lambda表達式
語法
最常用的：
Runnable runnable = () -> System.out.println("Hello!");
Thread t = new Thread(runnable);
t.start();
t.join();
你還可以這么寫：
Thread t = new Thread(() -> System.out.println("Hello!"));
t.start();
t.join();
帶有參數的呢？
Comparator<String> stringComparator = (s1, s2) -> s1.compareTo(s2); 擴展成一個完整的表達式： Comparator<String> stringComparator = (String s1, String s2) -> { System.out.println("Comparing..."); return s1.compareTo(s2); }; |
函數式接口
Lambda表達式使得你可以更緊湊地表達出單方法類。單一方法類也被稱作函數式接口，它可以通過
@FunctionalInterface來進行注解：
@FunctionalInterface public interface MyFunctionalInterface<T> { } // Usage MyFunctionalInterface<String> l = s -> s.startsWith("A"); |
方法引用
方法引用就是更緊湊，易懂的Lambda表達式，它適用于那些已經有名字的方法。來看一個簡單的例子：
public class Sample { public static void main(String[] args) { Runnable runnable = Sample::run; } private static void run() { System.out.println("Hello!"); } } |
還有一個：
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基礎
stream是一個元素序列，它支持串行及并行的操作。
遍歷列表
List<String> list = Arrays.asList("one", "two", "three", "four", "five", "six");
list.stream()
.forEach(s -> System.out.println(s));
過濾
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)); |
集合轉化成列表
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()); |
并行處理
用它來遍歷文件列表則再常見不過了：
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); } |
注意上面給出這個示例的同時，也暴露了并行處理的一些缺點。 Stream API進階
映射
遍歷文件后返回一個FileSize 對象：
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(); } } |
最終進行映射的代碼：
File[] files = new File("c:/windows").listFiles();
List<FileSize> result = Stream.of(files)
.map(FileSize::new)
.collect(Collectors.toList());
分組：
按文件名的第一個字母將FileSize對象分組
Map<String, List<FileSize>> result = Stream.of(files)
.map(FileSize::new)
.collect(Collectors.groupingBy(FileSize::getFirstLetter));
Reduce
找出文件夾下的最大最小文件：
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對象，只需要一個數值的話：
OptionalLong max = Stream.of(files)
.map(FileSize::new)
.mapToLong(fs -> fs.getSize())
.max();