Java 8 Pills

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Why Java why now

I recently followed a course1 on YouTube by Adib Saikali (NewCircle Instructor) about the key features introduced in Java > 8: it’s an old post regarding old stuff but…I collected some notes (mainly because I need to write down what I’m listening to to stay focused and learn new concepts) that I decided to share with you. This is to say, this post is for every one that had put aside Java for a while and is now looking for a quick overview of the key aspects - exactly like me some weeks ago - to improve his abilities in coding, taking advantage of the old (n.d.r.) features introduced a few years ago. For thus who missed the footnote before and want to jump directly to the lesson, here you can find the main source of the content of the next paragraphs.


Briefly, the basic steps of the Saikali course are: - Lambda expressions; - Functional interfaces; - Method references; - Default methods; - Collections Enhancements; - Streams;

I think that the arguments are really exposed in the perfect order, despite their relations and dependencies: for this reason, I will maintain this order and collect some snippets + doubts + questions arised during the course, except for the first two arguments.

Lambda expression

First of all, a lambda is not a method (or function) but an expression: this is to say, the lambda notation is a way to say to the compiler “take this code and create a method for me”. The concept of lambda is different from its implementation: the lambda expressions exist in many different languages and are implemented in many different ways. However, lambdas have the following characteristics: - a lambda expression define an anonymous functions, - can be assigned to variables, - can be passed to functions, - or returned from functions;

These features and how the compiler process the expressions make lambda expressions excellent to: - form the basis of the functional programming paradigm, - make parallel programming easier, - write more compact code, - create richer data structure collections, - and develop cleaner APIs;

The lambda expression is a concept, that has its own implementation flavor in different languages. But how lambdas are implemented in Java? Look at the code below:

List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

    x -> System.out.println(x)

integers.forEach(x -> {
    int y = x + 10;

forEach is a method that accepts a function as its input and calls the function for each value in the list. x -> System.out.println(x) is a lambda expression that defines an anonymous function with one parameter named x of type Integer. In the second lambda expression are defined multiple lines: moreover, you can create local variable inside the body of the lambda. You can specify (or not) the type for x parameter, because the Java compiler can do type inference. How does it work from a compiler point of view?

Lambda Lifecycle

More or less, the compiler transform the lambda expression in a static function and then call the generated function. The function should be static method, with a class wrapping around, and so on…it doesn’t matter for now. Instead, we are interest in the type of the lambda expressions: remember, they could be assigned to vars, passed to functions, or returned. This implies that lambda has a type. Initially Java engineers thought about a specific new function type, but in the end they didn’t create it. In the next paragraphs I try to cover the question “what is the type of a lambda function?” To do that, we need first to talk about functional interfaces.

Functional interface

A first simple incomplete definition for the functional interface is the following:

A functional interface is an interface with only one method.

I know what you are thinking: what kind of feature is that?! Of course, you can create how many interfaces you want with one method: rather this kind of interface is the most common and used in Java. For instance, the Runnable, Comparable and Callable interface are functional interfaces. They are so popular in lot of libraries, such as in Spring’s libraries: they contains TransactionCallback, RowMapper, StatementCallback and others. Then, they officially decided to call this kind of interface - functional interface - to formalize them inside the language. They also introduced a new optional annotation @FunctionalInterface to make the compiler able to produce an error if more than one method is added to a functional interface.

To conclude, each interface with one method is considered a functional interface by Java 8, even if it was compiled with a Java 1.0 compiler: the new lambda expressions will work with old libraries, without need to recompile. Ok wait a moment: what do lambda expressions have to do with functional interfaces?

List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

Consumer<Integer> consumer = x -> System.out.print(x);


In the example above I declared the type of the lambda expression x -> System.out.print(x); as Consumer<Integer>: for how Java works, this is a Class or an Interface because they are the unique two entities that define a type in Java grammar. In fact, Consumer<Integer> is a functional interface from the package java.util.function package. Why Consumer? The answer is simple, because Consumer<T> define the type of a function that takes an argument of type T and returns void - as System.out.print(x) call used in the one-line body of the lambda expression assigned to the variable consumer.

Back to lambda expressions

The type of the lambda expression is the same as the type of the functional interface that the lambda expression is assigned to.

At this point you could think about lambda expressions as anonymous inner class with one method: they are not (wait for it…).

