Java: static field in abstract class

Question

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 1

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...

Answer 2

public abstract class A {     private String str;     public String getStr() { return str;}     protected void setStr(String str) { this.str = str; } } 

Then you'll be able to have

B b = new B(); b.getStr(); 

The setter and getter are my addition, you can go by simply making the variable non-static.

Update If you want to have the static-per-subclass, then you can have:

protected static Map<Class, String> values; public abstract String getValue(); 

and then:

public String getValue() {     values.get(getClass()); } public void setValue(String value) {     values.set(getClass(), value); } 

But this is generally a bad idea.