Java double instance collection map HashMap treemap

The difference between Map and Collection

• The container elements in the Collection exist in isolation. The elements stored in the Collection are stored one by one

• The container elements in the Map exist in pairs. Each element consists of a key and a value. The corresponding value can be found through the key

• Containers in the Collection are called single column collections, and containers in the Map are called double column collections

• The collection in the Map cannot contain duplicate keys. Values can be repeated. Each key can only correspond to one value

• The commonly used containers in Map are HashMap and TreeMap

common method

HashMap is the interface implementation class of Map interface, which is implemented by hash algorithm. It is the most commonly used implementation class of Map interface. Because the underlying layer uses hash table to store data, it is required that the key cannot be repeated. If it is repeated, the new value will replace the old value. HashMap has very high efficiency in finding, deleting and modifying.

import java.util.*; public class HashMapTest { public static void main(String[] args) { //Instantiate HashMap container Map<String,String> map = new HashMap<>(); //1. Add the element and put the feature. If you encounter an element with the same key, the old one will be overwritten with the new one. //If the return value is overwritten, the old value will be returned, otherwise it will be null map.put("a", "A"); String value = map.put("a", "B"); System.out.println(value); //2. The first way to obtain elements is through keys. Disadvantages: if you want to obtain all value s in the Map, you need to know all keys. It's too troublesome //It needs to be called many times, okay System.out.println("----------------"); String val = map.get("a"); System.out.println(val); //The second method is to cooperate with the get method through keySet //keySet combines all the key s in the Map into one set System.out.println("----------------"); map.put("b", "B"); map.put("c", "C"); map.put("d", "D"); Set<String> keys = map.keySet(); for(String key:keys) { String v1 = map.get(key); System.out.println(key+"--"+v1); } //The third way is to obtain the Map.Entry type element through the entrySet method System.out.println("----------------"); //Entry is an internal interface. Through the external interface. Internal interface, the key value pairs are scattered and put into the Set Set<Map.Entry<String,String>> entrySet = map.entrySet(); //Traversal set for(Map.Entry<String,String> entry:entrySet) { String key = entry.getKey(); String v = entry.getValue(); System.out.println(key+"----"+v); } //3. For the union operation of map container, the generic types must be the same System.out.println("----------------"); Map<String,String> map1 = new HashMap<>(); map1.put("f", "F"); //Note that the value of key is c map1.put("c", "cc"); map1.putAll(map); Set<String> keys2 = map1.keySet(); //The result here is map1 and map for(String key:keys2) { String v2= map1.get(key); System.out.println(key+"--"+v2); } //4. Delete elements from map container System.out.println("----------------"); String val1 = map.remove("d"); Set<String> set2 = map.keySet(); for(String key:set2) { String v2 = map.get(key); System.out.println(key+"-----"+v2); } //5. Judge whether Key and value exist System.out.println("----------------"); boolean flag1 = map.containsKey("aaaa"); System.out.println(flag1); boolean flag2 = map.containsKey("a"); System.out.println(flag2); boolean flag3 = map.containsValue("B"); System.out.println(flag3); boolean flag4 = map.containsValue("BB"); System.out.println(flag4); } } /* A ---------------- B ---------------- a--B b--B c--C d--D ---------------- a----B b----B c----C d----D ---------------- a--B b--B c--C d--D f--F ---------------- a-----B b-----B c-----C ---------------- false true true false */

Underlying implementation and source code analysis of HashMap in Java_ lipengfei0427 blog - CSDN blog

Continue with the previous article

TreeMap and HashMap also implement the Map interface. Therefore, there is no difference in the usage of API. The efficiency of HashMap is higher than that of TreeMap. TreeMap is a container that can sort keys. TreeMap can be used when sorting keys. The bottom layer of TreeMap is based on red black tree.

When using TreeMap, you need to give a collation:

• The element itself implements comparison rules

• Comparison rules are implemented through comparators

//Self implementation comparison rules. There are notes on the Users class. Turn it over import java.util.*; public class TreeMapTest { public static void main(String[] args) { //Instantiate TreeMap Map<Users,String> map = new TreeMap<>(); Users u1 = new Users("li1",3); Users u2 = new Users("li2",4); map.put(u1,"li1"); map.put(u2, "li2"); Set<Users> keys = map.keySet(); for(Users key:keys) { System.out.println(key+"---"+map.get(key)); } } }

//The comparison rules are implemented through the comparator. Both the Student class and the StudentComparator comparator have them. Turn it over import java.util.*; public class TreeMapTest02 { public static void main(String[] args) { Map<Student,String> treemap = new TreeMap<>(new StudentComparator()); Student s1 = new Student("li1",18); Student s2 = new Student("li2",22); Student s3 = new Student("li1.5",22); treemap.put(s1,"li1"); treemap.put(s2, "li2"); treemap.put(s3,"li1.5"); Set<Student> keys = treemap.keySet(); for(Student key:keys) { System.out.println(key+"---"+treemap.get(key)); } } }

25 November 2021, 16:07 | Views: 7961