blogv2/_posts/常用的代码块.md

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title: Java常用代码块 date: 2020-10-28 tags: Java cover: https://static.jqwei.com/blog/img/IMG_3460.PNG comments: true

categories: 编程语言

java常用代码块

为了减少手写出现的错误，可以直接复制使用,大部分是经常使用的代码，对map操作，list 分组，排序，后续可以放到IDEA做成快捷键

对象	排序	分组	分页
list	√	√	√
map	√
set

map 根据Key排序

private static Map<String, Object> sortByKey(Map<String, Object> map) {
        Map<String, Object> result = new LinkedHashMap<>(map.size());
        map.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .forEachOrdered(e -> result.put(e.getKey(), e.getValue()));
        return result;
    }

map 进行遍历

    for (Map.Entry<String, Object> m : map.entrySet()) {
        System.out.println("key:" + m.getKey() + " value:" + m.getValue());
    }
    

list 对象根据某个字段分组

    //1 按照某个字段
    Map<Object,List<Object>> map = demoList.stream().collect(Collectors.groupingBy(Object::getAttr));
    //2 按照多个字段
    Map<String, List<Object>> groupBy = objectList.stream().collect(Collectors
    .groupingBy(o -> o.getAttr1() + "_" + o.getAttr2()));

list 用stream进行排序 .

    //根据单个属性排序
    List<String> stortList = list.stream().sorted().collect(Collectors.toList());
    //根据对象某个属性排序
    List<Object> sortList = objectList.stream().sorted(Comparator.comparing(Object::getAttr)
    .reversed()).collect(Collectors.toList());
    
    

list 用Guava进行拆分分页

    List<List<Long>> splitList = Lists.partition(list, 200);

jodaTime 两个日期相差小时，天 ，分钟

    DateTime dt1 = new DateTime("2020-09-05");
    dt1  = dt1.plusDays(1).plusMinutes(1).plusHours(1);
    DateTime dt2 = new DateTime("2020-09-06");
    
    System.out.println(dt1);
    System.out.println(dt2);
    
    //时间dt2-时间dt1 的差
    int hours = Hours.hoursBetween(dt1, dt2).getHours();
    int days = Days.daysBetween(dt1,dt2).getDays();
    int minutes = Minutes.minutesBetween(dt1,dt2).getMinutes();
    
    System.out.println(hours+" "+days+" "+minutes);