Java bytecode coercion with Java9 and the --release option

Test.java

import java.util.concurrent.ConcurrentHashMap;

public class Test {

  public static void main(String[] args) {
    ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
    map.put("A", "B");
    map.keySet();
  }

}
Java8 and 9:
$> javac Test.java
$> javap -c Test.class
      18: invokevirtual #7                  // Method java/util/concurrent/ConcurrentHashMap.keySet:()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;
(note the KeySetView which only got introduced in Java8!)

Java7 compatibility:
$> javac --release 7 Test.java
$> javap -c Test.class
      18: invokevirtual #7                  // Method java/util/concurrent/ConcurrentHashMap.keySet:()Ljava/util/Set;