Java Stream HOW TO .stream()

Question

I am trying and googling and can not get it right. How to write this with stream?

private final ConcurrentMap<UUID, Expression> alleMap = new ConcurrentHashMap<>(1000);

 private String[] getAllDatabases()
  {
     Set<String> allDatabases = new HashSet<>();
     for (Entry<UUID, Expression> entry : alleMap.entrySet())
     {
        allDatabases.add(entry.getValue().getChapter().getDatabaseName());
     }
     List<String> allDatabasesList = new ArrayList<>(allDatabases);
     String[] result = new String[allDatabases.size()];
     for (int i=0; i < result.length; i++)
     {
        result[i] = allDatabasesList.get(i);
     }  
     return result;
  }


alleMap.values().stream().???

The reason I need an Array is that I am writing a Swing Application and need the String[] for a JOptionPane question.

Answer 1

Breaking this down for you:

Set<String> allDatabases = new HashSet<>();
for (Entry<UUID, Expression> entry : alleMap.entrySet()) {
    allDatabases.add(entry.getValue().getChapter().getDatabaseName());
}

This is getting all unique database names for all chapters for expressions that are values in your map.

 List<String> allDatabasesList = new ArrayList<>(allDatabases);
 String[] result = new String[allDatabases.size()];
 for (int i=0; i < result.length; i++) {
    result[i] = allDatabasesList.get(i);
 }  

As far as I can tell this is all just to convert the set of database names to an array. If so, you could have just used allDatabases.toArray(new String[allDatabases.size()] . Streams have a similar method (though they take an IntFunction to create an array rather than the new array):

This translates to:

alleMap.values().stream()
    .map(Expression::getChapter)
    .map(Chapter::getDatabaseName)
    .distinct()
    .toArray(String[]::new);

Note the distinct to replicate the uniqueness of sets.

Answer 2

Starts with not wanting arrays. They are low-level, inflexible constructs that don't play nice with generics and in general shouldn't be used unless you really know what you are doing and are willing to accept a little pain to accomodate it.

Streams are a tool. Not a universal improvement. If there's a pressing need to go with streams, by all means. But don't replace everything with streams because you think that's somehow 'better'. Just do what you're comfy with:)

If you must:

You aren't using the keys at all, just the values. So why bother with entrySet? values() will get what you want with far less effort.

List<String> alleMap.values().stream()
  .map(e -> e.getChapter().getDataBaseName())
  .collect(Collectors.toList());

This is pretty basic; maybe drop the specific case you're working on, and follow some tutorials first, then pick this back up, you'll get the hang of it in no time.