Is there a way to override class variables in Java

Question

class Dad
{
    protected static String me = "dad";

    public void printMe()
    {
        System.out.println(me);
    }
}

class Son extends Dad
{
    protected static String me = "son";
}

public void doIt()
{
    new Son().printMe();
}

The function doIt will print "dad". Is there a way to make it print "son"?

Answer 1

Why would you want to override variables when you could easily reassign them in the subClasses.

I follow this pattern to work around the language design. Assume a case where you have a weighty service class in your framework which needs be used in different flavours in multiple derived applications.In that case , the best way to configure the super class logic is by reassigning its 'defining' variables.

public interface ExtensibleService{
void init();
}

public class WeightyLogicService implements ExtensibleService{
    private String directoryPath="c:\hello";

    public void doLogic(){
         //never forget to call init() before invocation or build safeguards
         init();
       //some logic goes here
   }

   public void init(){}    

}

public class WeightyLogicService_myAdaptation extends WeightyLogicService {
   @Override
   public void init(){
    directoryPath="c:\my_hello";
   }

}

Answer 2

If you are going to override it I don't see a valid reason to keep this static. I would suggest the use of abstraction (see example code). :

     public interface Person {
        public abstract String getName();
       //this will be different for each person, so no need to make it concrete
        public abstract void setName(String name);
    }

Now we can add the Dad:

public class Dad implements Person {

    private String name;

    public Dad(String name) {
        setName(name);
    }

    @Override
    public final String getName() {
    return name;
    }

    @Override
    public final void setName(String name) {
        this.name = name;
    }
}

the son:

public class Son implements Person {

    private String name;

    public Son(String name) {
        setName(name);
    }

    @Override
    public final String getName() {
        return name;
    }

    @Override
    public final void setName(String name) {
        this.name = name;
    }
}

and Dad met a nice lady:

public class StepMom implements Person {

    private String name;

    public StepMom(String name) {
        setName(name);
    }

    @Override
    public final String getName() {
        return name;
    }

    @Override
    public final void setName(String name) {
        this.name = name;
    }
}

Looks like we have a family, lets tell the world their names:

public class ConsoleGUI {

    public static void main(String[] args) {
        List<Person> family = new ArrayList<Person>();
        family.add(new Son("Tommy"));
        family.add(new StepMom("Nancy"));
        family.add(new Dad("Dad"));
        for (Person person : family) {
            //using the getName vs printName lets the caller, in this case the
            //ConsoleGUI determine versus being forced to output through the console. 
            System.out.print(person.getName() + " ");
            System.err.print(person.getName() + " ");
            JOptionPane.showMessageDialog(null, person.getName());
    }
}

}

System.out Output : Tommy Nancy Dad
System.err is the same as above(just has red font)
JOption Output:
Tommy then
Nancy then
Dad

Answer 3

Of course using private attributes, and getters and setters would be the recommended thing to do, but I tested the following, and it works... See the comment in the code

class Dad
{
    protected static String me = "dad";

    public void printMe()
    {
        System.out.println(me);
    }
}

class Son extends Dad
{
    protected static String me = "son";

    /* 
    Adding Method printMe() to this class, outputs son 
    even though Attribute me from class Dad can apparently not be overridden
    */

    public void printMe()
    {
        System.out.println(me);
    }
}

class Tester
{
    public static void main(String[] arg)
    {
        new Son().printMe();
    }
}

Sooo ... did I just redefine the rules of inheritance or did I put Oracle into a tricky situation ? To me, protected static String me is clearly overridden, as you can see when you execute this program. Also, it does not make any sense to me why attributes should not be overridable.

Answer 4

https://docs.oracle.com/javase/tutorial/java/IandI/hidevariables.html

It's called Hiding Fields

From the link above

Within a class, a field that has the same name as a field in the superclass hides the superclass's field, even if their types are different. Within the subclass, the field in the superclass cannot be referenced by its simple name. Instead, the field must be accessed through super, which is covered in the next section. Generally speaking, we don't recommend hiding fields as it makes code difficult to read.

Answer 5

You can create a getter and then override that getter. It's particularly useful if the variable you are overriding is a sub-class of itself. Imagine your super class has an Object member but in your sub-class this is now more defined to be an Integer.

class Dad
{
        private static final String me = "dad";

        protected String getMe() {
            return me;
        }

        public void printMe()
        {
                System.out.println(getMe());
        }
}

class Son extends Dad
{
        private static final String me = "son";

        @Override
        protected String getMe() {
            return me;
        }
}

public void doIt()
{
        new Son().printMe(); //Prints "son"
}

Answer 6

This looks like a design flaw.

Remove the static keyword and set the variable for example in the constructor. This way Son just sets the variable to a different value in his constructor.