Why not abstract fields?

Question

Why can't Java classes have abstract fields like they can with abstract methods?

For example: I have two classes that extend the same abstract base class. These two classes each have a method that is identical except for a String constant, which happens to be an error message, within them. If fields could be abstract, I could make this constant abstract and pull the method up into the base class. Instead, I have to create an abstract method, called getErrMsg() in this case, that returns the String, override this method in the two derived classes, and then I can pull up the method (which now calls the abstract method).

Why couldn't I just make the field abstract to begin with? Could Java have been designed to allow this?

Answer 1

You can do what you described by having a final field in your abstract class that is initialised in its constructor (untested code):

abstract class Base {      final String errMsg;      Base(String msg) {         errMsg = msg;     }      abstract String doSomething(); }  class Sub extends Base {      Sub() {         super("Sub message");     }      String doSomething() {          return errMsg + " from something";     } } 

If your child class "forgets" to initialise the final through the super constructor the compiler will give a warning an error, just like when an abstract method is not implemented.

Answer 2

I see no point in that. You can move the function to the abstract class and just override some protected field. I don't know if this works with constants but the effect is the same:

public abstract class Abstract {     protected String errorMsg = "";      public String getErrMsg() {         return this.errorMsg;     } }  public class Foo extends Abstract {     public Foo() {        this.errorMsg = "Foo";     }  }  public class Bar extends Abstract {     public Bar() {        this.errorMsg = "Bar";     } } 

So your point is that you want to enforce the implementation/overriding/whatever of errorMsg in the subclasses? I thought you just wanted to have the method in the base class and didn't know how to deal with the field then.