Question

I am coming from c# where this was easy, and possible.

I have this code:

public abstract class clsAbstractTable {

    public abstract String TAG;
    public abstract void init();

}

but Eclipse tells me I use illegal modifier.

I have this class:

public class clsContactGroups extends clsAbstractTable {


}

I want the variable and method defined in such way, that Eclipse to prompt me, I have unimplemented abstract variables and methods.

How do I need to define my abstract class so I should be prompted to implement the abstracts?

EDIT 1

I will create different classes for different db tables. Each class should have it’s own TABLENAME variable, no exception. I have to make sure this variable is static each time when I create a new class that extends the abstract class.

Then in the abstract class I will have a method eg: init();

If in this init() method I call TABLENAME, it should take the value from the sub-class.

something like this should also work out

String tablename=(clsAbstract)objItem.TABLENAME;
// where objItem can be any class that extended clsAbstract;

EDIT 2

I want a constant(static) defined in each class having it’s name defined in abstract.

• I define variable TABLENAME in abstract, but no value given.
• I create a clsContactGroups, I should be prompted to implement TABLENAME, this is where gets some data. eg: TABLENAME=”contactgroups”;
• I create a second class clsContacts, I should be prompted to implement TABLENAME, this is where gets some data. eg: TABLENAME=”contacts”;
  etc…

Answer 1

I think your confusion is with C# properties vs. fields/variables. In C# you cannot define abstract fields, even in an abstract class. You can, however, define abstract properties as these are effectively methods (e.g. compiled to get_TAG() and set_TAG(...)).

As some have reminded, you should never have public fields/variables in your classes, even in C#. Several answers have hinted at what I would recommend, but have not made it clear. You should translate your idea into Java as a JavaBean property, using getTAG(). Then your sub-classes will have to implement this (I also have written a project with table classes that do this).

So you can have an abstract class defined like this…

public abstract class AbstractTable {

    public abstract String getTag();
    public abstract void init();

    ...
}

Then, in any concrete subclasses you would need to define a static final variable (constant) and return that from the getTag(), something like this:

public class SalesTable extends AbstractTable {

    private static final String TABLE_NAME = "Sales";

    public String getTag() {
        return TABLE_NAME;
    }

    public void init() {
        ...
        String tableName = getTag();
        ...
    }

}

EDIT:

You cannot override inherited fields (in either C# or Java). Nor can you override static members, whether they are fields or methods. So this also is the best solution for that. I changed my init method example above to show how this would be used – again, think of the getXXX method as a property.