How to use java8-tutorial: Learn Java 8 Lambdas and Streams

Get the tutorial and what you need to run the examples

The tutorial is read online, and the README also gives a command to clone the repository to your own machine if you want a local copy: git clone https://github.com/winterbe/java8-tutorial.

You need Java installed to run the examples. The source says Java 8 or later is needed for running the examples. The guide itself is written as a read-through, so you do not install it the way you would a software tool.

Learn default methods and lambda expressions

Default methods let an interface include a non-abstract method implementation using the default keyword, so concrete classes only have to implement the abstract method. Lambda expressions replace verbose anonymous classes with a shorter syntax. The tutorial shows how a sort call that once needed an anonymous Comparator shrinks step by step into a single line such as names.sort((a, b) -> b.compareTo(a)).

• Start with the Formula interface example to see a default method in use.
• Compare the anonymous Comparator with the lambda version to see the reduction in code.
• Note that the compiler can infer parameter types, so you can usually leave them out.

Understand functional interfaces and method references

A functional interface contains exactly one abstract method, and each lambda of that type is matched to that method. The tutorial recommends adding the @FunctionalInterface annotation, which makes the compiler report an error if a second abstract method is added, though the code is still valid without it.

Method references using the :: keyword let you pass a static method, an object method or a constructor where a lambda would go. The tutorial shows Integer::valueOf, something::startsWith and Person::new as examples.

Work with lambda scopes

Lambdas can read local variables from the enclosing scope, but those variables must be effectively final. Writing to a local variable from inside a lambda is not allowed. Instance fields and static variables can be both read and written from within a lambda.

• Local variables: read-only, must be implicitly final.
• Instance fields and static variables: read and write access.

Use streams, Optionals and the Date API

The tutorial covers the new API surface, including streams with filter, sorted, map, match, count and reduce operations, parallel streams, Optionals, map extensions and the Date API with Clock, Timezones, LocalTime, LocalDate and LocalDateTime. Each part has its own short commented example that you can read in sequence.

Use the table of contents on the source page to jump to a topic you need, such as Streams or Date API, instead of reading from start to finish.