java通用构造函数 [英] java generic constructors

问题描述

我目前有以下代码从数据库中检索数据，然后创建一个 User 。这个代码在许多我的classe使用创建其他对象，如新闻，注释等...

它使用apache commons dbutils。

  final ResultSetHandler< User> handler = new ResultSetHandler< User>（）{
 
 @Override 
 public用户句柄（ResultSet rs）throws SQLException {
 
用户user = null; 
 if（rs.next（））{
 user = new User（）; 
 user.setId（rs.getInt（id））; 
 user.setUsername（rs.getString（username））; 
 user.setPassword（rs.getString（password））; 
} 
 return user; 
} 
}; 
 
 final用户user = run.query（
SELECT id，username，password FROM users WHERE username =？AND active = 2 LIMIT 1;，handler，
 username） ; 
  

可以包装 QueryRunner 在一个通用类中，并覆盖查询方法，所以处理程序使用ResultSet来实现泛型 T 。我会确保任何 T 类型将构造函数接受 ResultSet 。

像这样：

  public class QueryExecuter< T>扩展QueryRunner {
 private ResultSetHandler< T> _handler; 
 
 public QueryExecuter（）{// T类型用于测试haha 
 super（）; 
 handler = new ResultSetHandler< T>（）{
 
 @Override 
 public T handle（ResultSet rs）throws SQLException {
 
 T object = null ; 
 if（rs.next（））{
 object = new T（rs）; 
} 
 return object; 
} 
}; 
} 
} 
  

我不知道你会明白，

我想到了这个问题，但是我希望这样，我可以使用一个AbstractClass而不是所有的不同的对象将扩展的泛型类型，但似乎我不能写一个抽象构造函数。我必须创建一个静态方法返回对象的实例，如：

  public abstract class DatabaseEntity {
 public static abstract DatabaseEntity create（ResultSet rs）; //即使这样不起作用... 
} 
  

解决方案

可能，是的？但它是一个坏主意。

您可以：

  ResultSetHandler< T> {
 ResultSetHandler< T>（Class< T> clazz）{
 this.clazz = clazz; 
} 
 
 public T handle（ResultSet rs）throws SQLException {
 T object = null; 
 if（rs.next（））{
 object = clazz.getConstructor（ResultSet.class）.newInstance（rs）
} 
 return object; 
} 
} 
  

然而，混合域和数据库是一个坏主意。然而，更好的是定义一个基于结果集创建对象的abtract方法：

 抽象类ResultSetHandler< ; T> {
 
 protected abstract T create（ResultSet rs）; 
 
 public T handle（ResultSet rs）throws SQLException {
 T object = null; 
 if（rs.next（））{
 object = create（rs）; 
} 
 return object; 
} 
} 
  

然后，在实现类中，提供 create（）方法，而不是自己处理结果集，例如：

  h = new ResultSetHandler< Person>（）{
 protected Person create（ResultSet rs）{
 return new Person（rs.getString（name））; 
} 
} 
  

I currently have the following code that retrieves data from the database and then create a User. This code is used in many of my classe to create other objects such as News, Commentsetc...

It uses apache commons dbutils.

final ResultSetHandler<User> handler = new ResultSetHandler<User>() {

            @Override
            public User handle(ResultSet rs) throws SQLException {

                User user = null;
                if (rs.next()) {
                    user = new User();
                    user.setId(rs.getInt("id"));
                    user.setUsername(rs.getString("username"));
                    user.setPassword(rs.getString("password"));
                }
                return user;
            }
        };

        final User user = run.query(
                "SELECT id, username, password FROM users WHERE username = ? AND active = 2 LIMIT 1;", handler,
                username);

Would it be possible to wrap the QueryRunner in a generic class and override the query method so the handler instanciate the generic T with the ResultSet. I would make sure any Ttype would ahve a constructor accepting a ResultSet.

Like so :

        public class QueryExecuter<T> extends QueryRunner {
    private ResultSetHandler<T> _handler;

    public QueryExecuter(){//The T type was for testing haha
        super();
        handler = new ResultSetHandler<T>() {

            @Override
            public T handle(ResultSet rs) throws SQLException {

                T object = null;
                if (rs.next()) {
                    object = new T(rs);
                }
                return object;
            }
        };
    }
}

I don't know if you'll understand, but I hope so, ask me if you want more details or a better explanation.

EDIT

I thought I could use a AbstractClass instead of the generic type that all of the differents objects would extends but it seems like I can't write an abstract constructor. Will I have to make a static method that return an instance of the object like:

public abstract class DatabaseEntity {
    public static abstract DatabaseEntity create(ResultSet rs);//even this doesn't work...
}

解决方案

Possible, yes? But its a bad idea.

You could do:

class ResultSetHandler<T> {
  ResultSetHandler<T>(Class<T> clazz) {
    this.clazz = clazz;
  }

  public T handle(ResultSet rs) throws SQLException {
    T object = null;
    if (rs.next()) {
      object = clazz.getConstructor(ResultSet.class).newInstance(rs)
    }
    return object;
  }
}

Mixing domain and database is a bad idea, however. What would be better, however, would be to define an abtract method that creates the object based on the resultset:

abstract class ResultSetHandler<T> {

  protected abstract T create(ResultSet rs);

  public T handle(ResultSet rs) throws SQLException {
    T object = null;
    if (rs.next()) {
      object = create(rs);
    }
    return object;
  }
}

Then, in your implementing class, you only need to provide a create() method instead of handling the result set yourself, for example:

h = new ResultSetHandler<Person>() {
  protected Person create(ResultSet rs) {
    return new Person(rs.getString("name"));
  }
}

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