The provided code defines a Java class, `MyDerivedType`, and creates...

The provided code defines a Java class, MyDerivedType, and creates a static final map vMap within it. Let's break it down:

private static final Map<String, MyDerivedType> vMap =
  Stream.of(MyOrigEnum.values())
        .collect(Collectors.toMap(
          e -> e.getValue() + "",
          MyDerivedType::new,
          (e1, e2) -> e2,
          LinkedHashMap::new
        ));

Here is what the code does step by step:

1. Stream.of(MyOrigEnum.values()):

   • Retrieves all the values of an enum called MyOrigEnum (assumed to be defined elsewhere) by calling the values() method, which returns an array of the enum's constants.
   • Converts this array into a Stream for processing.

2. Collectors.toMap(... arguments ...):

   • Collects the stream elements into a map. The arguments provided to Collectors.toMap control how the keys and values are generated, how to handle collisions, and the type of map to use.

   The specific arguments passed are:

   1. e -> e.getValue() + "":

      • This is a lambda expression that defines how to generate the map's keys. It calls the getValue() method on the enum constants (likely a custom method defined in MyOrigEnum) and converts the result to a String by appending an empty "".

   2. MyDerivedType::new:

      • This is a constructor reference. It defines how to generate the map's values by creating new instances of MyDerivedType using its constructor. Each enum constant from the stream is passed to the constructor of MyDerivedType.

   3. (e1, e2) -> e2:

      • This is a merge function, which resolves key collisions (i.e., when two elements generate the same key). In this case, if there's a collision, e2 (the second value) is used, effectively ignoring e1 (the first value). This handles rare cases where getValue() might return the same value for multiple enum constants.

   4. LinkedHashMap::new:

      • Specifies that the resulting Map should be a LinkedHashMap. This type of map preserves the insertion order of the elements.

3. Final Result:

   • vMap becomes a LinkedHashMap where:
     • The keys are String representations of the getValue() result for each MyOrigEnum constant.
     • The values are new instances of MyDerivedType created for each MyOrigEnum constant.

Summary:

The code creates a Map (vMap) that associates each String representation of MyOrigEnum.getValue() to a corresponding new instance of MyDerivedType. The map is a LinkedHashMap to retain the order of the enum constants as they are encountered.