cambiaLetras.java

public class main {

    public static void cambiaLetras() {
        try {
            FileReader fr = new FileReader("texto.txt");
            BufferedReader br = new BufferedReader(fr);
            String str = null;
                       
            while ((str = br.readLine()) != null) {
                for (int i = 0; i < str.length(); i++) {
                    int charac = str.charAt(i);
                    if (charac != 32 && charac < 132) {
                        int characMayus = charac + 32;
                        System.out.print((char) characMayus);
                    }
                    if (charac != 32 && charac > 132) {
                        int characMinus = charac - 32;
                        System.out.print((char) characMinus);
                    }
                    
                    if (charac == 32) {
                        System.out.print((char) 32);
                    }
                    
                    if (charac == 10) {
                        System.out.println("\n");
                    }
                }
            }
            
        } catch (IOException ex) {
            System.out.println("No se encontró el archivo");
        }
    }
            
    public static void main(String[] args) {
        cambiaLetras();
    }
    
}