七、常用工具类

7.1 字符串处理

public class StringDemo {
    public static void main(String[] args) {
        String str = "Hello, Java!";

        // 常用方法
        System.out.println(str.length());           // 长度
        System.out.println(str.charAt(0));          // 获取字符
        System.out.println(str.indexOf("Java"));    // 查找位置
        System.out.println(str.substring(7));       // 截取子串
        System.out.println(str.substring(7, 11));   // 截取区间

        System.out.println(str.toLowerCase());      // 转小写
        System.out.println(str.toUpperCase());      // 转大写
        System.out.println(str.replace("Java", "World"));  // 替换

        System.out.println(str.startsWith("Hello")); // 判断开头
        System.out.println(str.endsWith("!"));       // 判断结尾
        System.out.println(str.contains("Java"));    // 是否包含

        // 字符串分割
        String[] parts = "a,b,c".split(",");

        // 去除空白
        String withSpaces = "  Hello  ";
        System.out.println(withSpaces.trim());

        // 字符串比较
        String s1 = "hello";
        String s2 = "hello";
        System.out.println(s1.equals(s2));      // 内容比较
        System.out.println(s1 == s2);            // 引用比较（不推荐）

        // StringBuilder：可变字符串
        StringBuilder sb = new StringBuilder();
        sb.append("Hello");
        sb.append(" ");
        sb.append("World");
        System.out.println(sb.toString());

        // 字符串拼接性能
        // 慢：使用+拼接大量字符串
        // 快：使用StringBuilder
    }
}

7.2 集合框架

import java.util.*;

public class CollectionDemo {
    public static void main(String[] args) {
        // List：有序可重复
        List<String> list = new ArrayList<>();
        list.add("苹果");
        list.add("香蕉");
        list.add("苹果");

        System.out.println(list.get(0));    // 获取
        System.out.println(list.size());    // 大小

        for (String item : list) {
            System.out.println(item);
        }

        // Set：无序不重复
        Set<String> set = new HashSet<>();
        set.add("苹果");
        set.add("香蕉");
        set.add("苹果");  // 不会重复添加

        System.out.println(set.size());  // 2

        // Map：键值对
        Map<String, Integer> map = new HashMap<>();
        map.put("苹果", 5);
        map.put("香蕉", 3);
        map.put("橙子", 4);

        System.out.println(map.get("苹果"));  // 5
        System.out.println(map.containsKey("香蕉"));  // true

        // 遍历Map
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }

        // 集合工具类
        Collections.sort(list);           // 排序
        Collections.reverse(list);        // 反转
        Collections.shuffle(list);        // 随机打乱
    }
}

7.3 日期时间API

import java.time.*;
import java.time.format.DateTimeFormatter;

public class DateTimeDemo {
    public static void main(String[] args) {
        // 当前日期时间
        LocalDate date = LocalDate.now();
        LocalTime time = LocalTime.now();
        LocalDateTime dateTime = LocalDateTime.now();

        System.out.println(date);    // 2024-01-15
        System.out.println(time);    // 14:30:25.123
        System.out.println(dateTime); // 2024-01-15T14:30:25.123

        // 创建指定日期
        LocalDate birth = LocalDate.of(2000, 1, 1);

        // 日期计算
        LocalDate tomorrow = date.plusDays(1);
        LocalDate nextMonth = date.plusMonths(1);
        LocalDate lastYear = date.minusYears(1);

        // 比较日期
        System.out.println(date.isAfter(birth));   // true
        System.out.println(date.isBefore(birth));  // false

        // 格式化
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        String formatted = dateTime.format(formatter);
        System.out.println(formatted);

        // 解析字符串
        LocalDate parsed = LocalDate.parse("2024-01-15");
    }
}

八、异常处理

8.1 try-catch-finally

import java.io.*;

public class ExceptionDemo {
    public static void main(String[] args) {
        try {
            // 可能抛出异常的代码
            int result = divide(10, 0);
            System.out.println("结果：" + result);

