The interface .... cannot define an initializer

Question

Why I can't do it:
public interface Constants
   java.util.HashMap BASE_HOLDERS = new java.util.HashMap();
   static
      BASE_HOLDERS.put("key1", "value2");
      BASE_HOLDERS.put("key2", "value3");
and receive: The interface Constants cannot define an initializer?
I don't want to have class (only interface). May you give me and advice?

Answer 1

That said, interface constants are an anti-pattern, if that's the route the OP is heading...
Effective Java - Joshua Bloch, Chapter 4, Item #17: Use interfaces only to define types.
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
In summary, interfaces should only be used to define types. They should not be used to export constants.
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