Map introduction
Commonly used maps include HashMap, TreeMap, LinkedHashMap
HashMap: The most commonly used Map, which stores data according to the HashCode value of the key, and can directly obtain its value according to the key, with a very fast access speed. HashMap allows only one record to have a key value of null at most (multiple records will be overwritten); the value of null is allowed to have a value of null. Non-thread safe
TreeMap: According to the key sorting, the default is to sort in ascending order. You can also specify the sorted comparator. When traversing TreeMap, the records obtained are sorted. TreeMap does not allow the value of key to be null. Non-thread safe
LinkedHashMap: Insert order. When traversing LinkedHashMap, the first record obtained must be inserted first. Inheriting HashMap, non-thread-safe
TreeMap sorting
TreeMap can only be sorted according to keys. TreeMap itself is a binary tree, and the order of elements is determined by the value of the key.
There is a Comparator within TreeMap by default. When new, you can override the default Comparator to define your own sorting rules, but you can only sort according to the key.
private static void sortTreeMap(){
Map<String,String> map =new TreeMap<>((k1,k2)->{
return k1.compareTo(k2);
});
map.put("a","2");
map.put("c","5");
map.put("d","6");
map.put("b","1");
map.forEach((k,v)->{
System.out.println(k+":"+v);
});
}
Output result
a:2
b:1
c:5
d:6
HashMap sorting
HashMap itself has no order and cannot be sorted directly
To sort, you can only convert it to list first, then sort the list, and then convert it to LinkedHasHMap
Sort by yourself, you can sort key or value
private static void sortMapValue(){
Map<String,String> map =new HashMap<>();
map.put("a","2");
map.put("c","5");
map.put("d","6");
map.put("b","1");
List<Map.Entry<String,String>> lstEntry=new ArrayList<>(map.entrySet());
Collections.sort(lstEntry,((o1, o2) -> {
return o1.getValue().compareTo(o2.getValue());
}));
lstEntry.forEach(o->{
System.out.println(o.getKey()+":"+o.getValue());
});
//If you have to return a map, new LinkedHashMap and put all the values in the list in turn.
/*LinkedHashMap<String,String> linkedHashMap=new LinkedHashMap<>();
(o->{
((),());
});*/
}
Output result
b:1
a:2
c:5
d:6