Java basic Map interface + Collections tool class, collections tool class

Java basic Map interface + Collections tool class, collections tool class

1. In Map, we mainly talk about two interfaces: HashMap and LinkedHashMap.

(1) how to obtain the sorted hashmap

Let's talk about the add, delete, modify, and query functions of Map:

/** Add Map set */Map <String, String> map = new HashMap <String, String> (); map. put ("Monday", "Monday"); map. put ("Saturday", "Sunday"); System. out. println (map);/** obtain the value through creation */String string = map. get ("Saturday"); System. out. println (string);/** modify value through creation */map. put ("Monday", "Mon"); System. out. println (map);/** Delete value through creation */String remove = map. remove ("Saturday"); System. out. println (remove); System. out. println (map );}

 

The above is the addition, deletion, modification, and query method of Map.

 

2. Map Traversal

1. First: Use the keySet () method to obtain all the data Set created and stored in the Set <> Set.

(1) enhanced

(2) iterator

Map <String, Character> map = new LinkedHashMap <String, Character> ();
Map. put ("1", '1 ');
Map. put ("2", '2 ');
Map. put ("3", '3 ');
Map. put ("4", '4 ');
Map. put ("5", '5 ');
Set <String> set = map. keySet ();

(1) enhanced

For (String string: set ){
System. out. println (string + "" + map. get (string ));
}

(2) iterator

Iterator <String> iterator = set. iterator ();
While (iterator. hasNext ()){
String str = iterator. next ();
Integer integer = map. get (str );
System. out. println (str + "" + integer );
}

The second method uses Map. Entry <K, V> to provide a nested interface during Map class design: Entry. The Entry encapsulates the correspondence between key-value pairs into objects.

GetKey () method: Get the key in the Entry object
GetValue () method: Get the value in the Entry object
EntrySet () method: This method is used to return all key-value pairs (Entry) objects in the Map Set and return them in the form of a Set.

(1) enhanced

Set <Entry <String, Character> entries = map. entrySet ();
For (Entry <String, Character> entry: entries ){
String str = entry. getKey ();
Character character = entry. getValue ();
System. out. println (str + "" + character );

}

(2) iterator

Set <Map. Entry <String, String> entries = map. entrySet ();
Iterator <Map. Entry <String, String> iterator = entries. iterator ();
While (iterator. hasNext ()){
Entry <String, String> entry = iterator. next ();
System. out. println (entry. getKey () + "" + entry. getValue ());
}

 

3. Nesting of Map

We pass in the reference type Person in the nested Map. here we need to rewrite the equlas and HashCode methods.

Private static void fun6 () {Map <Person, String> map1 = new LinkedHashMap <Person, String> (); map1.put (new Person ("Huang Xiaoming", 40 ), "Beijing"); map1.put (new Person ("AnlayBay", 36), "Beijing"); Map <Person, String> map2 = new LinkedHashMap <> (); map2.put (new Person ("", 40), "Liangshan"); map2.put (new Person ("Lin Chong", 40), "leopard Header"); Map <Person, string>, String> map = new LinkedHashMap <> (); map. put (map1, "group 1"); map. put (map2, "second group"); for (Entry <Map <Person, String>, String> zhu: map. entrySet () {String value = zhu. getValue (); for (Entry <Person, String> entry: zhu. getKey (). entrySet () {Person key = entry. getKey (); String value2 = entry. getValue (); System. out. println (value + ".. "+ key + ".. "+ value2) ;}}/ *** iterator traversal */private static void fun5 () {Map <Person, String> map1 = new LinkedHashMap <Person, string> (); map1.put (new Person ("Huang Xiaoming", 40), "Beijing"); map1.put (new Person ("AnlayBay", 36), "Beijing "); map <Person, String> map2 = new LinkedHashMap <> (); map2.put (new Person ("", 40), "Liangshan "); map2.put (new Person ("Lin Chong", 40), "leopard Header"); Map <Person, String>, String> map = new LinkedHashMap <> (); map. put (map1, "group 1"); map. put (map2, "group 2"); Set <Map <Person, String> set = map. keySet (); Iterator <Map <Person, String> iterator = set. iterator (); while (iterator. hasNext () {Map <Person, String> next = iterator. next (); String string = map. get (next); Set <Person> persons = next. keySet (); Iterator <Person> iterator2 = persons. iterator (); while (iterator2.hasNext () {Person next2 = iterator2.next (); String string2 = next. get (next2); System. out. println (string2 + "" + next2 + "" + string );}}}

Here I will add a set of tool classes

Collections

Public static void main (String [] args) {ArrayList <String> arrayList = new ArrayList <> (); arrayList. add ("a"); arrayList. add ("s"); arrayList. add ("g"); arrayList. add ("d"); // sort Collections. sort (arrayList); System. out. println (arrayList); // flip Collections. reverse (arrayList); System. out. println (arrayList); // disrupt the order of Collections. shuffle (arrayList); System. out. println (arrayList); // List for Binary Search <Integer> integers = new ArrayList <> (); Collections. addAll (integers, 1, 2, 3, 4, 5, 7, 8); System. out. println (integers); int binarySearch = Collections. binarySearch (integers, 6); System. out. println (binarySearch); // disrupt shuffle Collections. shuffle (integers); System. out. println (integers );}

Today, most of them are presented in code and I will not introduce them much. The set Module is nothing more than addition, deletion, modification, query, and traversal, in the course of my study, I have typed every set of methods for ten times to deepen my impression.