Java 8 简明教程

Java 8 Tutorial

转载:http://winterbe.com/posts/2014/03/16/java-8-tutorial/

March 16, 2014

“Java is still not dead—and people are starting to figure that out.”

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!

Default Methods for Interfaces

Java 8 enables us to add non-abstract method implementations to interfaces by utilizing the default keyword. This feature is also known as 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 calucation 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.

Lambda expressions

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 more shorter:

Collections.sort(names, (a, b) -> b.compareTo(a));

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.

Functional Interfaces

How does lambda expressions fit into Javas 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 ommited.

Method and Constructor References

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 bean 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.

Lambda Scopes

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.

Accessing local 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.

Accessing fields and static variables

In constrast 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);
        };
    }
}

Accessing Default Interface Methods

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);

Built-in Functional Interfaces

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

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

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

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

Consumers represents 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

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 = new Person("John", "Doe");
Person p2 = new Person("Alice", "Wonderland");

comparator.compare(p1, p2);             // > 0
comparator.reversed().compare(p1, p2);  // < 0

Optionals

Optionals are not functional interfaces, instead it's a nifty utility to prevent NullPointerException. It's an important concept for the next section, so let's have a quick look at how Optionals work.

Optional is a simple container for a value which may be null or non-null. Think of a method which may return a non-null result but sometimes return nothing. Instead of returning null you return an Optional in Java 8.

Optional<String> optional = Optional.of("bam");

optional.isPresent();           // true
optional.get();                 // "bam"
optional.orElse("fallback");    // "bam"

optional.ifPresent((s) -> System.out.println(s.charAt(0)));     // "b"

Streams

A java.util.Stream represents a sequence of elements on which one or more operations can be performed. Stream operations are either intermediate or terminal. While terminal operations return a result of a certain type, intermediate operations return the stream itself so you can chain multiple method calls in a row. Streams are created on a source, e.g. a java.util.Collection like lists or sets (maps are not supported). Stream operations can either be executed sequential or parallel.

Let's first look how sequential streams work. First we create a sample source in form of a list of strings:

List<String> stringCollection = new ArrayList<>();
stringCollection.add("ddd2");
stringCollection.add("aaa2");
stringCollection.add("bbb1");
stringCollection.add("aaa1");
stringCollection.add("bbb3");
stringCollection.add("ccc");
stringCollection.add("bbb2");
stringCollection.add("ddd1");

Collections in Java 8 are extended so you can simply create streams either by calling Collection.stream() or Collection.parallelStream(). The following sections explain the most common stream operations.

Filter

Filter accepts a predicate to filter all elements of the stream. This operation is intermediate which enables us to call another stream operation (forEach) on the result. ForEach accepts a consumer to be executed for each element in the filtered stream. ForEach is a terminal operation. It's void, so we cannot call another stream operation.

stringCollection
    .stream()
    .filter((s) -> s.startsWith("a"))
    .forEach(System.out::println);

// "aaa2", "aaa1"

Sorted

Sorted is an intermediate operation which returns a sorted view of the stream. The elements are sorted in natural order unless you pass a custom Comparator.

stringCollection
    .stream()
    .sorted()
    .filter((s) -> s.startsWith("a"))
    .forEach(System.out::println);

// "aaa1", "aaa2"

Keep in mind that sorted does only create a sorted view of the stream without manipulating the ordering of the backed collection. The ordering of stringCollection is untouched:

System.out.println(stringCollection);
// ddd2, aaa2, bbb1, aaa1, bbb3, ccc, bbb2, ddd1

Map

The intermediate operation map converts each element into another object via the given function. The following example converts each string into an upper-cased string. But you can also use map to transform each object into another type. The generic type of the resulting stream depends on the generic type of the function you pass to map.

stringCollection
    .stream()
    .map(String::toUpperCase)
    .sorted((a, b) -> b.compareTo(a))
    .forEach(System.out::println);

// "DDD2", "DDD1", "CCC", "BBB3", "BBB2", "AAA2", "AAA1"

Match

Various matching operations can be used to check whether a certain predicate matches the stream. All of those operations are terminal and return a boolean result.

boolean anyStartsWithA = 
    stringCollection
        .stream()
        .anyMatch((s) -> s.startsWith("a"));

System.out.println(anyStartsWithA);      // true

boolean allStartsWithA = 
    stringCollection
        .stream()
        .allMatch((s) -> s.startsWith("a"));

System.out.println(allStartsWithA);      // false

boolean noneStartsWithZ = 
    stringCollection
        .stream()
        .noneMatch((s) -> s.startsWith("z"));

System.out.println(noneStartsWithZ);      // true

Count

Count is a terminal operation returning the number of elements in the stream as a long.

long startsWithB = 
    stringCollection
        .stream()
        .filter((s) -> s.startsWith("b"))
        .count();

System.out.println(startsWithB);    // 3

Reduce

This terminal operation performs a reduction on the elements of the stream with the given function. The result is an Optional holding the reduced value.

