Code

import java.util.Scanner;

public class Phrase {

    private String phrase;

    public Phrase() {
        this.phrase = "";
    }

    public void saisir(){
        Scanner sc = new Scanner(System.in);
        System.out.println("Saisir la phrase");
        this.phrase = sc.next();
    }

    public void afficher(){
        System.out.println("Phrase : " + this.phrase);
    }

    public String miroir() {
        String chaine = "";
        for(int i = this.phrase.length()-1;i>=0;i--){
            chaine += this.phrase.charAt(i);
        }

        return chaine;
    }

    public void palindrome() {
        String chaine = this.miroir();
        if(chaine.equals(this.phrase)){
            System.out.println("C'est un palindrome");
        } else {
            System.out.println("Ce n'est pas un palindrome");
        }
    }

    public void crypter(int cle){
        String chaine = "";

        for(int i = 0;i<=this.phrase.length() - 1;i++){
                chaine += (char)(this.phrase.charAt(i) + cle);
            }

        System.out.println("Chaine cryptée  : " +chaine);

    }

    public void decrypter(int cle){
        String chaine ="";
        for(int i = 0;i<=this.phrase.length() - 1;i++){
            chaine += (char)(this.phrase.charAt(i) - cle);
        }

        System.out.println("Chaine décryptée  : " +chaine);
    }

    public void gerer() {
        Scanner sc = new Scanner(System.in);
        int choix = 0;
        do {
            System.out.println("_______ MENU PHRASE ___________");
            System.out.println("_1_ Saisir la phrase");
            System.out.println("_2_ Afficher la phrase");
            System.out.println("_3_ Miroir");
            System.out.println("_4_ Palindrome");
            System.out.println("_5_ Cryptage");
            System.out.println("_6_ Decryptage");
            System.out.println("_0_ Quitter");
            System.out.println("Votre choix ->");
            choix = sc.nextInt();
            if(choix <= 6 && choix >= 0){
                switch(choix){
                    case 1: this.saisir();break;
                    case 2: this.afficher();break;
                    case 3:
                        System.out.println("Miroir " + this.miroir());
                        break;
                    case 4:this.palindrome();break;
                    case 5: {
                        int cle;
                        System.out.println("Donner la cle :");
                        cle = sc.nextInt();
                        this.crypter(cle);
                    }
                        break;
                    case 6: {
                        int cle;
                        System.out.println("Donner la cle :");
                        cle = sc.nextInt();
                        this.decrypter(cle);
                    }
                        break;
                }
            } else {
                System.out.println("Choix incorrect");
            }
        } while(choix != 0);
    }
}