Java 11 Tutorial


Java 11 is around the corner and many people still use Java 8 in production. This example-driven tutorial covers the most important language and API features from Java 9 to 11. No walls of text, so let's dive right into the code. Enjoy!

Local Variable Type Inference

Java 10 has introduced a new language keyword var which optionally replaces the type information when declaring local variables (local means variable declarations inside method bodies).

Prior to Java 10 you would declare variables like this:

String text = "Hello Java 9";

Now you can replace String with var. The compiler infers the correct type from the assignment of the variable. In this case text is of type String:

var text = "Hello Java 10";

Variables declared with var are still statically typed. You cannot reassign incompatible types to such variables. This code snippet does not compile:

var text = "Hello Java 11";
text = 23;  // Incompatible types

You can also use final in conjunction with var to forbid reassigning the variable with another value:

final var text = "Banana";
text = "Joe";   // Cannot assign a value to final variable 'text'

Also var is not allowed when the compiler is incapable of infering the correct type of the variable. All of the following code samples result in compiler errors:

// Cannot infer type:
var a;
var nothing = null;
var lambda = () -> System.out.println("Pity!");
var method = this::someMethod;

Local variable type inference really shines with generics involved. In the next example current has a rather verbose type of Map<String, List<Integer>> which can be reduced to a single var keyword, saving you from typing a lot of boilerplate:

var myList = new ArrayList<Map<String, List<Integer>>>();

for (var current : myList) {
    // current is infered to type: Map<String, List<Integer>>
    System.out.println(current);
}

As of Java 11 the var keyword is also allowed for lambda parameters which enables you to add annotations to those parameters:

Predicate<String> predicate = (@Nullable var a) -> true;

Tip: In Intellij IDEA you can hover on a variable while holding CMD/CTRL to reveal the infered type of the variable (for keyboard junkies press CTRL + J).

HTTP Client

Java 9 introduced a new incubating HttpClient API for dealing with HTTP requests. As of Java 11 this API is now final and available in the standard libraries package java.net. Let's explore what we can do with this API.

The new HttpClient can be used either synchronously or asynchronously. A synchronous request blocks the current thread until the reponse is available. BodyHandlers define the expected type of response body (e.g. as string, byte-array or file):

var request = HttpRequest.newBuilder()
    .uri(URI.create("https://winterbe.com"))
    .GET()
    .build();
var client = HttpClient.newHttpClient();
HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(response.body());

The same request can be performed asynchronously. Calling sendAsync does not block the current thread and instead returns a CompletableFuture to construct asynchronous operation pipelines.

var request = HttpRequest.newBuilder()
    .uri(URI.create("https://winterbe.com"))
    .build();
var client = HttpClient.newHttpClient();
client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
    .thenApply(HttpResponse::body)
    .thenAccept(System.out::println);

We can omit the .GET() call as it's the default request method.

The next example sends data to a given URL via POST. Similiar to BodyHandlers you use BodyPublishers to define the type of data you want to send as body of the request such as strings, byte-arrays, files or input-streams:

var request = HttpRequest.newBuilder()
    .uri(URI.create("https://postman-echo.com/post"))
    .header("Content-Type", "text/plain")
    .POST(HttpRequest.BodyPublishers.ofString("Hi there!"))
    .build();
var client = HttpClient.newHttpClient();
var response = client.send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(response.statusCode());      // 200

The last sample demonstrates how to perform authorization via BASIC-AUTH:

var request = HttpRequest.newBuilder()
    .uri(URI.create("https://postman-echo.com/basic-auth"))
    .build();
var client = HttpClient.newBuilder()
    .authenticator(new Authenticator() {
        @Override
        protected PasswordAuthentication getPasswordAuthentication() {
            return new PasswordAuthentication("postman", "password".toCharArray());
        }
    })
    .build();
var response = client.send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(response.statusCode());      // 200

Collections

Collections such as List, Set and Map have been extended with new methods. List.of created a new immutable list from the given arguments. List.copyOf creates an immutable copy of the list.

var list = List.of("A", "B", "C");
var copy = List.copyOf(list);
System.out.println(list == copy);   // true

Because list is already immutable there's no practical need to actually create a copy of the list-instance, therefore list and copy are the same instance. However if you copy a mutable list, copy is indeed a new instance so it's garanteed there's no side-effects when mutating the original list:

var list = new ArrayList<String>();
var copy = List.copyOf(list);
System.out.println(list == copy);   // false

When creating immutable maps you don't have to create map entries yourself but instead pass keys and values as alternating arguments:

var map = Map.of("A", 1, "B", 2);
System.out.println(map);    // {B=2, A=1}

Immutable collections in Java 11 still use the same interfaces from the old Collection API. However if you try to modify an immutable collection by adding or removing elements, a java.lang.UnsupportedOperationException is thrown. Luckily Intellij IDEA warns via an inspection if you try to mutate immutable collections.

Streams

Streams were introduced in Java 8 and now receive three new methods. Stream.ofNullable constructs a stream from a single element:

Stream.ofNullable(null)
    .count()   // 0

The methods dropWhile and takeWhile both accept a predicate to determine which elements to abandon from the stream:

Stream.of(1, 2, 3, 2, 1)
    .dropWhile(n -> n < 3)
    .collect(Collectors.toList());  // [3, 2, 1]

Stream.of(1, 2, 3, 2, 1)
    .takeWhile(n -> n < 3)
    .collect(Collectors.toList());  // [1, 2]

If you're not yet familiar with Streams you should read my Java 8 Streams Tutorial.

Optionals

Optionals also receive a few quite handy new methods, e.g. you can now simply turn optionals into streams or provide another optional as fallback for an empty optional:

Optional.of("foo").orElseThrow();     // foo
Optional.of("foo").stream().count();  // 1
Optional.ofNullable(null)
    .or(() -> Optional.of("fallback"))
    .get();                           // fallback

Strings

One of the most basic classes String gets a few helper methods for trimming or checking whitespace and for streaming the lines of a string:

" ".isBlank();                // true
" Foo Bar ".strip();          // "Foo Bar"
" Foo Bar ".stripTrailing();  // " Foo Bar"
" Foo Bar ".stripLeading();   // "Foo Bar "
"Java".repeat(3);             // "JavaJavaJava"
"A\nB\nC".lines().count();    // 3

InputStreams

Last but not least InputStream finally gets a super useful method to transfer data to an OutputStream, a usecase that's very common when working with streams of raw data.

var classLoader = ClassLoader.getSystemClassLoader();
var inputStream = classLoader.getResourceAsStream("myFile.txt");
var tempFile = File.createTempFile("myFileCopy", "txt");
try (var outputStream = new FileOutputStream(tempFile)) {
    inputStream.transferTo(outputStream);
}

Other JVM features

These are the - in my opinion - most interesting language new API features when moving from Java 8 to 11. But the feature list doesn't end here. There's a lot more packed into the latest Java releases:

What's your favorite features? Let me know!

Where to go from here?

Many people (including me) are still using Java 8 in production. However as of the beginning of 2019 free support for JDK 8 ends. So this is a good time to migrate to Java 11 now. I wrote a migration guide how to move from Java 8 to 11 which hopefully helps you with your migration. You should also read my Java 8 and Stream API tutorials to learn more modern Java fundamentals. The source code of this tutorial is published on GitHub so feel free to play around with it (and leave a star if you like). You should also follow me on Twitter for more Java- and development-related stuff. Cheers!

Benjamin is Software Engineer, Full Stack Developer at Pondus, an excited runner and table foosball player. Get in touch on Twitter and GitHub.

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