Java 8 – Streams and Lambdas

Pipeline programming

Reactive programming is the new rage. Declare your logic and let it go! If you’ve worked with Scala or Ruby, you’re used combining different operations on your collection like mapping, sorting, filtering. When you can create functions (e.g. blocks) on the fly this becomes simple and readable. For instance mutation, filtering and sorting an array in Ruby is one line of:

['b', 'c', 'a', 'ab'].map(&:capitalize).select{|i|i.length == 1}.sort # => ["A", "B", "C"]


Luckily Java 8 also has the ability to create functions (lambdas). They are great for transforming your collections. Where creating a filtered version of a collection involved creating a new collection and a loop over the old one, with lambdas you can often replace this with a short and expressive statement

In it’s true statically half object oriented nature, Java has special functional classes to represent lambdas where primitives play a special part. Defining a function could like something like this:

ToIntFunction<? super String> alwaysOne = s -> 1;

Probably you will mostly use anonymous lambdas on collections. Collections (e.g. List, Map, Set) have new functions specifically designed for this. We’ll get to the Streams in a moment, first a short summary of the collection goodness you get with Java 8:

Java 8 adds these methods to all collections (iterables):

  • forEach – perform some action on each element
  • removeIf –  remove elements for witch the lambda return true

java.util.List also adds:

  • replaceAll – the mutable version of ‘map’ which doesn’t return the new collection (gotta love Java)
  • sort – mutates collection, lambda should return compareTo compatible results (e.g. 0, 1, -1)

java.util.Map adds:

  • computeIfPresent – replace or remove one entry with result of lambda but only if entry already existed
  • computeIfAbsent – sets a map value if it’s not already there (great for lazy initialising)
  • compute – combination of the above functions
  • merge – add or replace entry using the old value as input
  • replaceAll – same as List

Function pointers

Perhaps the weirdest syntactical change in Java 8 is the method reference operator :: In stead of using a plain lambda expression: -> s.toLowerCase)

you can refer to the method by it’s class and name directly:

A bit less verbose but it might not be obvious when to use the reference over the lambda.

Enter Streams

Apart from the useful functions per collection, the is the new functional kid on the Java block. They are designed specifically for lambda operators, parallel processing and chaining multiple transformations together. A simple example will make Java 8 get very close to the Scala/Ruby version:

Arrays.asList("a", "b").stream().map(String::toUpperCase).filter(s -> s.length() == 1).sorted()

A few things to note: generics are here to save the day, ‘s -> s.length’ is only possible because the type is already known. There is some type inference going on, so the input type (String in this case) can differ from the output type, which is very cool. Most notably Stream offers you two functions: map and reduce, renowned functions from the functional world. And since you can use concurrent computation by using parallel streams, you can easily write a fast single machine map/reduce using the stream API. If you’ve worked with Scala or Ruby you’ll realise how profound it is having these two functions available.

Not all lambdas are equal

Streams have two kinds of operations: intermediate (transforming, lazy) and terminal (value producing).  In its try OO style, in Java 8 the responsibility for collecting all those (parallel) transformations into some output is delegated to Collectors. In a not so great OO style but conveniently terminal operations like sum() are available directly on some of the stream types.

Types of Streams


Arrays.asList(“1”, “2”, “3”).stream().map(s -> new Integer(s)).filter(i -> i > 1).collect(Collectors.summingInt(Integer::intValue)); //  5

is equivalent to:

Arrays.asList(“1”, “2”, “3”).stream().mapToInt(Integer::valueOf).filter(i -> i > 1).sum(); //  5

If we weren’t producing Integers but more complex objects, the first variation would make more sense, but the second version creates an IntStream, which has convenient methods that  only make sense on a numeric stream such as sum(). Creating an IntStream directly from a Collection containing Integers is not possible but using the static functions of the IntStream interace (good Lord) you can do:

IntStream.of(1, 2, 3).filter(i -> i > 1).sum();

In short

The new lambdas in Java 8 have already been to good use in the default libraries. Streams are going to make your Java programming life very different, adding fast and expressive ways of filtering and transforming your data. When Java 8 is adopted expect your functions (or your colleagues functions) to start accepting and returning Streams in stead of collections. It will make your programs faster, shorter and more fun to write.

Further reading


Java 8 – Streams and Lambdas