Eclipse和类导入问题

Question

I am experiencing an odd behaviour using Eclipse: I am trying to use an ArrayList so if I try to import the class and then empoy it, I get a nasty error.

//... bunch of other imports

//Warning: unused import
import java.util.ArrayList;

public class TagHandler {
    public static <ArrayList>Tag getRows(int id) throws SQLException {
        String query = "SELECT * FROM Tag WHERE id = ?";

        //Unexpected type error
        ArrayList<Tag> list = null;

        ResultSet rs = null;

// ... more code ...
}

but if I instantiate an object using the absolute path of the package, Eclipse is ok with that.

//... bunch of other imports

public class TagHandler {
    public static <ArrayList>Tag getRows(int id) throws SQLException {
        String query = "SELECT * FROM Tag WHERE id = ?";

        //Works fine
        java.util.ArrayList<Tag> list = null;

        ResultSet rs = null;

// ... more code ...
}

I have written other classes and I have always imported the ArrayList class and used it without a hitch, so what could possibly be wrong?

Answer 1

You are declaring a generic type parameter for your method that shadows the ArrayList type

public static <ArrayList> Tag getRows(int id) throws SQLException {
               ^

If you use the simple name ArrayList , you are referring to that type variable, not the actual java.util.ArrayList type.

Your method was equivalent to

public static <T> Tag getRows(int id) throws SQLException {
    String query = "SELECT * FROM Tag WHERE id = ?";

    //Unexpected type error
    T<Tag> list = null;

    ResultSet rs = null;

which is not syntactically correct. You probably meant ArrayList<Tag> instead of <ArrayList>Tag .

Answer 2

That is because the declaration of your static method is incorrect

public static <ArrayList>Tag getRows(int id) throws SQLException

should be

public static ArrayList<Tag> getRows(int id) throws SQLException

try fixing your static method and importing ArrayList with an import at the top of the file

import java.util.ArrayList;

public class TagHandler {
  public static ArrayList<Tag> getRows(int id) throws SQLException {
    String query = "SELECT * FROM Tag WHERE id = ?";

    ArrayList<Tag> list = null;

    ResultSet rs = null;

// ... more code ...
}

Answer 3

应该

 public static ArrayList<Tag> getRows(int id) throws SQLException