Java中的方法返回null

Question

I made a class (OracleConnector2Test) that receives as parameter 2 values, makes a query in the base and returns the result of that query in the Eclipse console. In the class (OracleConnector2TestBuscaEstrategias) I instantiate the class (OracleConnector2Test), I pass the parameters ("179", "319") and have Eclipse display the result on the console screen. The values returned are correct, but in the Eclipse console the message:

Query Result: null.

I can not understand why the message (Query Result: null) if the query in the database returns 2 values (Facilities 179 and Consumer.GOV Clear TV 319).

Follow below the code in Java and Print Screen of Console Eclipse.

Class OracleConnector2Test

public class OracleConnector2Test {

String resultado;

public String returnDb(String Id_Estrategia1, String Id_Estrategia2) {
    // Connection URL Syntax: "jdbc:mysql://ipaddress:portnumber/db_name"
    String dbUrl = "jdbc:oracle:thin:@10.5.12.116:1521:desenv01";

    // Database Username
    String username = "bkofficeadm";

    // Database Password
    String password = "bkofficeadmdesenv01";

    // Query to Execute
    String query = "SELECT  DS_ESTRATEGIA, ID_ESTRATEGIA" + " FROM TB_BKOFFICE_ESTRATEGIA"
            + " WHERE ID_ESTRATEGIA  IN (" + Id_Estrategia1 + ", " + Id_Estrategia2 + ")";

    try {
        // Load mysql jdbc driver
        Class.forName("oracle.jdbc.driver.OracleDriver");

        // Create Connection to DB
        Connection con = DriverManager.getConnection(dbUrl, username, password);

        // Create Statement Object
        Statement stmt = con.createStatement();

        // Execute the SQL Query. Store results in ResultSet
        ResultSet rs = stmt.executeQuery(query);

        // While Loop to iterate through all data and print results
        while (rs.next()) {
            String DS_ESTRATEGIA = rs.getString(1);
            String ID_ESTRATEGIA = rs.getString(2);
            System.out.println(DS_ESTRATEGIA + "  " + ID_ESTRATEGIA);
        }
        // closing DB Connection
        con.close();
    } catch (ClassNotFoundException e) {

        e.printStackTrace();

    } catch (SQLException e) {

        e.printStackTrace();

    }
    return resultado;
  }
}

Class OracleConnector2TestBuscaEstrategias

public class OracleConnector2TestBuscaEstrategias {

public static void main(String[] args) {        
        String query;

        OracleConnector2Test t = new OracleConnector2Test();
        query = t.returnDb("179", "319");
        System.out.println("Query Result: " + query);       
   }
}

Answer 1

you did String resultado; then return resultado; never assigning anything to it.

Assign the result to resultdao .

something like this:

StringBuilder sb = new StringBuilder();
while (rs.next()) {
  String DS_ESTRATEGIA = rs.getString(1);
  String ID_ESTRATEGIA = rs.getString(2);
  System.out.println(DS_ESTRATEGIA + "  " + ID_ESTRATEGIA);
  sb.append(DS_ESTRATEGIA + "  " + ID_ESTRATEGIA + "\n");
}
resultdao = sb.toString();

Actually you'd be better off returning a List<String> :

List<String> resultdao = new ArrayList<>();

public List<String> returnDb(String Id_Estrategia1, String Id_Estrategia2) {
  ...
  while (rs.next()) {
    String DS_ESTRATEGIA = rs.getString(1);
    String ID_ESTRATEGIA = rs.getString(2);
    System.out.println(DS_ESTRATEGIA + "  " + ID_ESTRATEGIA);
    resultdao.add(DS_ESTRATEGIA + "  " + ID_ESTRATEGIA);
  }
  ...