lambda表达式
语法
最常用的：
Runnable runnable = () -> System.out.println("Hello!");
Thread t = new Thread(runnable);
t.start();
t.join();
你还可以这么写：
Thread t = new Thread(() -> System.out.println("Hello!"));
t.start();
t.join();
带有参数的呢？
Comparator<String> stringComparator = (s1, s2) -> s1.compareTo(s2); 扩展成一个完整的表达式： Comparator<String> stringComparator = (String s1, String s2) -> { System.out.println("Comparing..."); return s1.compareTo(s2); }; |
函数式接口
Lambda表达式使得你可以更紧凑地表达出单方法类。单一方法类也被称作函数式接口，它可以通过
@FunctionalInterface来进行注解：
@FunctionalInterface public interface MyFunctionalInterface<T> { } // Usage MyFunctionalInterface<String> l = s -> s.startsWith("A"); |
方法引用
方法引用就是更紧凑，易懂的Lambda表达式，它适用于那些已经有名字的方法。来看一个简单的例子：
public class Sample { public static void main(String[] args) { Runnable runnable = Sample::run; } private static void run() { System.out.println("Hello!"); } } |
还有一个：
public static void main(String[] args) { Sample sample = new Sample(); Comparator<String> stringLengthComparator = sample::compareLength; } private int compareLength(String s1, String s2) { return s1.length() - s2.length(); } |
Stream API基础
stream是一个元素序列，它支持串行及并行的操作。
遍历列表
List<String> list = Arrays.asList("one", "two", "three", "four", "five", "six");
list.stream()
.forEach(s -> System.out.println(s));
过滤
Predicate<String> lowerThanOrEqualToFour = s -> s.length() <= 4; Predicate<String> greaterThanOrEqualToThree = s -> s.length() >= 3; list.stream() .filter(lowerThanOrEqualToFour.and(greaterThanOrEqualToThree)) .forEach(s -> System.out.println(s)); |
排序
Predicate<String> lowerThanOrEqualToFour = s -> s.length() <= 4; Predicate<String> greaterThanOrEqualToThree = s -> s.length() >= 3; Comparator<String> byLastLetter = (s1, s2) -> s1.charAt(s1.length() - 1) - s2.charAt(s2.length() - 1); Comparator<String> byLength = (s1, s2) -> s1.length() - s2.length(); list.stream() .filter(lowerThanOrEqualToFour.and(greaterThanOrEqualToThree)) .sorted(byLastLetter.thenComparing(byLength)) .forEach(s -> System.out.println(s)); |
大小限制
Predicate<String> lowerThanOrEqualToFour = s -> s.length() <= 4; Predicate<String> greaterThanOrEqualToThree = s -> s.length() >= 3; Comparator<String> byLastLetter = (s1, s2) -> s1.charAt(s1.length() - 1) - s2.charAt(s2.length() - 1); Comparator<String> byLength = (s1, s2) -> s1.length() - s2.length(); list.stream() .filter(lowerThanOrEqualToFour.and(greaterThanOrEqualToThree)) .sorted(byLastLetter.thenComparing(byLength)) .limit(4) .forEach(s -> System.out.println(s)); |
集合转化成列表
Predicate<String> lowerThanOrEqualToFour = s -> s.length() <= 4; Predicate<String> greaterThanOrEqualToThree = s -> s.length() >= 3; Comparator<String> byLastLetter = (s1, s2) -> s1.charAt(s1.length() - 1) - s2.charAt(s2.length() - 1); Comparator<String> byLength = (s1, s2) -> s1.length() - s2.length(); List<String> result = list.stream() .filter(lowerThanOrEqualToFour.and(greaterThanOrEqualToThree)) .sorted(byLastLetter.thenComparing(byLength)) .limit(4) .collect(Collectors.toList()); |
并行处理
用它来遍历文件列表则再常见不过了：
public static void main(String[] args) { File[] files = new File("c:/windows").listFiles(); Stream.of(files) .parallel() .forEach(Sample::process); } private static void process(File file) { try { Thread.sleep(1000); } catch (InterruptedException e) { } System.out.println("Processing -> " + file); } |
注意上面给出这个示例的同时，也暴露了并行处理的一些缺点。 Stream API进阶
映射
遍历文件后返回一个FileSize 对象：
class FileSize { private final File file; private final Long size; FileSize(File file, Long size) { this.file = file; this.size = size; } File getFile() { return file; } Long getSize() { return size; } String getName() { return getFile().getName(); } String getFirstLetter() { return getName().substring(0, 1); } @Override public String toString() { return Objects.toStringHelper(this) .add("file", file) .add("size", size) .toString(); } } |
最终进行映射的代码：
File[] files = new File("c:/windows").listFiles();
List<FileSize> result = Stream.of(files)
.map(FileSize::new)
.collect(Collectors.toList());
分组：
按文件名的第一个字母将FileSize对象分组
Map<String, List<FileSize>> result = Stream.of(files)
.map(FileSize::new)
.collect(Collectors.groupingBy(FileSize::getFirstLetter));
Reduce
找出文件夹下的最大最小文件：
Optional<FileSize> filesize = Stream.of(files)
.map(FileSize::new)
.reduce((fs1, fs2) -> fs1.getSize() > fs2.getSize() ? fs1 : fs2);
如果你不需要FileSize对象，只需要一个数值的话：
OptionalLong max = Stream.of(files)
.map(FileSize::new)
.mapToLong(fs -> fs.getSize())
.max();