Jdk8的stream使用

1. Map操作：

• 获取map的key或者value放到list:

    private static void getMapKeyOrValueToList() {
        Map<String, Object> map = new HashMap<>();
        map.put("math", 96);
        map.put("english", 89);
        map.put("chanises", 90);
        map.put("phycicl", 86.5);
  
        // 获取所有的key to list<>
        List<String> list = map.keySet().stream().collect(Collectors.toList());
        System.out.println("map key to list :" + list);
  
        // 获取所有的key to set<>
        Set<String> set = map.keySet().stream().collect(Collectors.toSet());
        System.out.println("map key to set :" + set);
    }
  

• map将某个字段值转成大小写:

    private static void mapToLower() {
        List<String> lines = Arrays.asList("Spring", "node", "mkyong", "erer");
        // 法一
        lines = lines.stream().map(String::toLowerCase).collect(Collectors.toList());
        System.out.println("toLowerCase:" + lines);
        // 法二
        lines = lines.stream().map(m -> m.toLowerCase()).collect(Collectors.toList());
        System.out.println("toLowerCase:" + lines);
    
        lines = lines.stream().map(String::toUpperCase).collect(Collectors.toList());
        System.out.println("toUpperCase:" + lines);
    }
  

2.List操作：

• list分页:

     private static void ListPages(int pageRows, int pageIndex) {
        List<String> persons = new ArrayList();
        for (int i = 1; i <= 10000; i++) {
            persons.add("name" + i);
        }
    	//use stream() of skip() and limit()
        List<String> result = persons.stream().skip((pageRows * (pageIndex - 1))).limit(pageRows).collect(Collectors.toList());
        System.out.println("--------" + result);
    }
  

• list过滤+排序:

    private static void ListFilterAndSort() {
        List<String> lines = Arrays.asList("spring", "node", "mkyong", "erer");
  
        List<String> result1 = lines.stream() // convert list to stream
                .filter(line -> !"mkyong".equals(line)) // filter the line which equals to "mkyong"
                .sorted((s1, s2) -> s1.compareTo(s2)).collect(Collectors.toList()); // collect the output and  convert streams to a list
        result1.forEach(System.out::println);
    }
  

• list去重+求和：

    private static void ListDistinct() {
        List<String> lines = Arrays.asList("spring", "node", "mkyong", "erer", "node");
  
        System.out.println("去重：" + lines.stream().distinct().collect(Collectors.toList()));
        System.out.println("过滤：" + lines.stream().filter(line -> !"mkyong".equals(line)).collect(Collectors.toList()));
  
        // String tString = "我认为#我认为#沃尔沃若";
        // Stream<String> ssStream = Stream.of(tString.split("#")).peek(System.out.print(tString));
  
        List<Integer> nums = Arrays.asList(1, 1, null, 2, 3, 4, null, 5, 6, 7, 8, 9, 10);
        System.out.println("求和:" + nums.stream().filter(num -> num != null).distinct().mapToInt(num -> num * 2)
                .peek(System.out::println).skip(2).limit(4).sum());
  
        // System.out.println("sum is:" + nums.stream().filter(i -> System.out.println("hello" +
        // i)).reduce().get());
  

  }