Building Enums

Overview

The BEnum base class is used to define enumerated types. An enum is composed of a fixed set of int/String pairs called its range. The int identifiers are called ordinals and the String identifiers are called tags. Enum ranges are managed by the BEnumRange class.

There are three subclasses of BEnum. BBoolean is a special case which models a boolean primitive. The BDynamicEnum class is used to manage weakly typed enums which may store any ordinal and range. Strongly typed enums may be defined at compile time by subclassing BFrozenEnum. The Niagara Framework builds a BFrozenEnum's range using the following set of introspection rules:

Meet the common rules applicable to all BObjects;
Meet the common rules applicable to all BSimples (although BFrozenEnum is not required to declare a DEFAULT field);
Define a set of public static final fields which reference instances of the BFrozenEnum's range. Each of these BFrozenEnum must declare a unique ordinal value. By convention ordinals should start at zero and increment by one. Each of these BFrozenEnum must have a type exactly equal to the declaring class.
There can be no way to create other instances of the BFrozenEnum outside of the fields declaring its range. This means no other instances declared in static fields, returned by a static method, or instantiable through non-private constructors.
There must be at least one BFrozenEnum declared in the range.
The default value of a BFrozenEnum is the first instance, by convention with an ordinal value of zero.
By convention a public static final int field is defined for each BFrozenEnum in the range to provide access to the ordinal value.

Example

The following source provides a complete example of the implementation for BOrientation:

  
/*
 * Copyright 2000 Tridium, Inc. All Rights Reserved.
 */
package javax.baja.ui.enum;

import javax.baja.sys.*;

/**
 * BOrientation defines a widget's orientation as
 * either horizontal or vertical.
 */
public final class BOrientation
  extends BFrozenEnum
{
  
  public static final int HORIZONTAL = 0;
  public static final int VERTICAL = 1;
  
  public static final BOrientation horizontal = new BOrientation(HORIZONTAL);
  public static final BOrientation vertical = new BOrientation(VERTICAL);

  public Type getType() { return TYPE; }
  public static final Type TYPE = Sys.loadType(BOrientation.class);
  
  public static BOrientation make(int ordinal)
  {
    return (BOrientation)horizontal.getRange().get(ordinal);
  }
  
  public static BOrientation make(String tag)
  {
    return (BOrientation)horizontal.getRange().get(tag);
  }
  
  private BOrientation(int ordinal) { super(ordinal); }
  
}