private Boolean checkData(String litra){
	/*
	 * elegxei an einai swsti i timi kai an oxi tote emfanizei minima lathous
	 */
	if(litra.length()<1){
		JOptionPane.showMessageDialog(null, "Den Dwsate arithmo litrwn");
		return false;
	}
	try{  
	    double d = Double.parseDouble(litra);  
	}catch(NumberFormatException nfe){
		JOptionPane.showMessageDialog(null, "Dwste Egkiro arithmo!");
	    return false;  
	}  
	
	return true;
}