CODEWITHSUNDEEP
javaoop
SBmore
SBmore

Reputation: 619

Generating a list of certain instance variables

I'm creating a program that writes data to a database. I've created classes for each table and I wanted the class to be able to create its own INSERT statement. Since the member variables will be the columns of the database I thought it would be a good idea to grab them with obj.getClass().getDeclaredFields() and write them into an INSERT statement. I didn't want to duplicate code so I made a Table class that has a buildSqlInsertQuery() and used that to extend the other classes.

The issue is that there could be other instance variables that I need in these classes (such as tableName) that I don't want to become part of the INSERT statement. I also want to assign values to the columns so I don't think I can just put them in an array without duplicating (so if I add a column I would also need to add it to the array).

Here is what I have at the moment, which will include the tableName in the INSERT query:

Table.java

public class Table {
    public String buildSqlInsertQuery(Object obj, String tableName) {
        Field[] fields = obj.getClass().getDeclaredFields();
        String[] fieldNames = new String[fields.length];
        String[] placeholders = new String[fields.length];
        String placeholder = "?";

        for (int x = 0; x < fields.length; x += 1) {
            fieldNames[x] = fields[x].getName();
            placeholders[x] = placeholder;
        }

        return joinInsertQuery(tableName, fieldNames, placeholders);
    }

    private String joinInsertQuery(String tableName, String[] fieldNames, String[] placeholders) {
        String query = "INSERT INTO " + tableName + " (";
        query += StringUtils.join(fieldNames, ",");
        query += ") VALUES (";
        query += StringUtils.join(placeholders, ",");
        query += ")";

        return query;
    }
}

Addresses.java

public class Addresses extends Table {
    private final static String tableName = "Addresses";
    public String idCustomers;
    public String address1;
    public String address2;
    public String address3;
    public String city;
    public String state;
    public String zip;
    public String country;

    public Addresses() {
        System.out.println(buildSqlInsertQuery(this, this.tableName));
    }
}

Is there a way that I can flag certain instance variables that should be considered a column (such as with an annotation) or am I going about this completely wrong?

Upvotes: 2

Views: 71

Answers (2)

Olivier Meurice
Olivier Meurice

Reputation: 562

You may create your own annotation and annotate your fields. Then you can find annotated fields with reflection.

for (Field field : c.getDeclaredFields()) {
  if (field.isAnnotationPresent(Annotation)) {
    set.add(field);
  }
}

Upvotes: 1

Debojit Saikia
Debojit Saikia

Reputation: 10632

One way, which you have mentioned, is to use annotation to identify the instance variable as table name. Just define an annotation as this:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface TableAnnotation {

}

And annotate the field having the name of the table with this:

public class Addresses {

@TableAnnotation
private String tableName = "Addresses";
public String idCustomers;
public String address1;
...
}

Now in your Table class, use reflection (which you are already doing) to figure out the field with this particular annotation:

if(field.isAnnotationPresent(TableAnnotation.class)){
      System.out.println("This field " + field.getName() + " is the name of   table");
}

Upvotes: 2

Related Questions