Specify object type of a returned array list dynamically

问题

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();
    }
}

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.");
       }
    }    
}

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;
    }
}

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 ?

Best Regards, Sandro

回答1:

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>.

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).

回答2:

Please refer Spring's JdbcTemplate class. There is required functionality that you can use even without using Spring at all. Or you can just use it as a helpful guide to build your own persistent layer implementation.

来源：https://stackoverflow.com/questions/15270133/specify-object-type-of-a-returned-array-list-dynamically

标签

java

types

casting

arraylist