public static final String s3;

这是一个Java中的常量声明，使用了public static final关键字来定义一个不可修改的字符串类型常量s3。由于使用了final关键字，这个变量值只能被赋值一次，并且一旦被赋值就不能再被修改。使用public关键字则表示这个常量可以被其他类访问，使用static关键字则表示这个常量是一个静态变量，可以通过类名来访问。

相关问题

已定义Student类，如下所示： public class Student implements Serializable{ private static final long serialVersionUID = 1L; //序列化串行标识 private String name; private int age; private String address; public Student(String name, int age, String address) { super(); this.name = name; this.age = age; this.address = address; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } @Override public String toString() { return "Student [name=" + name + ", age=" + age + ", address=" + address + "]"; } } 要求定义5个学生对象，数据如下：学生1：("李四", 25, "河南省郑州市")，学生2：("韩梅梅", 19, "焦作市武陟县")，学生3：("李磊", 20, "郑州市中原区")，学生4：("张明", 21, "修武县方庄")，学生5：("吴林", 18, "开封市祥符区")。 请将这5个学生对象数据写入到“d:\\学生信息.txt”文件中，并在程序中反序列化输出5条学生信息。

代码如下：

import java.io.*;
import java.util.ArrayList;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        // 创建5个学生对象
        Student s1 = new Student("李四", 25, "河南省郑州市");
        Student s2 = new Student("韩梅梅", 19, "焦作市武陟县");
        Student s3 = new Student("李磊", 20, "郑州市中原区");
        Student s4 = new Student("张明", 21, "修武县方庄");
        Student s5 = new Student("吴林", 18, "开封市祥符区");
        // 将学生对象添加到列表中
        students.add(s1);
        students.add(s2);
        students.add(s3);
        students.add(s4);
        students.add(s5);
        try {
            // 将学生对象列表写入文件
            ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("d:\\学生信息.txt"));
            oos.writeObject(students);
            oos.close();
            // 读取文件中的学生对象列表并输出信息
            ObjectInputStream ois = new ObjectInputStream(new FileInputStream("d:\\学生信息.txt"));
            List<Student> studentsRead = (List<Student>) ois.readObject();
            for (Student student : studentsRead) {
                System.out.println(student);
            }
            ois.close();
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

输出结果如下：

Student [name=李四, age=25, address=河南省郑州市]
Student [name=韩梅梅, age=19, address=焦作市武陟县]
Student [name=李磊, age=20, address=郑州市中原区]
Student [name=张明, age=21, address=修武县方庄]
Student [name=吴林, age=18, address=开封市祥符区]

优化下面代码package Y; import java.io.*; import java.net.*; import java.time.*; import java.util.*; public class Shiyan5Server { public static void main(String[] args) { ServerSocket server = null; Sy5ServerThread thread; Socket you = null; while(true) { try { server = new ServerSocket(2010); } catch (IOException e) { System.out.println("正在监听"); // TODO: handle exception } try { System.out.println("等待客户呼叫"); you = server.accept(); System.out.println("客户的地址"+you.getInetAddress()); } catch (Exception e) { // TODO: handle exception System.out.println("正在等待客户"); } if (you != null) { new Sy5ServerThread(you).start(); } } } } class Sy5ServerThread extends Thread{ Socket socket; DataOutputStream out = null; DataInputStream in = null; String s = null; Sy5ServerThread(Socket t) { socket = t; try { out = new DataOutputStream(socket.getOutputStream()); in = new DataInputStream(socket.getInputStream()); } catch (Exception e) { // TODO: handle exception } } public void run() { while(true) { try { String S1 = "几点了?"; String S2 = "今天是几号?"; String S3 = "今天是星期几?"; String st = in.readUTF(); if (st.equals(S1)) { LocalTime time = LocalTime.now(); String Time = String.format("%tR", time); out.writeUTF(Time); } else if(st.equals(S2)) { LocalDate date = LocalDate.now(); String Time = String.format("%tm月%td日", date,date); out.writeUTF(Time); }else if (st.equals(S3)) { LocalDate week = LocalDate.now(); String Time = String.format("%tA", week); out.writeUTF(Time); }else { System.out.println("服务结束"); break; } } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); }} } }

以下是代码的优化：

1. 将ServerSocket的创建放在while循环的外部，否则每次循环都会重新创建一个ServerSocket对象，浪费资源。
2. 在catch语句中添加具体的异常处理代码，避免出现异常时程序无法正常执行。
3. 在Sy5ServerThread类的构造函数中，将out和in的初始化放在try语句块中，避免出现异常时无法正确初始化。
4. 将字符串常量定义为静态常量，避免在运行时重复创建对象，浪费资源。
5. 在while循环中添加try-catch语句，避免出现异常时程序中断。

改进后的代码如下：

package Y;

import java.io.*;
import java.net.*;
import java.time.*;
import java.util.*;

public class Shiyan5Server {

    private static final String S1 = "几点了?";
    private static final String S2 = "今天是几号?";
    private static final String S3 = "今天是星期几?";
    
    public static void main(String[] args) {
        ServerSocket server = null;
        Socket you = null;
        try {
            server = new ServerSocket(2010);
            System.out.println("正在监听");
        } catch (IOException e) {
            e.printStackTrace();
        }
        while (true) {
            try {
                System.out.println("等待客户呼叫");
                you = server.accept();
                System.out.println("客户的地址" + you.getInetAddress());
                new Sy5ServerThread(you).start();
            } catch (Exception e) {
                System.out.println("正在等待客户");
                e.printStackTrace();
            }
        }
    }
}

class Sy5ServerThread extends Thread {

    private static final String S1 = "几点了?";
    private static final String S2 = "今天是几号?";
    private static final String S3 = "今天是星期几?";
    
    private Socket socket;
    private DataOutputStream out;
    private DataInputStream in;

    Sy5ServerThread(Socket t) {
        socket = t;
        try {
            out = new DataOutputStream(socket.getOutputStream());
            in = new DataInputStream(socket.getInputStream());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void run() {
        while (true) {
            try {
                String st = in.readUTF();
                if (st.equals(S1)) {
                    LocalTime time = LocalTime.now();
                    String Time = String.format("%tR", time);
                    out.writeUTF(Time);
                } else if (st.equals(S2)) {
                    LocalDate date = LocalDate.now();
                    String Time = String.format("%tm月%td日", date, date);
                    out.writeUTF(Time);
                } else if (st.equals(S3)) {
                    LocalDate week = LocalDate.now();
                    String Time = String.format("%tA", week);
                    out.writeUTF(Time);
                } else {
                    System.out.println("服务结束");
                    break;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}