Variable capture

Look at the code below:

List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

int localVar = 10;

    x -> System.out.println(x + localVar)

A lambda expression can interact with variables defined outside the body of the lambda: using variables outside is called variable capture. But… the value of the variable declared outside must be final. What does it mean? If you copy the code above it will work: why? Because the compiler will handle the lack of the statement final, and consider localVar as final (and also static). The signature generated from the example will be something like this:

public static void generatedName(Integer x, final int var) {
    System.out.println(x + var);

Private field

Look at the code below:

    x -> {
        System.out.println(x + this.var);
        if (this == main) {
            System.out.println("Compiler has passed this of enclosing method as the first parameter of method generated from the lambda expression");

    new Consumer<Integer>() {
        private int state = 10;

        public void accept(Integer x) {
            int y = this.state + Main.this.var + x;
            System.out.println("Anonymous class that implements Consumer Interface and accept overriding: " + y);
The two example show a difference: in the first there is no private field. This is because there is no ability for you to define private fields inside a lambda expression: they are just the body of the signature defined in a functional interface the lambda expression will be assigned to. Remember also that a lambda expression has always a type: this because it can only be assigned to a variable, or passed as a parameter or returned by a method and, in each of these cases, you need a type. What about the this statement? Try the code to see what happens (in any it is explained in the next paragraph).

Lambda (functional typed) vs Anonymous Inner Class

Let summarize the main differences between a lambda expression and an anonymous inner class: - inner classes can have state in the form of class level instance variables lambdas can not, - inner classes can have multiple methods, while lambdas only have a single method body, - this points to the object instance for an anonymous Inner class BUT points to the enclosing object for a lambda;

This is why lambda expressions are basically different from anonymous inner class.

Available functional interfaces

Is it really necessary to create a functional interface for each lambda expression that we want to use in our code? The answer is no, because java.util.function.* package contains 43 (wow!) commonly used functional interfaces:

######┬áBreak point Until now we saw how the functional interface provide a way to expose the lambda expression to outside world and use them in our code. Let’s move one step forward, introducing method references.

Method references

Basically a lambda is a way to define an anonymous function. The question now is: what about functions I already written? Do I have to rewrite them? Look at the following code:

private static void myAlreadyWrittenFunction(Integer i) {

Consumer<Integer> consumerLambda = x -> System.out.println(x);

Consumer<Integer> consumerOldFunction = Main::myAlreadyWrittenFunction;

There’s a new notation with double colons that is the syntax used to reference a method. So, the short answer to the previous question is no, because method references can be used to pass an existing function in a place where a lambda is expected.

Reference a static method

In the consumerOldFunction we are saying to the compiler “use the myAlreadyWrittenFunction as the implementation of the accept method (signature) defined in the functional interface Consumer<Integer>. Obviously, the signature of the referenced method needs to match the signature of functional interface method (and yes, it will handle also overloading and generics). In this first scenario we referenced a static method, but there are four different types of method references: - static method reference, - constructor reference, - specific object instance reference, - and specific arbitrary object of a particular type reference;

Reference a constructor

Look at the following code:

Function<String, Integer> firstMapper = x -> new Integer(x);

Function<String, Integer> secondMapper = Integer::new;

You can use method reference to reference a constructor: this is handy when working with streams (the last topics of this article). With the notation Function<String, Integer> secondMapper = Integer::new; we are asking the compiler to create for us a function that takes as angurment a string, returns an integer and in the body of that method invoque the new constructor of the Integer, passing the string parameter provided to that method. Cool right?!

NOTE: we use the Function<T, R> functional interface provided by the java.util.function.*: it defines a function that takes an argument of Type T and returns a result of type R, as our Integer constructor;

Reference to a specific object instance method

Look at the following code:

Consumer<Integer> firstConsumer = x -> System.out.println(x);

Consumer<Integer> secondConsumer = System.out::println;

With the notation Consumer<Integer> secondConsumer = System.out::println; we tell the compiler that the lambda body signature should match the method println and that the lambda expression should result in a call to System.out.println(x).

