Question

I have a software setup with 2 layers, a core layer and a customer specific layer. The core layer has defined constants which the customer specific layer should be able to extend. More specific:

public class CoreConstants
{ 
  public static long CORE_CONSTANT_1 = 1;
  public static long CORE_CONSTANT_2 = 2;
}

The customer specific layer should be able to add constants which are only used in the customer specific layer. Idea:

public class CustomerConstants extends CoreConstants
{
  public static long CUSTOMER_CONSTANT_1 = 1000; // starting range = 1000
}

Is there a more common way to handle this?

More info: The reason for inheritance is to define the starting range of the customer specific constants. In the CoreConstants class I could set the starting value for customer specific constants. Customer specific constants could then be defined like:

public static long CUSTOMER_CONSTANT_1 = customStartValue + 1;
public static long CUSTOMER_CONSTANT_2 = customStartValue + 2;

Answer 1

Integer constants are generally better replaced with enums, and you can achieve what you want using interfaces on enums.

interface CoreConstant {
    int intValue();
}

enum CoreConstants implements CoreConstant {
    CORE_CONSTANT_1(1),
    CORE_CONSTANT_2(2);
    private final int intValue;
    public CoreConstants(int intValue) { this.intValue = intValue; }
    public int intValue() { return intValue; }
}

interface CustomerConstant extends CoreConstant {}

enum CustomerConstants implements CustomerConstant {
    CUSTOMER_CONSTANT_1(1000);
    private final int intValue;
    public CustomerConstants(int intValue) { this.intValue = intValue; }
    public int intValue() { return intValue; }    
}

You could perhaps improve the design by using delegation within the enums, using an IntConstant class. Unfortunately for your case, you cannot extend an enum. The result is a bit of code duplication in the enum classes.

Otherwise, if you want to stay with the public static int model, then use an interface instead of a class, and finalize the constants.

interface CoreConstants {
    public static final int CORE_CONSTANT_1 = 1;
}