Section 41st: Class set framework in Java

Class set Framework in Java

The class set framework, then what is the class set framework, what kind of collections there are, and the infrastructure of the class set framework.

The class set framework is a collection of classes and interfaces that java.util are used by users to store and manage objects, in which we mainly learn three main classes, namely, collections, lists, and mappings.

Collections, lists, mappings

set is a collection in which the objects in the collection are not in order, and are objects that have no duplicates. simple: unordered, no repetition.

lists are listed , the objects in the list are sorted by order, and there are duplicate objects.

Map is a map in which each element in the collection has a key object and a value object, and the key in the map cannot be duplicated, and the value can be duplicated.

Class Collection Framework Body

Composition and classification of set frame

Top Iterator and collection

import java.util.List;import java.util.ArrayList;public class Demo{ public static void main(String args[]){ // 创建列表的实现类对象  ArrayList<String> arrayList = new ArrayList<String>(); // 添加数据 arrayList.add("a"); arrayList.add("b"); arrayList.add("c");// 移除 arrayList.remove(1); // 获取数据 String s = arrayList.get(1); System.out.println(s); // 获取长度 int a = arrayList.size(); // 打印所有元素 for(int i=0; i<arrayList.size(); i++){  String s = arrayList.get(i);  System.out.println(s); } }}

A class collection framework is a class of jdk classes and interfaces that are provided to master collections, lists, mappings, collections that are unordered, the elements in the collection are not allowed to be duplicates, the list is ordered, the elements in the list are allowed to be duplicated, the mappings are stored in key-value pairs, the keys are non-repeatable, and the values can be duplicated.

Collection and Iterator interfaces

CollectionYes List and Set the parent class

CollectionMethod of the interface

Method	Description
int size()	Returns the number of elements in the collection
boolean add(Object obj)	To add an object to the collection
remove(Object obj)	Remove an element from the collection
void clear()	Delete all objects in a collection
boolean isEmpty()	Determines whether the collection is empty

Set and HashSet are used for the set implementation class

import java.util.Set;import java.util.HashSet;public class Demo{ public static void main(String args[]){  HashSet<String> hashSet = new HashSet<String>(); Set<String> set = hashSet; // 实现类向上转型为接口 // 同理 Set<String> set = new HashSet<String>(); // 添加元素 set.add("a");  set.add("b"); set.add("c"); set.add("d"); // 获取集合中的元素长度 int a = set.size(); }}

IteratorInterface Iterator Object

Iterator - > Collection -> Set - > HashSet
hasNext() next()
it.hasNext()Whether there are elements
it.next()remove element

import java.util.Set;import java.util.HashSet;import java.util.Iterator;public class Demo{ public static void main(String args[]){  HashSet<String> hashSet = new HashSet<String>(); Set<String> set = hashSet; // 实现类向上转型为接口 // 同理 Set<String> set = new HashSet<String>(); // 添加元素 set.add("a");  set.add("b"); set.add("c"); set.add("d"); // 迭代器 iterator() 遍历整个set// 通过迭代器取出集合中的对象 Iterator<String> it = set.iterator<String>(); while(it.hasNext){  String s = it.next();  System.out.println(s); } }}

Setand HashSet for Set the implementation class to use
Iterator, Collection, Set, HashSet
Iterator, Collection, List--ArrayList
hasNext()Determine if there is a next element
next()Remove the element and move the cursor to the next

Use of Map and HASHMAP (implementation class of map)

MapFor a map, each element in the map has a key object and a value object, in which the Map key is not repeatable and the value can be duplicated. From jdk1.2 having this class.

java.utilInterface Map<K,V>K - the type of keys maintained by this mapV - the type of mapped values

Mapis an object that keys values establishes a mapping relationship between a key and a value, one that cannot be map duplicated, and each can only be keys key uniquely mapped to a value.

Method	Description
put(K key, V value)	mapAdd a key-value pair to the
get(Object key)	Get value

import java.util.Map;import java.util.HahMap;public class Demo{ public static void main(String args[]){  HashMap<String,String> hasMap = new HashMap<String,String>(); Map<String,String> map = hasMap; // 向上转型 // 同理 Map<Stirng,String> map = new HashMap<String,String>(); // put() 用来存放键值对 map.put("1","a"); map.put("2","b"); map.put("3","c");// 获取长度 int i = map.size();// 获取键为2的值 String s = map.get("2"); }}

The Map Middle key is not repeatable, and the value can be repeated. If you map add the same key, the new value overwrites the old value.

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Conclusion

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Section 41st: Class set framework in Java