public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    /** The value is used for character storage. */
    private final char value[];

/*
登錄系統(tǒng)
*/
public class UserLogin {
	public static void main(String [] args) {
		String username = "hxf1688";
		String password = "2e5y8hxf";
		for(int i=0; i < 3; i++) {
			Scanner sc = new Scanner(System.in);      //創(chuàng)建sc為輸入類
			System.out.println("請輸入用戶名：");
			String user = sc.nextLine();
			System.out.println("請輸入密碼：");
			String psw = sc.nextLine();
			if(username.equals(user)&&password.equals(psw)) {
				System.out.println("登錄成功");
				break;
			}
			else {
				if(i==2) {
					System.out.println("您輸入錯(cuò)誤次數(shù)過多，請聯(lián)系管理員");
				}
				else {
					System.out.println("輸入失敗，您還有"+(2-i)+"嘗試機(jī)會");
				}
			}
		}
	}
}

StringBuilder s1 = StringBuilder(hello);
s2 = s1.append("world");
(s1 == s2)    //

import java.util.ArrayList;
public class Arraylist1 {
    public static void main(String[] args) {
        ArrayList<String> al =new ArrayList<>();
        al.add("ding");
        al.add("yu");
        al.add("meng");             //增加元素
        al.add(3,"wo"); //增加指定索引元素
        System.out.println(al.remove("wo")); // 刪除元素
        System.out.println(al.remove(2));//  刪除指定索引元素
        System.out.println(al.set(1,"sun"));//  修改指定索引元素
        System.out.println(al.get(0)); // 返回指定索引處的元素
        System.out.println(al.get(1));
        System.out.println(al.size()); //集合長度
        System.out.println(al);
    }
}

import java.util.ArrayList;
import java.util.Scanner;
public class Studenttext {
    public static void main(String[] args) {
        boolean isFlag=true;
        ArrayList<Student> s = new ArrayList<>();
        while (isFlag) {
            System.out.println("------歡迎來到遠(yuǎn)遠(yuǎn)學(xué)生管理軟件------\n");
            System.out.println("      1  添加學(xué)生");
            System.out.println("      2  刪除學(xué)生");
            System.out.println("      3  修改學(xué)生");
            System.out.println("      4  查看所有學(xué)生");
            System.out.println("      5  退出");
            System.out.println("      請輸入你的選擇: ");
            char selection = Student.readMenuSelection();
            switch (selection){
                case '1':
                    addStudent(s);
                    break;
                case '2':
                    removeStudent(s);
                    break;
                case '3':
                    setStudent(s);
                    break;
                case '4':
                    findStudent(s);
                    break;
                case '5':
                    System.out.print("確認(rèn)是否退出(Y/N)：");
                    char isExit = Student.readConfirmSelection();
                    if (isExit == 'Y') {
                        isFlag = false;
                        break;
                    }
            }
        }
    }

    public static void findStudent(ArrayList<Student> s) {
        if (s.size()==0){
            System.out.println("無數(shù)據(jù),請先錄入學(xué)生信息");
            return;
        }
        System.out.println("學(xué)號\t\t姓名\t\t年齡\t\t居住地");
        for (int i = 0; i < s.size() ; i++) {
            Student a=s.get(i);
            System.out.println(a.getSid()+"\t\t"+a.getName()+"\t\t"+a.getAge()+"歲\t"+a.getAddress());
        }
}            

 public static void setStudent(ArrayList<Student> s) {
        Scanner sc = new Scanner(System.in);
        System.out.println("請輸入你要修改的學(xué)生學(xué)號:");
        String sid = sc.nextLine();
        int index = -1;
        for (int i = 0; i < s.size(); i++) {
            Student b = s.get(i);
            if (b.getSid().equals(sid)) {
                index = i;
                break;
            }
        }
        if (index == -1) {
            System.out.println("你輸入的學(xué)號不存在,請重新輸入");
        } else {
            System.out.println("請輸入學(xué)生新姓名");
            String name = sc.nextLine();
            System.out.println("請輸入學(xué)生新年齡");
            String age = sc.nextLine();
            System.out.println("請輸入學(xué)生新居住的");
            String address = sc.nextLine();
            Student a = new Student();
            a.setSid(sid);
            a.setName(name);
            a.setAge(age);
            a.setAddress(address);
            for (int i = 0; i < s.size(); i++) {
                Student b = s.get(i);
                if (b.getSid().equals(sid)) {
                    s.set(i, a);
                    break;
                }
            }
            System.out.println("修改成功");
        }
    }