Java: static field in abstract class
I just start out with an example, that explains it best:
public abstract class A{
static String str;
}
public class B extends A{
public B(){
str = "123";
}
}
public class C extends A{
public C(){
str = "abc";
}
}
public class Main{
public static void main(String[] args){
A b = new B();
A c = new C();
System.out.println("b.str = " + b.str);
System.out.println("c.str = " + c.str);
}
}
This will print out:
b.str = abc
c.str = abc
But I would like a solution where each subclass that instantiate the super class, has their own class variable, at the same time I want to be able to reference that class variable through the identifier, or a method call, defined in the abstract super class.
So I would like the output to be:
b.str = 123
c.str = abc
Is that doable?
Answer
If you want classes B and C to have separate static variables, you'll need to declare the variables in those classes. Basically, static members and polymorphism don't go together.
Note that accessing static members through references is a really bad idea in terms of readability - it makes it look like it depends on the value of the reference, when it doesn't really. So your current code won't even compile when you've moved str down to B and C. Instead, you'll need
System.out.println("b.str = " + B.str);
System.out.println("c.str = " + C.str);
If you really need to access the value polymorphically (i.e. through an instance of A) then one option is to make a polymorphic getter:
public class A {
public abstract String getStr();
}
public class B extends A {
private static String str = "b";
@Override public String getStr() {
return str;
}
}
(and the same for C).
That way you get the behaviour you want in terms of not having a separate variable per instance, but you can still use it polymorphically. It's a little odd for an instance member to return a static value like this, but you're using the value for polymorphism of type, basically...