Java Lambda Streams and Groovy Clouses Comparisons

This Blog post will look at some proverbial operations on List data structure and make some comparison between Java 8/9 and Groovy syntax.  So firstly, the data structure.  It's just a simple Rugby player who has name and a rating.

Java

class RugbyPlayer {
    private String name;
    private Integer rating;
    
    RugbyPlayer(String name, Integer rating) {
        this.name = name;
        this.rating = rating;
    }

    public String toString() {
        return name + "," + rating;
    }
        
    public String getName() {
        return name;
    }
        
    public Integer getRating() {
        return rating;
    }
}

//...
//...
List<RugbyPlayer> players = Arrays.asList(
    new RugbyPlayer("Tadgh Furlong", 9),
    new RugbyPlayer("Bundee AKi", 7),
    new RugbyPlayer("Rory Best", 8),
    new RugbyPlayer("Jacob StockDale", 8)
);

Groovy

@ToString
class RugbyPlayer {
    String name
    Integer rating
}
//...
//...
List<RugbyPlayer> players = [
    new RugbyPlayer(name: "Tadgh Furlong", rating: 9),
    new RugbyPlayer(name: "Bundee AKi", rating: 7),
    new RugbyPlayer(name: "Rory Best", rating: 8),
    new RugbyPlayer(name: "Jacob StockDale", rating: 8)
]

Find a specific record

Java

// Find Tadgh Furlong
Optional<RugbyPlayer> result = players.stream()
    .filter(player -> player.getName().indexOf("Tadgh")  >= 0)
    .findFirst();      
String outputMessage = result.isPresent() ? result.get().toString() : "not found";

System.out.println(outputMessage);

Groovy

println players.find{it.name.indexOf("Tadgh") >= 0}

Comments

• The Java lambda has just one parameter: player.  This doesn't need to be typed as its type can be inferred.  Note: If there were two parameters in the parameter list, parenthesis would be needed around the parameter list.
• In Java, a stream must be created from the List first before the functional operation can be applied.  
• A lambda is then used to before performing a function which returns an Optional
• The lambda definition doesn't need a return statement.  It also doesn't need {} braces or one of those semi-colons to complete a Java statement.  However, you can use {} if you want to and if you want to, you must include the ; and the return statement.  Note: if you lambda is more than one line, you don't get a choice, you must use {}.   It is recommended best practise to keep Lambda's short and to just one line. 
• Java 8 supports fluent APIs for pipeline Stream operations.  This is also supported in Groovy Collection operations.
• In Java a player variable is specified for the Lambda.  The Groovy closure doesn't need to specify a variable.  It can just use "it" which is the implicit reference to the parameter (similar to _ in Scala).  
• The Java filter API takes a parameters of type Predicate.   A Functional Interface means: can be used as the assignment target for a lambda expression or method reference.  Predicate, is type of Functional interface.  It's one abstract method is: boolean test(T t).    In this case, in the lamda, the player corresponds to t.  The body definition should evaluate to a true or a false, in our case player.getName().indexOf("Tadgh") will always evaluate to true or false.  True corresponds to a match. 
• Java 8 has other types of Functional Interfaces:
• Function – it takes one argument and returns a result
• Consumer – it takes one argument and returns no result (represents a side effect)
• Supplier – it takes not argument and returns a result
• Predicate – it takes one argument and returns a boolean
• BiFunction – it takes two arguments and returns a result
• BinaryOperator – it is similar to a BiFunction, taking two arguments and returning a result. The two arguments and the result are all of the same types
• UnaryOperator – it is similar to a Function, taking a single argument and returning a result of the same type
• Java 8 can infer the type for the lambda input parameters. Note if you have to specify the parameter type,  the declaration must be in brackets which adds further verbosity.
• Groovy can println directly.  No System.out needed, and no need for subsequent braces.
• Like Java, Groovy doesn't need the return statement.  However, this isn't just for closures, in Groovy it extends to every method.    Whatever is evaluated as the last line is automatically returned. 
• Groovy has no concept of a Functional interface.  This can mean if you forget to ensure your last expression is an appropriate boolean expression, you get unexpected results and bugs at runtime.
• The arrow operator is used in both Groovy and Java to mean effectively the same thing - separating parameter list from body definition. In Groovy it is only needed it you need to declare the parameters (the default it, doesn't suffice). Note: In Scala, => is used.