            // 读取文件
            FileReader file = new FileReader("test.txt");
            BufferedReader reader = new BufferedReader(file);
            String line = reader.readLine();
            System.out.println(line);
            reader.close();

        } catch (ArithmeticException e) {
            // 处理算术异常
            System.out.println("算术异常：" + e.getMessage());
        } catch (FileNotFoundException e) {
            // 处理文件未找到异常
            System.out.println("文件未找到：" + e.getMessage());
        } catch (IOException e) {
            // 处理IO异常
            System.out.println("IO异常：" + e.getMessage());
        } catch (Exception e) {
            // 处理其他所有异常
            System.out.println("其他异常：" + e.getMessage());
        } finally {
            // 无论是否异常都会执行
            System.out.println("finally块执行");
        }
    }

    public static int divide(int a, int b) {
        if (b == 0) {
            throw new ArithmeticException("除数不能为0");
        }
        return a / b;
    }
}

8.2 自定义异常

// 自定义异常
class AgeException extends Exception {
    public AgeException(String message) {
        super(message);
    }
}

public class CustomExceptionDemo {
    public static void checkAge(int age) throws AgeException {
        if (age < 0) {
            throw new AgeException("年龄不能为负数");
        }
        if (age > 150) {
            throw new AgeException("年龄不能超过150岁");
        }
        System.out.println("年龄有效：" + age);
    }

    public static void main(String[] args) {
        try {
            checkAge(200);
        } catch (AgeException e) {
            System.out.println("年龄验证失败：" + e.getMessage());
        }
    }
}

九、文件I/O

9.1 文件读写

import java.io.*;
import java.nio.file.*;

public class FileIODemo {
    public static void main(String[] args) {
        // 写入文件
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"))) {
            writer.write("第一行");
            writer.newLine();
            writer.write("第二行");
            writer.newLine();
            System.out.println("写入成功");
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 读取文件
        try (BufferedReader reader = new BufferedReader(new FileReader("output.txt"))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        // NIO方式（Java 7+）
        try {
            // 写入
            Files.write(Paths.get("nio_output.txt"), "Hello NIO".getBytes());

            // 读取
            byte[] data = Files.readAllBytes(Paths.get("nio_output.txt"));
            System.out.println(new String(data));

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

十、实战案例：学生成绩管理系统

import java.util.*;

public class StudentManager {
    // 学生类
    static class Student {
        private int id;
        private String name;
        private Map<String, Double> scores;

        public Student(int id, String name) {
            this.id = id;
            this.name = name;
            this.scores = new HashMap<>();
        }

        public void addScore(String subject, double score) {
            scores.put(subject, score);
        }

        public double getAverage() {
            if (scores.isEmpty()) return 0;
            double sum = 0;
            for (double score : scores.values()) {
                sum += score;
            }
            return sum / scores.size();
        }

        public void display() {
            System.out.println("学号：" + id + "，姓名：" + name);
            System.out.println("成绩：");
            for (Map.Entry<String, Double> entry : scores.entrySet()) {
                System.out.println("  " + entry.getKey() + ": " + entry.getValue());
            }
            System.out.println("平均分：" + String.format("%.2f", getAverage()));
            System.out.println("------------------------");
        }

        // Getters
        public int getId() { return id; }
        public String getName() { return name; }
    }

    // 成绩管理系统
    private Map<Integer, Student> students;
    private Scanner scanner;

    public StudentManager() {
        students = new HashMap<>();
        scanner = new Scanner(System.in);
    }

    public void addStudent() {
        System.out.println("\n--- 添加学生 ---");
        System.out.print("学号：");
        int id = scanner.nextInt();
        scanner.nextLine();  // 消耗换行符
        System.out.print("姓名：");
        String name = scanner.nextLine();

        if (students.containsKey(id)) {
            System.out.println("学号已存在！");
            return;
        }

