Bai tap String

Baitap.java

import java.util.*;

public class Baitap {
    private static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) {
        for (; ; ) {
            System.out.println("Input String");
            String s = scanner.nextLine();
            s = s.trim().replaceAll("\\s+", " ");//Dong nay de xoa het dau cach thua (de dem tu)
            Baitap bt = new Baitap();
            System.out.println("Ket qua cau 1: " + bt.f1(s));
            System.out.println("Ket qua cau 2: " + bt.f2(s));
            System.out.println("Ket qua cau 3: " + bt.f3(s));
            System.out.println("Ket qua cau 4: " + bt.f4(s));
            System.out.println("Ket qua cau 5: " + bt.f5(s));
        }
    }

    private double f4(String s) {
        System.out.println("Nhap n:");
        String n = scanner.nextLine();
        return Double.parseDouble(n)*Double.parseDouble(s);//parse: chuyen tu string sang Double. Int cac kieu
    }
    private double f5(String s){
        System.out.println("Nhap s2:");
        String s2  =scanner.nextLine();
        return Double.parseDouble(s) + Double.parseDouble(s2);
    }

    private StringBuilder f1(String s) {
        //chuyen vtri chan thanh lower, le thanh upper
        String s1 = s.toLowerCase(), s2 = s.toUpperCase();
        StringBuilder sb = new StringBuilder();//xem lai stringbuilder nhe
        for (int i = 0; i < s.length(); i++) sb.append(i % 2 == 0 ? s1.charAt(i) : s2.charAt(i));
        return sb;
    }

    private StringBuilder f2(String s) {
        //chuyen chu o vtri chan thanh lower, le thanh upper

        String s1 = s.toLowerCase(), s2 = s.toUpperCase();
        String[] splt1 = s1.split("\\s"), splt2 = s2.split("\\s");//chia string ra thanh array
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < splt1.length; i++)//cai nay la thuat toan thoi kcj dau, tu nghi nhe :3
            sb.append(i != splt1.length - 1 ? i % 2 == 0 ? splt1[i] + " " : splt2[i] + " " : i % 2 == 0 ? splt1[i] : splt2[i]);
        return sb;//dai khai la toi chia thanh array roi dung sb nhap tung mang vao string
        //thuc ra co the co them dong String str = sb.toString(); roi return str, bien f2 thanh ham String duoc
    }

    private StringBuilder f3(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            if (i % 3 == 2) sb.append(s.charAt(i) + "-");//neu char o vi tri 2, 5, 8, vv thi se build kem them ktu "-"
            else sb.append(s.charAt(i));//neu ko thi thoi
        }
        return sb;
    }

}