Optional<String> reduced =
    stringCollection
        .stream()
        .sorted()
        .reduce((s1, s2) -> s1 + "#" + s2);

reduced.ifPresent(System.out::println);
// "aaa1#aaa2#bbb1#bbb2#bbb3#ccc#ddd1#ddd2"

Parallel Streams

As mentioned above streams can be either sequential or parallel. Operations on sequential streams are performed on a single thread while operations on parallel streams are performed concurrent on multiple threads.

The following example demonstrates how easy it is to increase the performance by using parallel streams.

First we create a large list of unique elements:

int max = 1000000;
List<String> values = new ArrayList<>(max);
for (int i = 0; i < max; i++) {
    UUID uuid = UUID.randomUUID();
    values.add(uuid.toString());
}

Now we measure the time it takes to sort a stream of this collection.

Sequential Sort

long t0 = System.nanoTime();

long count = values.stream().sorted().count();
System.out.println(count);

long t1 = System.nanoTime();

long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
System.out.println(String.format("sequential sort took: %d ms", millis));

// sequential sort took: 899 ms

Parallel Sort

long t0 = System.nanoTime();

long count = values.parallelStream().sorted().count();
System.out.println(count);

long t1 = System.nanoTime();

long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
System.out.println(String.format("parallel sort took: %d ms", millis));

// parallel sort took: 472 ms

As you can see both code snippets are almost identical but the parallel sort is roughly 50% faster. All you have to do is change stream() to parallelStream().

Map

As already mentioned maps don't support streams. Instead maps now support various new and useful methods for doing common tasks.

Map<Integer, String> map = new HashMap<>();

for (int i = 0; i < 10; i++) {
    map.putIfAbsent(i, "val" + i);
}

map.forEach((id, val) -> System.out.println(val));

The above code should be self-explaining: putIfAbsent prevents us from writing additional if null checks; forEach accepts a consumer to perform operations for each value of the map.

This example shows how to compute code on the map by utilizing functions:

map.computeIfPresent(3, (num, val) -> val + num);
map.get(3);             // val33

map.computeIfPresent(9, (num, val) -> null);
map.containsKey(9);     // false

map.computeIfAbsent(23, num -> "val" + num);
map.containsKey(23);    // true

map.computeIfAbsent(3, num -> "bam");
map.get(3);             // val33

Next, we learn how to remove entries for a a given key, only if it's currently mapped to a given value:

map.remove(3, "val3");
map.get(3);             // val33

map.remove(3, "val33");
map.get(3);             // null

Another helpful method:

map.getOrDefault(42, "not found");  // not found

Merging entries of a map is quite easy:

map.merge(9, "val9", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9

map.merge(9, "concat", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9concat

Merge either put the key/value into the map if no entry for the key exists, or the merging function will be called to change the existing value.

Date API

Java 8 contains a brand new date and time API under the package java.time. The new Date API is comparable with the Joda-Time library, however it's not the same. The following examples cover the most important parts of this new API.

Clock

Clock provides access to the current date and time. Clocks are aware of a timezone and may be used instead of System.currentTimeMillis() to retrieve the current milliseconds. Such an instantaneous point on the time-line is also represented by the class Instant. Instants can be used to create legacy java.util.Date objects.

Clock clock = Clock.systemDefaultZone();
long millis = clock.millis();

Instant instant = clock.instant();
Date legacyDate = Date.from(instant);   // legacy java.util.Date

Timezones

Timezones are represented by a ZoneId. They can easily be accessed via static factory methods. Timezones define the offsets which are important to convert between instants and local dates and times.

System.out.println(ZoneId.getAvailableZoneIds());
// prints all available timezone ids

ZoneId zone1 = ZoneId.of("Europe/Berlin");
ZoneId zone2 = ZoneId.of("Brazil/East");
System.out.println(zone1.getRules());
System.out.println(zone2.getRules());

// ZoneRules[currentStandardOffset=+01:00]
// ZoneRules[currentStandardOffset=-03:00]

LocalTime

LocalTime represents a time without a timezone, e.g. 10pm or 17:30:15. The following example creates two local times for the timezones defined above. Then we compare both times and calculate the difference in hours and minutes between both times.

LocalTime now1 = LocalTime.now(zone1);
LocalTime now2 = LocalTime.now(zone2);

System.out.println(now1.isBefore(now2));  // false

long hoursBetween = ChronoUnit.HOURS.between(now1, now2);
long minutesBetween = ChronoUnit.MINUTES.between(now1, now2);

System.out.println(hoursBetween);       // -3
System.out.println(minutesBetween);     // -239

LocalTime comes with various factory method to simplify the creation of new instances, including parsing of time strings.