Reference to a specific arbitrary object of a particular type

Finally, look at the following code:

Function<String, String> firstMapper = x -> x.toUpperCase();

Function<String, String> secondMapper = String::toUpperCase;

With the notation Function<String, String> secondMapper = String::toUpperCase; we tell the compiler to invoke the toUpperCase method on the parameter that is passed to the lambda expression. So invoking secondMapper.apply("def") will call the lambda expression derived method and inside that will call the toUpperCase method on the parameter “def”

The all idea of the method reference feature is that if you already have a method ready, you can create a lambda using method reference, and then use lambda wherever you want in your code. At this point, we are ready to talk about default methods and, finally, provide a complete definition of the functional interface.

Default methods

Until now I wrote a little bit about lambda expressions and arrow notation -> and method references. A natural place to use lambda expressions is with the Java collections framework. The collection framework is defined with interfaces such as Iterable, Collection, Map, List, Set, etc. Think about that: adding a new method such forEach we used in the previous examples to, let me say, the Iterable interface, will mean that all existing implementations of Iterable will break. Why? Because they don’t implement the forEach method introduced in the interface: so what? All codes already compiled will not work with the new version of Java. Unacceptable. This problem is known as the the interface evolution problem: how can published interfaces be evolved without breaking existing implementations? Default methods provide a solution to this big problem:

A default method defined in a Java Interface<T> has an implementation (provided in the interface) and is inherited by all classes that implement the Interface<T>.

Look at the example:

public class Main implements Matteo {
    public static void main(String[] args) {
        new Main().printMatteo();

interface Matteo {
    default void printMatteo() {

Question: can you override a default method? Yes, of course, simply overriding the method. Furthermore, if you a default method myMethod() defined in an interface A, an interface B that extend A and override the implementation of myMethod() provided by A, if you have a class MyClass that implement interface B, without any implementation of myMethod(), the implementation provided in interface B will be used, or

The closest implementation in hierarchy will be used.

Creating a conflict

Let be A and B two different interfaces that provide two default methods with the same signature. Let be MyClass a class that implements both A and B: if you create this situation the compiler arises an error that sounds like duplicate default methods implementations. How to solve this conflict? Of course, with overriding. Further, you can call the implementation in the interface, with notation A.super.myMethod(). Instead, same level implementation implies a compiler error, and the override is mandatory. NOTE: a default method should not be final…because it doesn’t make so much sense.

What about our functional interface?! Do you remember the old incomplete definition?

A functional interface is an interface with only one method.

The complete definition is

A functional interface is an interface with only one __non-default__ method.

Let’s move one step forward, introducing collections enhancements.

Collections Enhancements

Java 8 uses lambda expressions and default methods to improve the exising Java collections frameworks and add a lot of functions to them. We already seen example of these methods, such as the forEach - an example of internal iteration: it delegates the looping to a library function (such as forEach, as we said), and the loop body processing to a lambda expression. Further, this method is really important because it allows the library function we are delegating to implement the logic needed to execute the iteration on muliple cores, if desired. Some of the new methods provided by the standard library are:

The streams related methods mentioned above are: - default Spliterator spliterator(); - default Stream stream(); - default Stream parallelStream();

But…what is a stream exactly?


Streams are a functional programming desing pattern for processing sequences of elements - sequentially or in parallel. When examining java programs we always run into code along the following lines: - run a database query to get a list of objects, - iterate over the list to compute a single result, - iterate over the list to generate a new data structure such as another list, map, set, etc, - or iterate over the list and …;

Boring. Well, streams are the implementation of a concept (a design pattern) in the same way lambdas are: they can be implemented in many programming languages. In the next paragraphs I talk about - more or less - about how they work in Java (without talking in details about how they are implemented).

int sumOdd =
                     .filter(o -> o % 2 == 1)
                     .mapToInt(o -> o)

In the example the program creates a stream instance from a source (a Java collection), add a filter operation to the stream intermediate operations pipeline, add a map operation to the stream intermediate operations pipeline and add a terminal operation (sum) that kicks off the stream processing: what the hell is going on?! Let’s first talk about a stream composition.

The stream composition

A stream has three elements: - a source that the stream can pull objects from, - a pipeline of operations that will execute on the elements of the stream, - a terminal operation that pulls values down the stream;

The stream lifecycle

A stream lifecycle has the following steps: - a creation step, ih which a stream get created from a source object such as a collection, file, or generator, - a configuration step, in which a stream get configured with a collection of pipeline operations, - a execution step, done when stream terminal operation is invoked which starts pulling objects trough the operations pipeline of the stream, - and a cleanup step, in which stream can only be used once;

It is important to remember that stream execution is lazy, that means that until you call a terminal operation the stream doesn’t do anythings. It doesn’t loop.