 

Find specific records

 

Java

// Find all players with a rating over 8
List<RugbyPlayer> ratedPlayers = players.stream()
    .filter(player -> player.getRating() >= 8)
    .collect(Collectors.toList());
ratedPlayers.forEach(System.out::println);

Groovy

println players.findAll{it.rating >= 8}

Comments

• In the Java version, the iterable object ratedPlayers has its forEach method invoked.   This method takes a FunctionalInterface of type Consumer (see Jdoc here).  Consumer, methods a function which takes an input parameter but returns nothing, it is void.  
• In Java, the stream.filter() will return another stream. Stream.collect() is one of Java 8's Stream terminal methods. It performs mutable fold operations on the data elements held inside the stream instance return by the filter method.  
• Collectors.toList () returns a Collector which collects all Stream elements into a List.
• When using the toList() collector, you can't assume the type of List that will be used.  If you want more control you need to use the toCollection().  For example: 
• .collect(toCollection(LinkedList::new)
• Note: We could have omitted the .collect() operation and invoked forEach straight on the stream.   This would make the Java code shorter.  

players.stream()
   .filter(player -> player.getRating() >= 8)
   .forEach(System.out::println);

• System.out::println is a method reference - a new feature in Java 8.   It is syntactic sugar to reduce the verbosity of some lambdas.  This is essentially saying, for every element in ratedPlayers, execute, System.out.println, passing in the the current element as a parameter.
• Again less syntax from Groovy.  The function can operate on the collection, there is no need to create a Stream.  
• We could have just printed the entire list in the Java sample, but heck I wanted to demo forEach and method reference.

 

Map from object type to another

Java

// Map the Rugby players to just names. 
// Note, the way we convert the list to a stream and then back again to a to a list using the collect API. 
System.out.println("Names only...");
List<String> playerNames = players
    .stream()
    .map(player -> player.getName())
    .collect(Collectors.toList());
playerNames.forEach(System.out::println);

Groovy:

println players.collect{it.name}

Comments

• A stream is needed to be created first before executing the Lambda.  Then the collect() method is invoked on the Stream - this is needed to convert it back to a List. This makes code more verbose. 

• That said, if all you are doing is printing the list, you can just do...

  players.stream()
     .map(player -> player.getName())
     .forEach(System.out::println);
  

 

Perform a Reduction calculation

Java

System.out.println("Max player rating only...");
Optional<Integer> maxRatingOptional = players.
   stream()
   .map(RugbyPlayer::getRating)
   .reduce(Integer::max);
String maxRating = maxRatingOptional.isPresent() ? maxRatingOptional.get().toString() : "No max";
System.out.println("Max rating=" + maxRating);

Groovy

def here = players.inject(null){ 
    max, it -> 
        it.rating > max?.rating ? it : max
} 

Comments

• In the Java version, the reduce operation is invoked on the Stream.  There are three different versions of this method.   In this version, no initial value is specified meaning and an Optional type is returned.  The input parameter of type BinaryOperator.  Because BinaryOperator is a functional interface it means a lamda expression or method reference can be used to specify its value.  In this case, the method reference Integer.max() is used.
• The null safe operator is used in the Groovy inject closure - so that the first comparsion will work 
• In Java, it is possible to avoid the isPresent check on the optional by just doing...

  players.stream()
     .map(RugbyPlayer::getRating)
     .reduce(Integer::max);
     .map(Objects::toString).orElse("No max")
  

Summary

• Groovy is still far more terse
• However, some of the function operations in Java are lazily run.  For example map(), filter() which are considered intermediate.  Intermediate operations produce antoher Stream.  They won't execute unless a terminal function e.g. forEach, collect, reduce is invoked on the stream.  Terminal functions are value or side-effect producing. 
• Intermediate operations can either be stateless or stateful.  Stateless operations like map() or filter() can operate on elements independently. Stateful operations like distinct() or sorted() may incorporate data from previously seen elements.  
• The elements of a stream are only visited once during the life of a stream. Like an Iterator, a new stream must be generated to revisit the same elements of the source.
• In Java processing streams lazily allows for two performances efficiencies:
• fusing of multiple operations to minimise passes of the data
• avoiding examination of the data unless it is necessary.  A stream() may have an infinite number of elements, but a findFirst() or limit() operation will mean that only data that needs to checked will be.
• This may the code more verbose in cases, but it also means it can be more performant.
• Groovy also offers some lazy functions. 

Full Java code here. Full Groovy code here.