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Lesson 4 Mapping for enums
Objective Describe and use the Java produced by the Mapping for IDL enums.

Java produced by the Mapping for IDL enums

The following section describes how to use the Java produced by the mapping for IDL enums, which is a type that can take one of a given set of values using the IDL of Corba.

Java enum mapping

An IDL enum is a type that can take one of a given set of values. Java mapping makes the possible values accessible as both objects and as ints mapped to the index of the value in the set. This mapping covers both possible ways in which people have typically represented enumerated types in Java.
An IDL enum X is mapped to a Java class also called X. This Java class has a final static int field and a final static X field for every label in the enum. It has a constructor (nonpublic) that takes an int index and creates the corresponding X object (using the constructor). This class also has an instance method value() to get the index int for an X object and a static from -int() method to covert from an int to the corresponding object. In addition to the enum class, helper and holder classes, XHelper and XHolder, are also generated.

Example
Let's look at an example in the following enum-example that defines and uses an enum in IDL and the resulting Java.
ForecastType is mapped to ForecastType.java as well as corresponding ForecastTypeHelper.java and ForecastTypeHolder.java files.


Enums Forecast Type Weather
1) IDL enum ForecastType is mapped to a corresponding Java class
1) IDL enum ForecastType is mapped to a corresponding Java class
File 1: `enum.idl` 
// Enum example - Weather Service
module Module4
{
    enum ForecastType {hot, cold};

    interface WeatherService
    {
        ForecastType getForecast(in string city);
    };
}

File 2: Java class corresponding to the IDL enum 
package Module4;

public final class ForecastType
    implements org.omg.CORBA.portable.IDLEntity
{
    private int value = -1;
    public static final int _hot = 0;
    public static final ForecastType hot = new ForecastType(_hot);
    public static final int _cold = 1;
    public static final ForecastType cold = new ForecastType(_cold);

    public int value() { return value; }

    public static ForecastType from_int(int value)
    {
        switch (value) {
            case _hot: return hot;
            case _cold: return cold;
            default: return null;
        }
    }

    private ForecastType(int i) { value = i; }
}

The IDL file defines an enum and an interface for a weather service module. The Java class implements functionality to map the enum from the IDL file to corresponding Java constants and methods.

2) The enum label hot is mapped to a corresponding public static int value
2) 
package Module4;
public final class ForecastType implements org.omg.CORBA.portable.IDLEntity

The enum label hot is mapped to a corresponding public static int value
The image contains code and comments from two different files:
File 1: `enum.idl` 
// Enum example - Weather Service
module Module4
{
   enum ForecastType {hot, cold};

   interface WeatherService
   {
       ForecastType getForecast(in string city);
   };
}

File 2: Java class corresponding to the IDL enum 
package Module4;

public final class ForecastType
    implements org.omg.CORBA.portable.IDLEntity
{
    private int value = -1;
    public static final int _hot = 0;
    public static final ForecastType hot = new ForecastType(_hot);
    public static final int _cold = 1;
    public static final ForecastType cold = new ForecastType(_cold);

    public int value() { return value; }

    public static ForecastType from_int(int value)
    {
        switch (value) {
            case _hot: return hot;
            case _cold: return cold;
            default: return null;
        }
    }

    private ForecastType(int i) { value = i; }
}

The IDL file defines an enum and an interface for a weather service module. The Java class implements functionality to map the enum from the IDL file to corresponding Java constants and methods.

3) The enum label cold is mapped to a corresponding public static int value and public static object
3) File 1: `enum.idl` 
// Enum example - Weather Service
module Module4
{
    enum ForecastType {hot, cold};

    interface WeatherService
    {
        ForecastType getForecast(in string city);
    };
}

File 2: Java class corresponding to the IDL enum 
package Module4;

public final class ForecastType
    implements org.omg.CORBA.portable.IDLEntity
{
    private int value = -1;
    public static final int _hot = 0;
    public static final ForecastType hot = new ForecastType(_hot);
    public static final int _cold = 1;
    public static final ForecastType cold = new ForecastType(_cold);

    public int value() { return value; }

    public static ForecastType from_int(int value)
    {
        switch (value) {
            case _hot: return hot;
            case _cold: return cold;
            default: return null;
        }
    }

    private ForecastType(int i) { value = i; }
}

The IDL file defines an enum and an interface for a weather service module. The Java class implements functionality to map the enum from the IDL file to corresponding Java constants and methods. The enum label cold is mapped to a corresponding public static int value and public static object. Converting from the int to the object is supported by the from_int() method.


Using enum classes

An important aspect of the enum mapping is that it guarantees that there is only one instance of a given enum object. All the enum objects are the static final members of the class, they are singletons. This means that equality tests will work correctly for enums, and it is such equality testing that lies at the heart of most enum usage. Let us look at a sample client fragment for the above: 

//Initialize ORB ..
  WeatherService wService = null;
  //Obtain reference to remote WeatherService..
  ForecastType nyForecast = wService.getForecast
    ("New York");
  if(nyForecast == ForecastType.hot)
  {
   System.out.println("The big apple is in the oven.");
  }
  // ...

As you can see in the usage of ForecastType.hot, the enum objects can be used simply by referring to the static fields of the enum class.
In the next lesson, we will recap the module thus far and focus on writing Java code based on IDL using sequences, structs, and enums.

idl To Java Mapping for Enums - Quiz

Click on the Quiz link to test your knowledge of IDL-to-Java mapping for enums.
idl To Java Mapping for Enums - Quiz

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