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Modern Java - A Guide to Java 8
This article was originally posted on my blog.
You should also read my Java 11 Tutorial (including new language and API features from Java 9, 10 and 11).
Welcome to my introduction to Java 8. This tutorial guides you step by step through all new language features. Backed by short and simple code samples you'll learn how to use default interface methods, lambda expressions, method references and repeatable annotations. At the end of the article you'll be familiar with the most recent API changes like streams, functional interfaces, map extensions and the new Date API. No walls of text, just a bunch of commented code snippets. Enjoy!
Java 8 enables us to add non-abstract method implementations to interfaces by utilizing the default keyword. This feature is also known as virtual extension methods.
Here is our first example:
interface Formula {
double calculate(int a);
default double sqrt(int a) {
return Math.sqrt(a);
}
}
Besides the abstract method calculate the interface Formula also defines the default method sqrt. Concrete classes only have to implement the abstract method calculate. The default method sqrt can be used out of the box.
Formula formula = new Formula() {
@Override
public double calculate(int a) {
return sqrt(a * 100);
}
};
formula.calculate(100); // 100.0
formula.sqrt(16); // 4.0
The formula is implemented as an anonymous object. The code is quite verbose: 6 lines of code for such a simple calculation of sqrt(a * 100). As we'll see in the next section, there's a much nicer way of implementing single method objects in Java 8.
Let's start with a simple example of how to sort a list of strings in prior versions of Java:
List<String> names = Arrays.asList("peter", "anna", "mike", "xenia");
Collections.sort(names, new Comparator<String>() {
@Override
public int compare(String a, String b) {
return b.compareTo(a);
}
});
The static utility method Collections.sort accepts a list and a comparator in order to sort the elements of the given list. You often find yourself creating anonymous comparators and pass them to the sort method.
Instead of creating anonymous objects all day long, Java 8 comes with a much shorter syntax, lambda expressions:
Collections.sort(names, (String a, String b) -> {
return b.compareTo(a);
});
As you can see the code is much shorter and easier to read. But it gets even shorter:
Collections.sort(names, (String a, String b) -> b.compareTo(a));
For one line method bodies you can skip both the braces {} and the return keyword. But it gets even shorter:
names.sort((a, b) -> b.compareTo(a));
List now has a sort method. Also the java compiler is aware of the parameter types so you can skip them as well. Let's dive deeper into how lambda expressions can be used in the wild.
How does lambda expressions fit into Java's type system? Each lambda corresponds to a given type, specified by an interface. A so called functional interface must contain exactly one abstract method declaration. Each lambda expression of that type will be matched to this abstract method. Since default methods are not abstract you're free to add default methods to your functional interface.
We can use arbitrary interfaces as lambda expressions as long as the interface only contains one abstract method. To ensure that your interface meet the requirements, you should add the @FunctionalInterface annotation. The compiler is aware of this annotation and throws a compiler error as soon as you try to add a second abstract method declaration to the interface.
Example:
@FunctionalInterface
interface Converter<F, T> {
T convert(F from);
}
Converter<String, Integer> converter = (from) -> Integer.valueOf(from);
Integer converted = converter.convert("123");
System.out.println(converted); // 123
Keep in mind that the code is also valid if the @FunctionalInterface annotation would be omitted.
The above example code can be further simplified by utilizing static method references:
Converter<String, Integer> converter = Integer::valueOf;
Integer converted = converter.convert("123");
System.out.println(converted); // 123
Java 8 enables you to pass references of methods or constructors via the :: keyword. The above example shows how to reference a static method. But we can also reference object methods:
class Something {
String startsWith(String s) {
return String.valueOf(s.charAt(0));
}
}
Something something = new Something();
Converter<String, String> converter = something::startsWith;
String converted = converter.convert("Java");
System.out.println(converted); // "J"
Let's see how the :: keyword works for constructors. First we define an example class with different constructors:
class Person {
String firstName;
String lastName;
Person() {}
Person(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
}
Next we specify a person factory interface to be used for creating new persons:
interface PersonFactory<P extends Person> {
P create(String firstName, String lastName);
}
Instead of implementing the factory manually, we glue everything together via constructor references:
PersonFactory<Person> personFactory = Person::new;
Person person = personFactory.create("Peter", "Parker");
We create a reference to the Person constructor via Person::new. The Java compiler automatically chooses the right constructor by matching the signature of PersonFactory.create.
Accessing outer scope variables from lambda expressions is very similar to anonymous objects. You can access final variables from the local outer scope as well as instance fields and static variables.
We can read final local variables from the outer scope of lambda expressions:
final int num = 1;
Converter<Integer, String> stringConverter =
(from) -> String.valueOf(from + num);
stringConverter.convert(2); // 3
But different to anonymous objects the variable num does not have to be declared final. This code is also valid:
int num = 1;
Converter<Integer, String> stringConverter =
(from) -> String.valueOf(from + num);
stringConverter.convert(2); // 3
However num must be implicitly final for the code to compile. The following code does not compile:
int num = 1;
Converter<Integer, String> stringConverter =
(from) -> String.valueOf(from + num);
num = 3;
Writing to num from within the lambda expression is also prohibited.
In contrast to local variables, we have both read and write access to instance fields and static variables from within lambda expressions. This behaviour is well known from anonymous objects.
class Lambda4 {
static int outerStaticNum;
int outerNum;
void testScopes() {
Converter<Integer, String> stringConverter1 = (from) -> {
outerNum = 23;
return String.valueOf(from);
};
Converter<Integer, String> stringConverter2 = (from) -> {
outerStaticNum = 72;
return String.valueOf(from);
};
}
}
Remember the formula example from the first section? Interface Formula defines a default method sqrt which can be accessed from each formula instance including anonymous objects. This does not work with lambda expressions.
Default methods cannot be accessed from within lambda expressions. The following code does not compile:
Formula formula = (a) -> sqrt(a * 100);
The JDK 1.8 API contains many built-in functional interfaces. Some of them are well known from older versions of Java like Comparator or Runnable. Those existing interfaces are extended to enable Lambda support via the @FunctionalInterface annotation.
But the Java 8 API is also full of new functional interfaces to make your life easier. Some of those new interfaces are well known from the Google Guava library. Even if you're familiar with this library you should keep a close eye on how those interfaces are extended by some useful method extensions.
Predicates are boolean-valued functions of one argument. The interface contains various default methods for composing predicates to complex logical terms (and, or, negate)
Predicate<String> predicate = (s) -> s.length() > 0;
predicate.test("foo"); // true
predicate.negate().test("foo"); // false
Predicate<Boolean> nonNull = Objects::nonNull;
Predicate<Boolean> isNull = Objects::isNull;
Predicate<String> isEmpty = String::isEmpty;
Predicate<String> isNotEmpty = isEmpty.negate();
Functions accept one argument and produce a result. Default methods can be used to chain multiple functions together (compose, andThen).
Function<String, Integer> toInteger = Integer::valueOf;
Function<String, String> backToString = toInteger.andThen(String::valueOf);
backToString.apply("123"); // "123"
Suppliers produce a result of a given generic type. Unlike Functions, Suppliers don't accept arguments.
Supplier<Person> personSupplier = Person::new;
personSupplier.get(); // new Person
Consumers represent operations to be performed on a single input argument.
Consumer<Person> greeter = (p) -> System.out.println("Hello, " + p.firstName);
greeter.accept(new Person("Luke", "Skywalker"));
Comparators are well known from older versions of Java. Java 8 adds various default methods to the interface.
Comparator<Person> comparator = (p1, p2) -> p1.firstName.compareTo(p2.firstName);
Person p1 = n