用Java制作一个泛型函数

Question

我有两种方法可以从数据库中获取数据。它们仅在请求和返回类型上有所不同。是否有可能以某种方式将它们组合起来以使代码不会重复？

public ObservableList<Car> getAllCars() {
    ObservableList<Car> list = FXCollections.observableArrayList();
    Connection connection = MyConnection.connection;
    String query = "SELECT * FROM cars";
    try {
        Statement statement = connection.createStatement();
        ResultSet rs = statement.executeQuery(query);
        while (rs.next()) {
            Car car = new Car(rs.getInt("ID"), rs.getInt("STYLE_ID"),
                    rs.getString("MAKE"), rs.getString("MODEL"),
                    rs.getInt("YEAR"), rs.getInt("PRICE"));
            list.add(car);
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return list;
}

public ObservableList<Style> getAllStyles() {
    ObservableList<Style> list = FXCollections.observableArrayList();
    Connection connection = MyConnection.connection;
    String query = "SELECT * FROM styles";
    try {
        Statement statement = connection.createStatement();
        ResultSet rs = statement.executeQuery(query);
        while (rs.next()) {
            Style style = new Style(rs.getInt("ID"), rs.getString("NAME"));
            list.add(style);
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return list;
}

Answer 1

您可以先编写一个函数式接口，表示将ResultSet 转换为某种类型T 的函数：

@FunctionalInterface
interface ResultConverter<T> {
    T convert(ResultSet resultSet) throws SQLException;
}

然后你可以写一个通用函数，接受ResultConverter<T>：

public <T> ObservableList<T> getAll(ResultConverter<T> converter, String query) {
    ObservableList<T> list = FXCollections.observableArrayList();
    Connection connection = MyConnection.connection;
    try {
        Statement statement = connection.createStatement();
        ResultSet rs = statement.executeQuery(query);
        while (rs.next()) {
            T t = converter.convert(rs);
            list.add(t);
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return list;
}

那么你可以将不重复的部分留在getAllStyles和getAllCars中：

// getAllCars:
return getAll(rs -> new Car(rs.getInt("ID"), rs.getInt("STYLE_ID"),
                rs.getString("MAKE"), rs.getString("MODEL"),
                rs.getInt("YEAR"), rs.getInt("PRICE")),
              "SELECT * FROM cars");

// getAllStyles:
return getAll(rs -> new Style(rs.getInt("ID"), rs.getString("NAME")),
              "SELECT * FROM styles");