1
public class Box<T> {
2
    private T content;
3

4
    public void set(T content) {
5
        this.content = content;
6
    }
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8
    public T get() {
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        return content;
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    }
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}
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13
// 使用
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Box<String> stringBox = new Box<>();
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stringBox.set("Hello");
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String value = stringBox.get();

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public interface Comparable<T> {
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    int compareTo(T other);
3
}
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5
public class Person implements Comparable<Person> {
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    private String name;
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8
    @Override
9
    public int compareTo(Person other) {
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        return this.name.compareTo(other.name);
11
    }
12
}

1
public class Utility {
2
    public static <T> void swap(T[] array, int i, int j) {
3
        T temp = array[i];
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        array[i] = array[j];
5
        array[j] = temp;
6
    }
7
}
8

9
// 使用
10
String[] names = {"Alice", "Bob", "Charlie"};
11
Utility.swap(names, 0, 2);

1
// 没有泛型的情况 - 容易出错
2
List list = new ArrayList();
3
list.add("Hello");
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list.add(123);
5
String str = (String) list.get(1); // 运行时ClassCastException!
6

7
// 使用泛型 - 编译时检查
8
List<String> stringList = new ArrayList<>();
9
stringList.add("Hello");
10
// stringList.add(123); // 编译错误！
11
String str = stringList.get(0); // 不需要强制转换

1
// 泛型前
2
Map map = new HashMap();
3
map.put("name", "张三");
4
String name = (String) map.get("name"); // 需要强制转换
5

6
// 泛型后
7
Map<String, String> userMap = new HashMap<>();
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userMap.put("name", "张三");
9
String name = userMap.get("name"); // 直接获取，无需转换

1
// 一个泛型类可以处理多种类型
2
Box<Integer> intBox = new Box<>();
3
Box<String> stringBox = new Box<>();
4
Box<Person> personBox = new Box<>();

1
// 用户列表
2
List<User> users = new ArrayList<>();
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users.add(new User("张三", 25));
4
users.add(new User("李四", 30));
5

6
// 配置映射
7
Map<String, String> config = new HashMap<>();
8
config.put("database.url", "jdbc:mysql://localhost:3306/test");
9
config.put("database.username", "root");
10

11
// 响应结果
12
Set<Long> processedIds = new HashSet<>();
13
processedIds.add(1001L);
14
processedIds.add(1002L);

1
// 通用响应类
2
public class ApiResponse<T> {
3
    private int code;
4
    private String message;
5
    private T data;
6

7
    public static <T> ApiResponse<T> success(T data) {
8
        ApiResponse<T> response = new ApiResponse<>();
9
        response.code = 200;
10
        response.message = "成功";
11
        response.data = data;
12
        return response;
13
    }
14

15
    public static <T> ApiResponse<T> error(String message) {
16
        ApiResponse<T> response = new ApiResponse<>();
17
        response.code = 500;
18
        response.message = message;
19
        return response;
20
    }
21

22
    // getter/setter...
23
}
24

25
// 使用示例
26
@RestController
27
public class UserController {
28

29
    @GetMapping("/user/{id}")
30
    public ApiResponse<User> getUser(@PathVariable Long id) {
31
        User user = userService.findById(id);
32
        return ApiResponse.success(user);
33
    }
34

35
    @GetMapping("/users")
36
    public ApiResponse<List<User>> getUsers() {
37
        List<User> users = userService.findAll();
38
        return ApiResponse.success(users);
39
    }
40
}

1
// 通用DAO接口
2
public interface BaseDao<T, ID> {
3
    T save(T entity);
4
    T findById(ID id);
5
    List<T> findAll();
6
    void deleteById(ID id);
7
    List<T> findByExample(T example);
8
}
9

10
// 具体实现
11
@Repository
12
public class UserDao implements BaseDao<User, Long> {
13

14
    @Override
15
    public User save(User user) {
16
        // 保存逻辑
17
        return entityManager.merge(user);
18
    }
19

20
    @Override
21
    public User findById(Long id) {
22
        return entityManager.find(User.class, id);
23
    }
24

25
    // 其他方法实现...
26
}
27

28
// Service层使用
29
@Service
30
public class UserService {
31
    @Autowired
32
    private UserDao userDao;
33

34
    public User createUser(User user) {
35
        return userDao.save(user);
36
    }
37
}

1
public class PageResult<T> {
2
    private List<T> data;        // 当前页数据
3
    private long total;          // 总记录数
4
    private int page;            // 当前页码
5
    private int size;            // 每页大小
6
    private int totalPages;      // 总页数
7

8
    public PageResult(List<T> data, long total, int page, int size) {
9
        this.data = data;
10
        this.total = total;
11
        this.page = page;
12
        this.size = size;
13
        this.totalPages = (int) Math.ceil((double) total / size);
14
    }
15

16
    // getter/setter...
17
}
18

19
// 使用示例
20
@Service
21
public class ProductService {
22

23
    public PageResult<Product> getProducts(int page, int size) {
24
        List<Product> products = productDao.findByPage(page, size);
25
        long total = productDao.count();
26
        return new PageResult<>(products, total, page, size);
27
    }
28
}

1
public class QueryBuilder<T> {
2
    private StringBuilder sql = new StringBuilder();
3
    private List<Object> parameters = new ArrayList<>();
4

5
    public QueryBuilder<T> select(String fields) {
6
        sql.append("SELECT ").append(fields);
7
        return this;
8
    }
9

10
    public QueryBuilder<T> from(String table) {
11
        sql.append(" FROM ").append(table);
12
        return this;
13
    }
14

15
    public QueryBuilder<T> where(String condition, Object value) {
16
        sql.append(" WHERE ").append(condition);
17
        parameters.add(value);
18
        return this;
19
    }
20

21
    public QueryBuilder<T> and(String condition, Object value) {
22
        sql.append(" AND ").append(condition);
23
        parameters.add(value);
24
        return this;
25
    }
26

27
    public String build() {
28
        return sql.toString();
29
    }
30

31
    public List<Object> getParameters() {
32
        return parameters;
33
    }
34
}
35

36
// 使用示例
37
QueryBuilder<User> userQuery = new QueryBuilder<User>()
38
    .select("*")
39
    .from("users")
40
    .where("age > ?", 18)
41
    .and("status = ?", "ACTIVE");
42

43
String sql = userQuery.build();
44
List<Object> params = userQuery.getParameters();