Stream Sources

There are different type of streams: in the next paragraphs I show same simple examples of different streams.

Number Stream Source

Look at the code:

System.out.print("Long Stream Source\n");

LongStream.range(0, 5).forEach(System.out::println);

The example shows a range stream from 0 to 5, starting from a LongSource from* package in standard library. Remember that a stream can be anything: a stream is a design pattern. What it does? It splits out elements from a collection.

String Stream Source

Look at the code:

List<String> cities = Arrays.asList("toronto", "ottawa", "montreal", "vancouver");;

The example shows a stream from a list of String.

Collection Stream Source

Look at the code:

long length = "ABC".chars().count();

Consumer<Long> printer = System.out::println;


A character stream source is called using chars() and then count().

File System Streams

Look at the code:

String workDir = System.getProperty("user.dir");
Path workDirPath = FileSystems.getDefault().getPath(workDir);

System.out.println("Directory Listing Stream");

System.out.println("Depth First Directory Walking Stream");

The line Files.list(Path object from Java 7) gives a stream to work directly on list. The line Files.walk(Path object from Java 7) gives a stream to work directly on list with depth first search.

A first summary example

Before going on, try to understand what the code below does:

String workDir = System.getProperty("user.dir");
Path workDirPath = FileSystems.getDefault().getPath(workDir);

String className = Main.class.getName().replace(".", "/") + ".java";

Files.find(workDirPath, 10,
    (fileName, attributes) -> fileName.endsWith(className)).forEach(path -> {
      try {
      } catch (Exception e) {}

Did you understand what the code does? The example simply print out it self. Briefly, the call to find method returns a stream, so it is possible to call forEach method on it: we know that the forEach terminal does something to each things that it founds. The things it founds are pointed by path variable, passed as parameter to the lambda expression in the brackets. Inside this, we use another stream of Files called lines that out each lines of the file as a stream: once again, the call to forEach print out each of the elements splitted out by the lines stream.

How about stream terminals?

We said that until the terminal operation is invoked, the stream doesn’t do anything. You might be thinking “what kind of terminal could I use to execute my stream pipeline?“. There are many types of terminals for stream:

But remember: nothing happens until the terminal operation is invoked (think about updating, and so on).

Reduction terminal operations

Look at the following:

List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

Optional<Integer> result;
result =, y) -> x - y);


result = -> x));

Integer res =, (x, y) -> x + y);

Can you explain the min? Actually, I didn’t understand the min: you can write a comment below if you want XD.

Mutable reduction operations

Look at the following:

List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

Set<Integer> result =;

Integer[] a =[]::new);;

Integer res =, (x, y) -> x + y);

Collectors are used to get all the things that came out of a stream and collect them into a structure: the Collectors class defines many useful collectors such as List, Set, Map, groupingBy, partitioningBy, etc.

Search terminal operation

Look at the following:

List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

Optional<Integer> result =;

Boolean res = -> x == 5);

res = -> x > 3);

Optional<Integer> anyres =;

Note the findAny: result of findAny call is unpredictable. In a sense, if stream is executed, for instance, parallel, then the first call that ends will return the result so you don’t know exactly a priori which will be the returned value. It just means “find any”.

Generic terminal operation

forEach is an example of generic terminal operation.

Streams pipeline rules

What about streams pipeline call? - streams can process elements sequentially; - streams can process elements in parallel;

Thus means that streams operations are not allowed to modify the stream source otherwise bad things happens XD. It is simple: a stream operate over a source. Don’t modify the source! sounds like a best practise.

Intermediate stream operations

There are two classes of intermediate stream operations:

For instance, if you want a stream to sort elements you obviously needs to know the order of sorting. Instead, if you have a parallel operations with threads that compete with each others, you are not interested in state of each elements. Let’s have a look at sample example: -> x < 4).forEach(System.out::println);

integers = -> x + x).collect(Collectors.toList());

IntSummaryStatistics intSummaryStatistics = -> x).summaryStatistics();

A repo with all the examples in the article is here and the main source is here.

Thank you everybody for reading!

  1. The course is entitled Java 8 Lambda Expressions & Streams and is available for free here. Thank you again Adib Saikali for your lesson! [return]

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