
            }

            if(maxRand != 0){
                randNumber = (int) (Math.random() * maxRand);
                intent = new Intent(this,ShowResultsActivity.class);
                startActivity(intent);   //This starts a new activity
            } else {
                   // maxRand == 0 ===> ERRORS, update your textField here
                   textField.setText("Invalid Argument, Please try again.");
            }
}