TreeMap排序

实现TreeMap后默认为key升序排序，如果要实现key其他排序规则，可以使用Comparator对象作为参数，前提是key为可以排序的类型（String，int等类型)

Map<String,People> map = new TreeMap<>(new Comparator<String>() {
    @Override
    public int compare(final String o1, final String o2) {
        return o2.compareTo(o1);
    }
});

如果要实现TreeMap的value值排序，可以将Map转List后排序，在将排序后的List转回Map时，需要在toMap里增加LinkedHashMap::new，如果不指定LinkedHashMap，会使排序的效果丢失

People people1 = new People("1","1");
People people2 = new People("2","6");
People people3 = new People("3","6");
People people4 = new People("3","4");
Map<String,People> map = new TreeMap<>();
map.put("6",people1);
map.put("9",people2);
map.put("2",people3);
map.put("1",people4);
//Map转List
List<Map.Entry<String,People>> entryList = new ArrayList<>(map.entrySet());
//排序List，按照value里的no字段升序排序
List<Map.Entry<String,People>> sortList = entryList.stream().sorted(Comparator.comparing(o1 -> o1.getValue().getNo())).collect(Collectors.toList()); 
//List转Map，不指定LinkedHashMap，当前sortMap排序效果会丢失，转后的Map默认按照key升序
Map<String,People> sortMap = sortList.stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
//List转Map，指定LinkedHashMap::new，转后的Map排序效果仍然在
Map<String,People> sortMap1 = sortList.stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,(i1,i2)->i2,LinkedHashMap::new)); 
System.out.println("entryList:"+  JsonUtils.toJson(entryList));
System.out.println("sortList:"+  JsonUtils.toJson(sortList));
System.out.println("sortMap:"+  JsonUtils.toJson(sortMap));
System.out.println("sortMap1:"+  JsonUtils.toJson(sortMap1));

      
//People类
static class People{
    public People(String no,String age) {
        this.no = no;
        this.age = age;
    }
    public String no;
    public String age;

    public String getNo() {
        return no;
    }

    public String getAge() {
        return age;
    }

}