"https://docs.oracle.com/javase/8/docs/api/java/util/Map.html?is-external=true#isEmpty--" title="class or interface in java.util">isEmpty</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Map.html?is-external=true#keySet--" title="class or interface in java.util">keySet</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Map.html?is-external=true#merge-K-V-java.util.function.BiFunction-" title="class or interface in java.util">merge</a>, <a href="https://docs.oracle.com/javase/