Abstract variables in Java?

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

Define a constructor in the abstract class which sets the field so that the concrete implementations are per the specification required to call/override the constructor.

E.g.

public abstract class AbstractTable {
    protected String name;

    public AbstractTable(String name) {
        this.name = name;
    }
}

When you extend AbstractTable, the class won't compile until you add a constructor which calls super("somename").

public class ConcreteTable extends AbstractTable {
    private static final String NAME = "concreteTable";

    public ConcreteTable() {
        super(NAME);
    }
}

This way the implementors are required to set name. This way you can also do (null)checks in the constructor of the abstract class to make it more robust. E.g:

public AbstractTable(String name) {
    if (name == null) throw new NullPointerException("Name may not be null");
    this.name = name;
}