Java 入门教程（十二）：Stream API 与综合实战

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// ===== StudentSystem.java 完整可编译 =====
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import java.io.IOException;
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import java.nio.charset.StandardCharsets;
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import java.nio.file.Files;
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import java.nio.file.Path;
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import java.nio.file.StandardOpenOption;
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import java.util.*;
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import java.util.stream.Collectors;
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public class StudentSystem {
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    // —— 数据模型（record + 封装）——
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    record Student(String id, String name, String className, double score) {
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        String toCsv() { return String.join(",", id, name, className, String.valueOf(score)); }
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        static Student fromCsv(String line) {
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            String[] p = line.split(",", -1);
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            if (p.length != 4) return null;
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            try { return new Student(p[0], p[1], p[2], Double.parseDouble(p[3])); }
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            catch (NumberFormatException e) { return null; }
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        }
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    }
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    // —— 业务异常（第九篇）——
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    static class SysException extends RuntimeException {
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        SysException(String msg) { super(msg); }
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    }
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    // —— 服务层：集合 + Stream（第八篇 + 本篇）——
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    static class Service {
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        private final Map<String, Student> data = new LinkedHashMap<>();
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        private final Path file = Path.of("students.csv");
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        Service() { load(); }
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        Student add(String name, String className, double score) {
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            if (score < 0 || score > 100)
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                throw new SysException("成绩必须在 0~100: " + score);
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            String id = "S" + (data.size() + 1001);
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            Student s = new Student(id, name, className, score);
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            data.put(id, s);
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            save();
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            return s;
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        }
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        void remove(String id) {
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            if (data.remove(id) == null)
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                throw new SysException("学号不存在: " + id);
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            save();
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        }
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        List<Student> ranking() {
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            return data.values().stream()
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                    .sorted(Comparator.comparingDouble(Student::score).reversed())
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                    .collect(Collectors.toList());
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        }
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        Map<String, Double> classAverages() {
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            return data.values().stream()
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                    .collect(Collectors.groupingBy(Student::className,
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                             Collectors.averagingDouble(Student::score)));
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        }
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        List<Student> search(String kw) {
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            return data.values().stream()
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                    .filter(s -> s.name().contains(kw) || s.id().contains(kw))
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                    .collect(Collectors.toList());
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        }
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        // —— 持久化（第十篇）——
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        private void save() {
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            try {
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                Files.write(file,
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                        data.values().stream().map(Student::toCsv).toList(),
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                        StandardCharsets.UTF_8,
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                        StandardOpenOption.CREATE,
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                        StandardOpenOption.TRUNCATE_EXISTING);
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            } catch (IOException e) {
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                throw new SysException("保存失败: " + e.getMessage());
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            }
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        }
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        private void load() {
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            if (!Files.exists(file)) return;
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            try {
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                Files.readAllLines(file, StandardCharsets.UTF_8).stream()
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                     .map(Student::fromCsv)
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                     .filter(Objects::nonNull)
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                     .forEach(s -> data.put(s.id(), s));
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            } catch (IOException e) {
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                System.err.println("加载失败: " + e.getMessage());
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            }
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        }
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    }
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    // —— 控制台交互（流程控制 + 异常处理）——
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    public static void main(String[] args) {
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        Service svc = new Service();
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        Scanner sc = new Scanner(System.in);
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        System.out.println("=== 学生管理系统 ===");
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        System.out.println("add 姓名 班级 成绩 | del 学号 | rank | stat | search 关键词 | list | quit");
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        while (true) {
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            System.out.print("\n> ");
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            String line = sc.nextLine().trim();
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            if (line.isEmpty()) continue;
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            String[] p = line.split("\\s+");
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            String cmd = p[0];
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            try {
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                switch (cmd) {
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                    case "quit" -> { System.out.println("再见！"); return; }
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                    case "add" -> {
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                        if (p.length < 4) throw new SysException("用法: add 姓名 班级 成绩");
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                        Student s = svc.add(p[1], p[2], Double.parseDouble(p[3]));
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                        System.out.println("✅ 已添加 " + s.id() + " " + s.name());
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                    }
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                    case "del" -> {
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                        svc.remove(p[1]);
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                        System.out.println("🗑 已删除");
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                    }
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                    case "rank" -> svc.ranking().forEach(s ->
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                            System.out.printf("  %-6s %-6s %-4s %.1f%n",
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                                    s.id(), s.name(), s.className(), s.score()));
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                    case "stat" -> {
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                        svc.classAverages().forEach((cls, avg) ->
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                                System.out.printf("  %s: 平均 %.1f%n", cls, avg));
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                        System.out.println("  全员均分: " +
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                                svc.ranking().stream()
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                                   .mapToDouble(Student::score).average().orElse(0));
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                    }
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                    case "search" -> svc.search(p[1]).forEach(s ->
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                            System.out.println("  " + s));
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                    case "list" -> svc.ranking().forEach(s ->
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                            System.out.println("  " + s));
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                    default -> System.out.println("未知命令");
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                }
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            } catch (SysException e) {
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                System.out.println("❌ " + e.getMessage());
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            } catch (Exception e) {
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                System.out.println("输入格式错误: " + e.getMessage());
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            }
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        }
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    }
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}