        Student student = new Student(id, name);
        students.put(id, student);
        System.out.println("学生添加成功！");
    }

    public void addScore() {
        System.out.println("\n--- 添加成绩 ---");
        System.out.print("学号：");
        int id = scanner.nextInt();
        scanner.nextLine();

        Student student = students.get(id);
        if (student == null) {
            System.out.println("学生不存在！");
            return;
        }

        System.out.print("科目：");
        String subject = scanner.nextLine();
        System.out.print("成绩：");
        double score = scanner.nextDouble();
        scanner.nextLine();

        student.addScore(subject, score);
        System.out.println("成绩添加成功！");
    }

    public void displayAll() {
        System.out.println("\n--- 学生列表 ---");
        if (students.isEmpty()) {
            System.out.println("暂无学生数据");
            return;
        }

        for (Student student : students.values()) {
            student.display();
        }
    }

    public void searchStudent() {
        System.out.println("\n--- 搜索学生 ---");
        System.out.print("学号：");
        int id = scanner.nextInt();
        scanner.nextLine();

        Student student = students.get(id);
        if (student == null) {
            System.out.println("学生不存在！");
        } else {
            student.display();
        }
    }

    public void deleteStudent() {
        System.out.println("\n--- 删除学生 ---");
        System.out.print("学号：");
        int id = scanner.nextInt();
        scanner.nextLine();

        Student removed = students.remove(id);
        if (removed != null) {
            System.out.println("学生 " + removed.getName() + " 已删除");
        } else {
            System.out.println("学生不存在！");
        }
    }

    public void showStatistics() {
        System.out.println("\n--- 统计信息 ---");
        if (students.isEmpty()) {
            System.out.println("暂无学生数据");
            return;
        }

        double total = 0;
        double max = 0;
        double min = 100;
        Student bestStudent = null;
        Student worstStudent = null;

        for (Student student : students.values()) {
            double avg = student.getAverage();
            total += avg;
            if (avg > max) {
                max = avg;
                bestStudent = student;
            }
            if (avg < min) {
                min = avg;
                worstStudent = student;
            }
        }

        double classAvg = total / students.size();
        System.out.println("班级人数：" + students.size());
        System.out.println("班级平均分：" + String.format("%.2f", classAvg));
        if (bestStudent != null) {
            System.out.println("最高分：" + bestStudent.getName() + " (" + 
                             String.format("%.2f", max) + "分)");
        }
        if (worstStudent != null) {
            System.out.println("最低分：" + worstStudent.getName() + " (" + 
                             String.format("%.2f", min) + "分)");
        }
    }

    public void run() {
        while (true) {
            System.out.println("\n" + "=".repeat(40));
            System.out.println("学生成绩管理系统");
            System.out.println("=".repeat(40));
            System.out.println("1. 添加学生");
            System.out.println("2. 添加成绩");
            System.out.println("3. 显示所有学生");
            System.out.println("4. 搜索学生");
            System.out.println("5. 删除学生");
            System.out.println("6. 统计信息");
            System.out.println("7. 退出系统");
            System.out.print("请选择操作（1-7）：");

            int choice = scanner.nextInt();
            scanner.nextLine();

            switch (choice) {
                case 1:
                    addStudent();
                    break;
                case 2:
                    addScore();
                    break;
                case 3:
                    displayAll();
                    break;
                case 4:
                    searchStudent();
                    break;
                case 5:
                    deleteStudent();
                    break;
                case 6:
                    showStatistics();
                    break;
                case 7:
                    System.out.println("感谢使用学生成绩管理系统！");
                    return;
                default:
                    System.out.println("无效选择，请重新输入");
            }
        }
    }

    public static void main(String[] args) {
        StudentManager manager = new StudentManager();
        manager.run();
    }
}

对于初学者而言，最重要的是理解面向对象思想和坚持动手实践。不要停留在阅读文档，尽快创建你的第一个Java程序，然后逐步增加功能。随着一个个小项目的完成，你对Java的理解会越来越深入，编写代码也会越来越得心应手。
来源：
https://app-ac8abncezqpt.appmiaoda.com