Java8新特性Stream之Collectors

Java8新特性Stream之Collectors

Java8新特性Stream之Collectors(toList()、toSet()、toCollection()、joining()、partitioningBy()、collectingAndT)

将流中的数据转成集合类型：

一、将数据收集进一个列表(Stream 转换为 List，允许重复值，有顺序)

//1.将数据收集进一个列表(Stream 转换为 List，允许重复值，有顺序)
//创建流
Stream<String> language = Stream.of("java", "python", "C++","php","java");
List<String> listResult = language.collect(Collectors.toList());
result.forEach(System.out::println);

//2.stream()代替流
List<String> list = Arrays.asList("java", "python", "C++","php","java");
      List<String> listResult = list.stream().collect(Collectors.toList());
      listResult.forEach(System.out::println);

二、将数据收集进一个集合(Stream 转换为 Set，不允许重复值，没有顺序)

//1.将数据收集进一个集合(Stream 转换为 Set，不允许重复值，没有顺序)
Stream<String> language = Stream.of("java", "python", "C++","php","java");
Set<String> setResult = language.collect(Collectors.toSet());
setResult.forEach(System.out::println);

三、用自定义的实现Collection的数据结构收集

List<String> list = Arrays.asList("java", "python", "C++","php","java");  
//用LinkedList收集
List<String> linkedListResult = list.stream().collect(Collectors.toCollection(LinkedList::new));
linkedListResult.forEach(System.out::println);
System.out.println("--------------");

//用CopyOnWriteArrayList收集
List<String> copyOnWriteArrayListResult = list.stream().collect(Collectors.toCollection(CopyOnWriteArrayList::new));
copyOnWriteArrayListResult.forEach(System.out::println);
System.out.println("--------------");

//用TreeSet收集
TreeSet<String> treeSetResult = list.stream().collect(Collectors.toCollection(TreeSet::new));
treeSetResult.forEach(System.out::println);

四、对Stream的字符串拼接

List<String> list = Arrays.asList("java", "python", "C++","php","java");
//直接将输出结果拼接
System.out.println(list.stream().collect(Collectors.joining()));
//每个输出结果之间加拼接符号“|”
System.out.println(list.stream().collect(Collectors.joining(" | ")));
//输出结果开始头为Start--，结尾为--End，中间用拼接符号“||”
System.out.println(list.stream().collect(Collectors.joining(" || ", "Start--", "--End")));

五、其他还有partitioningBy()，分类成一个key为True和Flase的Map。例如

List<String> list = Arrays.asList("java", "python", "C++","php","java");
Map<Boolean, List<String>> result = list.stream().collect(partitioningBy(s -> s.length() > 2));

六、collectingAndThen()，收集之后继续做一些处理。例如

List<String> list = Arrays.asList("java", "python", "C++","php","java");
//收集后转换为不可变List
ImmutableList<String> collect = list.stream().collect(Collectors.collectingAndThen(Collectors.toList(), ImmutableList::copyOf));