Abstract variables in Java?

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) {
    Objects.requireNonNull(name, "Name may not be null");
    this.name = name;
}

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.