RecordMap

RecordMap is a map implementation that mostly supports the java.util.Map API but uses a record RecordEntry instead of Map.Entry for its entry set.

Java 20 introduces the ability to use record pattern in enhanced for but sadly Map.Entry is not a record so this code does not compile

  Map<String, String> map = ...
  for(Map.Entry(var key, var value) : map.entrySet()) {
    ...
  }

At least not yet.

The class RecordMap solves that issue, the following compiles with Java 20

  RecordMap<String, String> map = ...
  for(RecordEntry(var key, var value) : map.entrySet()) {
    ...
  }

Obviously there is a trick somewhere because Map.entrySet() is typed Set<Map.Entry<K,V>> and Set<RecordEntry<K,V>> can not be a subtype of Set<Map.Entry<K, V>> because parametrized types are not covariant in Java.

This works because

• at compile time, the actual code is using a raw type Map and then the signature of the methods are later patched by rewriting the bytecode.
• at runtime, the implementation of the entry set returned by entrySet() throws an UnsupportedoperationException for any methods that takes an entry as parameter.

In practice, it makes RecordMap a valid implementation of Map by making Set<RecordEntry<K,V>> entrySet() an override of Set<Map.Entry<K,V>> entrySet().