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

User · Answer

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

User · Answer

As there is no implementation of a variable it can t be abstract

User · Answer

No  Java doesn t support abstract variables  It doesn t really make a lot of sense  either   What specific change to the  implementation  of a variable to you expect a sub class to do   When I have a abstract String variable in the base class  what should the sub class do to make it non-abstract

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt Observer gt  observerList       public AttackBehavior attackBehavior      public FlyBehavior flyBehavior       public Duck             observerList   new ArrayList lt Observer gt               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

User · Answer

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

User · Answer

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

User · Answer

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     lt something else gt            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

User · Answer

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

User · Answer

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

[java] Abstract variables in Java?

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