动态指定返回数组列表的对象类型 [英] Specify object type of a returned array list dynamically

问题描述

我有以下类，它需要一个接口，并执行一些功能：

I have following class which takes an interface and execute some functions:

public class MSSQLHandler {

    IMSSQLStatement statement;

    public MSSQLHandler(IMSSQLStatement statement) {
        this.statement = statement;
    }

    public void invoke() throws SQLException {
        statement.executeStatement();
    }

    public List<?> getDataList() throws SQLException {
        return statement.getDataList();
    }
}

该接口由一个抽象类实现的：

The interface is implemented by an abstract class:

public abstract class MSSQLStatement implements IMSSQLStatement {

    protected Connection conn = null;
    protected ResultSet rs = null;

    protected abstract String createStatement() throws SQLSyntaxErrorException;

    public MSSQLStatement(Connection conn) {    
       this.conn = conn;
    }

    public void executeStatement() throws SQLException {    
       Statement st = conn.createStatement();
       String sql = createStatement();
       if(sql != null) {
          rs = st.executeQuery(createStatement());
       } else {
          throw new SQLException("Method 'createStatement()' has to be implemented.");
       }
    }    
}

被传递给处理类的类（或接口）从上方扩展抽象类：

The class (or the interface) which is passed to the handler class extend the abstract class from above:

public class MSSQLTaskStatement extends MSSQLStatement {

    public MSSQLTaskStatement(Connection conn) {
    super(conn);
    }

    private String projectName = null;

    public void setProjectName(String projectName) {
    this.projectName = projectName;
    }

    protected String createStatement() throws SQLSyntaxErrorException {
      // Create SQL query
    }

    @Override
    public List<MyObjectData> getDataList() throws SQLException {
      // Wrap results into a data object and save it to an array list
      List<MyObjectData> l = new ArrayList<MyObjectData>()
      while(rs.next()) {
         MyObjectData o = new MyObjectData();
         o.setColumn1(rs.getString(1))
         l.add(o);
      }
      return l;
    }
}

问题是，是否有可能通过对象类型（MyObjectData ）的覆盖的返回列表的 getDataList（）从类方法 MSSQLTaskStatement 来处理程序类公开名单＆LT;＆GT; getDataList（）抛出的SQLException 方法？

The question is whether it is possible to pass the object type (MyObjectData) of the returned list of the overridden getDataList() method from the class MSSQLTaskStatement to the handler class public List<?> getDataList() throws SQLException method ?

最好的问候，
桑德罗

Best Regards, Sandro

推荐答案

添加一个类型参数＆LT; T＆GT; 或＆LT;吨延伸ObjectDataBaseClass＆GT ; 到 IMSSQLStatement 和 MSSQLStatement ，在IMSSQLStatement方法getDataList更改为列表＆LT; T＆GT; getDataList（），并使用公共类MSSQLTaskStatement扩展MSSQLStatement＆LT; MyObjectData方式＆gt;

Add a type parameter <T> or <T extends ObjectDataBaseClass> to IMSSQLStatement and MSSQLStatement, change the method getDataList in IMSSQLStatement to List<T> getDataList() and use public class MSSQLTaskStatement extends MSSQLStatement<MyObjectData>.

然后，如果你的 MSSQLHandler 有一个字段 IMSSQLStatement＆LT; MyObjectData＆GT;声明，其自身的 getDataList（）可以输入，安全地返回列表＆LT; MyObjectData＆GT; （也可以使 MSSQLHandler 通用也一样，如果你想与不建立在 MyObjectData 语句中使用它）

Then, if your MSSQLHandler has a field IMSSQLStatement<MyObjectData> statement, its own getDataList() can type-safely return a List<MyObjectData> (or you can make MSSQLHandler generic too, if you want to use it with statements that do not build on MyObjectData).

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