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...

Solution 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.

Solution 2

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;
}

Solution 3

No such thing as abstract variables in Java (or C++).

If the parent class has a variable, and a child class extends the parent, then the child doesn't need to implement the variable. It just needs access to the parent's instance. Either get/set or protected access will do.

"...so I should be prompted to implement the abstracts"? If you extend an abstract class and fail to implement an abstract method the compiler will tell you to either implement it or mark the subclass as abstract. That's all the prompting you'll get.

Solution 4

The best you could do is have accessor/mutators for the variable.
Something like getTAG()
That way all implementing classes would have to implement them.

Abstract classes are used to define abstract behaviour not data.

Solution 5

Just add this method to the base class

public abstract class clsAbstractTable {

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

}

Now every class that extends the base class (and does not want to be abstract) should provide a TAG

You could also go with BalusC's answer

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