Abstract variables in Java?

问题

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...

回答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.

回答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;
}

回答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.

回答4:

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

回答5:

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.

回答6:

As there is no implementation of a variable it can't be abstract ;)

回答7:

Why do you want all subclasses to define the variable? If every subclass is supposed to have it, just define it in the superclass. BTW, given that it's good OOP practice not to expose fields anyway, your question makes even less sense.

回答8:

Change the code to:

public abstract class clsAbstractTable {
  protected String TAG;
  public abstract void init();
}

public class clsContactGroups extends clsAbstractTable {
  public String doSomething() {
    return TAG + "<something else>";
  }
}

That way, all of the classes who inherit this class will have this variable. You can do 200 subclasses and still each one of them will have this variable.

Side note: do not use CAPS as variable name; common wisdom is that all caps identifiers refer to constants, i.e. non-changeable pieces of data.

回答9:

To add per-class metadata, maybe an annotation might be the correct way to go.

However, you can't enforce the presence of an annotation in the interface, just as you can't enforce static members or the existence of a specific constructor.

回答10:

Use enums to force values as well to keep bound checks:

enum Speed {
    HIGH, LOW;
}
private abstract  class SuperClass {
    Speed speed;
    SuperClass(Speed speed) {
        this.speed = speed;
    }
}
private class ChildClass extends SuperClass {
    ChildClass(Speed speed) {
        super(speed);
    }
}

回答11:

In my experiment, Java abstract class does need to specify abstract keyword. Reversely, error that "abstract modifier cannot be put here" will be prompted. You can specify abstract attributes just like ordinary attributes.

public abstract class Duck implements Quackable, Observable {
    // observerList should keep the list of observers watching this duck 
    List<Observer> observerList;

    public AttackBehavior attackBehavior;
    public FlyBehavior flyBehavior;

    public Duck() {
        observerList = new ArrayList<Observer>();
    }
}

And in subclass, you can directly use these attributes this.flyBehavior or this.attackBehavior. You don't need to rewrite the attributes in attribute field.