Question

What is wrong with this source code?

 1  package Core;
 2  
 3  import java.sql.*;
 4  
 5  public class Course extends Model {
 6      
 7      protected int id;
 8      protected String title;
 9      
10      public Course(int id) {
11          this.id = id;
12      }
13      
14       public static Course getNew(ResultSet res) {
15          try {
16              Course c;
17              c = new Course(res.getInt("course_id"));
18              return c;
19          } catch(SQLException e) {
20              return null;
21          }
22      }
23      
24      @Override
25      public String toString() {
26          return this.title;
27      }
28  }

I’ve set breakpoints on lines 17, 18 and 11 (mentioned in the order of the execution flow), the object is certainly constructed, yet on line 18 it will return a null instead of the object.

What am I missing?

Please don’t tell me how bad static methods are, I know that. Help me understand why is this happenning and how to fix it.

I’m using netbeans + glassfish + java 7 (openjdk) on linux x64.

Addendum

Ok so the problem may be in the way I’m calling it – I’m using reflection and generics.

The caller looks like this

 1  package Core;
 2  
 3  import java.sql.*;
 4  import java.util.*;
 5  import java.lang.reflect.*;
 6  
 7  /**
 8   * Represents storable elements in a database
 9   */
10  public abstract class Model {
11      /**
12       * this attribute is common to all models of the app
13       * this way we can reuse the same connection for
14       * everything (good or bad, depending on the project's requirements; in
15       * our case it's good - this should be a simple application, no mysql
16       * replication and things like that)
17       */
18      protected static Connection conn = null;
19      
20       public static void setConnection(Connection c) {
21          Model.conn = c;
22      }
23       
24       public static void initStatements() throws SQLException {
25           
26       }
27       
28       protected <T extends Model> HashMap<String, Model> resultsetMap(Class<T> cls, ResultSet res) {
29           HashMap<String, Model> data = new HashMap<String, Model>();
30           Method m;
31           try {
32              m = cls.getMethod("getNew", ResultSet.class);
33              
34           } catch(Exception e) {
35               return null;
36           }
37           
38           
39           try {
40               while(res.next()) {
41                   T obj = (T) m.invoke(null, res);
42                   data.put(obj.toString(), obj);
43               }
44           }
45           catch(Exception e) {
46               return null;
47           }
48           return data;
49       }
50       
51       @Override
52       public abstract String toString();
53       
54       public static Model getNew(ResultSet res) {
55           return null;
56       }
57  }

On line 41 I’m calling the above method getNew(). I end up with nulls in the map, both for the keys and the values.

The method on line 28 is called like this

return this.<Course>resultsetMap(Course.class, res);

Sorry for all the mess, I’ve found the (stupid, as usual) cause: the title of the Course class is left unset.

Answer 1

If you reach line 18, then it can’t be null. But if you are debugging an older version, or you just don’t reach line 18, it can be that you enter the catch block with an exception.

So, rule of thumb – don’t discard exceptions. For a start, use ex.printStackTrace() to see it in the console.

Then try directly return new Course(res.getInt("course_id"));