LocalTime late = LocalTime.of(23, 59, 59);
System.out.println(late);       // 23:59:59

DateTimeFormatter germanFormatter =
    DateTimeFormatter
        .ofLocalizedTime(FormatStyle.SHORT)
        .withLocale(Locale.GERMAN);

LocalTime leetTime = LocalTime.parse("13:37", germanFormatter);
System.out.println(leetTime);   // 13:37

LocalDate

LocalDate represents a distinct date, e.g. 2014-03-11. It's immutable and works exactly analog to LocalTime. The sample demonstrates how to calculate new dates by adding or substracting days, months or years. Keep in mind that each manipulation returns a new instance.

LocalDate today = LocalDate.now();
LocalDate tomorrow = today.plus(1, ChronoUnit.DAYS);
LocalDate yesterday = tomorrow.minusDays(2);

LocalDate independenceDay = LocalDate.of(2014, Month.JULY, 4);
DayOfWeek dayOfWeek = independenceDay.getDayOfWeek();
System.out.println(dayOfWeek);    // FRIDAY

Parsing a LocalDate from a string is just as simple as parsing a LocalTime:

DateTimeFormatter germanFormatter =
    DateTimeFormatter
        .ofLocalizedDate(FormatStyle.MEDIUM)
        .withLocale(Locale.GERMAN);

LocalDate xmas = LocalDate.parse("24.12.2014", germanFormatter);
System.out.println(xmas);   // 2014-12-24

LocalDateTime

LocalDateTime represents a date-time. It combines date and time as seen in the above sections into one instance. LocalDateTime is immutable and works similar to LocalTime and LocalDate. We can utilize methods for retrieving certain fields from a date-time:

LocalDateTime sylvester = LocalDateTime.of(2014, Month.DECEMBER, 31, 23, 59, 59);

DayOfWeek dayOfWeek = sylvester.getDayOfWeek();
System.out.println(dayOfWeek);      // WEDNESDAY

Month month = sylvester.getMonth();
System.out.println(month);          // DECEMBER

long minuteOfDay = sylvester.getLong(ChronoField.MINUTE_OF_DAY);
System.out.println(minuteOfDay);    // 1439

With the additional information of a timezone it can be converted to an instant. Instants can easily be converted to legacy dates of type java.util.Date.

Instant instant = sylvester
        .atZone(ZoneId.systemDefault())
        .toInstant();

Date legacyDate = Date.from(instant);
System.out.println(legacyDate);     // Wed Dec 31 23:59:59 CET 2014

Formatting date-times works just like formatting dates or times. Instead of using pre-defined formats we can create formatters from custom patterns.

DateTimeFormatter formatter =
    DateTimeFormatter
        .ofPattern("MMM dd, yyyy - HH:mm");

LocalDateTime parsed = LocalDateTime.parse("Nov 03, 2014 - 07:13", formatter);
String string = formatter.format(parsed);
System.out.println(string);     // Nov 03, 2014 - 07:13

Unlike java.text.NumberFormat the new DateTimeFormatter is immutable and thread-safe.

For details on the pattern syntax read here.

Annotations

Annotations in Java 8 are repeatable. Let's dive directly into an example to figure that out.

First, we define a wrapper annotation which holds an array of the actual annotations:

@interface Hints {
    Hint[] value();
}

@Repeatable(Hints.class)
@interface Hint {
    String value();
}

Java 8 enables us to use multiple annotations of the same type by declaring the annotation @Repeatable.

Variant 1: Using the container annotation (old school)

@Hints({@Hint("hint1"), @Hint("hint2")})
class Person {}

Variant 2: Using repeatable annotations (new school)

@Hint("hint1")
@Hint("hint2")
class Person {}

Using variant 2 the java compiler implicitly sets up the @Hints annotation under the hood. That's important for reading annotation informations via reflection.

Hint hint = Person.class.getAnnotation(Hint.class);
System.out.println(hint);                   // null

Hints hints1 = Person.class.getAnnotation(Hints.class);
System.out.println(hints1.value().length);  // 2

Hint[] hints2 = Person.class.getAnnotationsByType(Hint.class);
System.out.println(hints2.length);          // 2

Although we never declared the @Hints annotation on the Person class, it's still readable via getAnnotation(Hints.class). However, the more convenient method is getAnnotationsByType which grants direct access to all annotated @Hint annotations.

Furthermore the usage of annotations in Java 8 is expanded to two new targets:

@Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE})
@interface MyAnnotation {}

That's it

My guide to Java 8 programming ends here. There's certainly alot of more stuff to figure out. It's up to you to explore all the other great additions in JDK 1.8, e.g. Arrays.parallelSort, StampedLock and CompletableFuture - just to name a few.

I hope this post was helpful to you and you enjoyed reading it. The full source code of the tutorial samples is hosted on GitHub. Feel free to fork the repository or send me your feedback via Twitter.