“Java is still not dead—and people are starting to figure that out.”
这是对(https://github.com/winterbe/java8-tutorial) 的中文翻译
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No walls of text, just a bunch of commented code snippets. Enjoy!
没有文字，只有一堆注释的代码片段。请享受！
- 接口的默认方法
- [Lambda 表达式](#Lambda 表达式)
- 函数式接口
- 方法和构造函数引用
- [Lambda 作用域](#Lambda 作用域)
- 内置功能接口
- Optionals
- Streams
- Parallel Streams
- Maps
- Date API
- Annotations
- Where to go from here?
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); // 4.0The 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); // 123Keep 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); // 123Java 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.
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); // 3But 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); // 3However 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 PersonConsumers 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 = new Person("John", "Doe");
Person p2 = new Person("Alice", "Wonderland");
comparator.compare(p1, p2); // > 0
comparator.reversed().compare(p1, p2); // < 0Optionals 来历
到目前为止，臭名昭著的空指针异常是导致Java应用程序失败的最常见原因。以前，为了解决空指针异常，Google公司著名的Guava项目引入了Optional类，Guava通过使用检查空值的方式来防止代码污染，它鼓励程序员写更干净的代码。受到Google Guava的启发，Optional类已经成为Java 8类库的一部分。
Optional实际上是个容器：它可以保存类型T的值，或者仅仅保存null。Optional提供很多有用的方法，这样我们就不用显式进行空值检测。更多详情请参考官方文档。
原文翻译
Optionals 不是功能接口，但能极好而有效的防止NullPointerException。这是下一节的一个重要概念，所以让我们看看如何使用。
Optional 被定义为一个简单的容器，其值可能是null或者不是null。在Java 8之前一般某个函数应该返回非空对象但是偶尔却可能返回了null，而在Java 8中，不推荐你返回null而是返回Optional。
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"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 sequentially or parallely.
Streams are extremely powerful, so I wrote a separate Java 8 Streams Tutorial. You should also check out Stream.js, a JavaScript port of the Java 8 Streams API.
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 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 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, ddd1The 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"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); // trueCount 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); // 3This 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"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 concurrently 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.
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 mslong 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 msAs 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().
As already mentioned maps do not directly support streams. There's no stream() method available on the Map interface itself, however you can create specialized streams upon the keys, values or entries of a map via map.keySet().stream(), map.values().stream() and map.entrySet().stream().
Furthermore maps 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); // val33Next, we learn how to remove entries for 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); // nullAnother helpful method:
map.getOrDefault(42, "not found"); // not foundMerging 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); // val9concatMerge 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.
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 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 time in milliseconds since Unix EPOCH. 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.DateTimezones 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 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); // -239LocalTime comes with various factory methods 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); // 13:37LocalDate 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 subtracting 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); // FRIDAYParsing 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); // 2014-12-24LocalDateTime 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); // 1439With 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 2014Formatting 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 - 07:13Unlike java.text.NumberFormat the new DateTimeFormatter is immutable and thread-safe.
For details on the pattern syntax read here.
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.
@Hints({@Hint("hint1"), @Hint("hint2")})
class Person {}@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 information 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); // 2Although 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 {}My programming guide to Java 8 ends here. If you want to learn more about all the new classes and features of the JDK 8 API, check out my JDK8 API Explorer. It helps you figuring out all the new classes and hidden gems of JDK 8, like Arrays.parallelSort, StampedLock and CompletableFuture - just to name a few.
I've also published a bunch of follow-up articles on my blog that might be interesting to you:
- Java 8 Stream Tutorial
- Java 8 Nashorn Tutorial
- Java 8 Concurrency Tutorial: Threads and Executors
- Java 8 Concurrency Tutorial: Synchronization and Locks
- Java 8 Concurrency Tutorial: Atomic Variables and ConcurrentMap
- Java 8 API by Example: Strings, Numbers, Math and Files
- Avoid Null Checks in Java 8
- Fixing Java 8 Stream Gotchas with IntelliJ IDEA
- Using Backbone.js with Java 8 Nashorn
You should follow me on Twitter. Thanks for reading!