What is a Map collection? Characteristics of Map collections
Jun 20, 2017 am 10:15 AM
Characteristics of Map collection:
1. It is a double-column collection. When assigning values, key and value must be assigned at the same time.
2. It is an unordered collection (storing and removing elements) The order may be inconsistent)
3. The key value cannot be repeated, but the value can be repeated
4. One key can only correspond to one vlaue
5. When defining a collection, the data type key and value can use the same data type. You can also use different data types
Characteristics of Map collection
java.util.Map
The first way to traverse a Map collection
The first way to traverse a Map collection: to find a value through a key
There is a method in the Map collection: keySet
Set
Steps to traverse the Map collection:
1. Define a Map collection and add elements to the collection
2. Call the Map collection The method keySet in the Map collection stores the keys in the Map collection into a Set collection
3. Traverse the Set collection and obtain all the keys in the Map collection
4. Use the get method of the Map collection to search through the obtained keys Value
?1?public?static?void?main(String[]?args)?{?2?????????//1.定義一個Map集合,往集合中添加元素?3?????????Map<String,String>?map?=?new?HashMap<String,String>();?4?????????map.put("a",?"1");?5?????????map.put("b",?"2");?6?????????map.put("c",?"3");?7?????????map.put("d",?"4");?8?????????//2.調(diào)用Map集合中的方法keySet,把Map集合中的健存儲到一個Set集合中?9?????????Set<String>?set?=?map.keySet();10?????????//System.out.println(set.getClass());11?????????//3.遍歷Set集合,獲取Map集合所有的健12?????????//使用迭代器遍歷13?????????Iterator<String>?it?=?set.iterator();14?????????while(it.hasNext()){15?????????????String?key?=?it.next();16?????????????//4.通過獲取到的健,使用Map集合的方法get查找值17?????????????String?value?=?map.get(key);18?????????????System.out.println(key+"..."+value);19?????????}20?????????System.out.println("----------------");21?????????//使用增強(qiáng)for遍歷22?????????for(String?key?:?set){23?????????????//4.通過獲取到的健,使用Map集合的方法get查找值24?????????????String?value?=?map.get(key);25?????????????System.out.println(key+"..."+value);26?????????}27?????????System.out.println("----------------");28?????????//使用增強(qiáng)for遍歷29?????????for(String?key?:?map.keySet()){30?????????????//4.通過獲取到的健,使用Map集合的方法get查找值31?????????????String?value?=?map.get(key);32?????????????System.out.println(key+"..."+value);33?????????}34?????}The second way of Map collection traversal
The second way of Map collection traversal: the way of traversing key-value pairs
Map There is a method in the collection: entrySet
Set
Traversal steps:
1. Define a Map collection and add elements to the collection
2. Call the entrySet method in the Map collection to add each mapping relationship in the Map collection ( Marriage certificate) into the Set collection
3. Traverse the Set collection and obtain each mapping relationship Entry
4. Use the methods getKey and getValue in Entry
?1?public?static?void?main(String[]?args)?{?2?????????//1.定義一個Map集合,往集合中添加元素?3?????????Map<String,String>?map?=?new?HashMap<String,String>();?4?????????map.put("a",?"1");?5?????????map.put("b",?"2");?6?????????map.put("c",?"3");?7?????????map.put("d",?"4");?8?????????/*?9??????????*?2.調(diào)用Map集合中的方法entrySet,把Map集合中的每一個映射關(guān)系(結(jié)婚證)放入到Set集合中10??????????*?成員內(nèi)部類的訪問方式:外部類.內(nèi)部類(Map.Entry)11??????????*/12?????????Set<Map.Entry<String, String>>?set?=?map.entrySet();13?????????//3.遍歷Set集合,獲取每一個映射關(guān)系Entry<K,V>14?????????//使用迭代器遍歷Set集合15?????????Iterator<Map.Entry<String, String>>?it?=?set.iterator();16?????????while(it.hasNext()){17?????????????Map.Entry<String, String>?entry?=?it.next();18?????????????//4.使用Entry<K,V>中的方法getKey和getValue獲取健和值19?????????????String?key?=?entry.getKey();20?????????????String?value?=?entry.getValue();21?????????????System.out.println(key+"..."+value);22?????????}23?????????System.out.println("---------------------");24?????????//使用增強(qiáng)for遍歷Set集合25?????????for(Map.Entry<String, String>?entry:set){26?????????????//4.使用Entry<K,V>中的方法getKey和getValue獲取健和值27?????????????String?key?=?entry.getKey();28?????????????String?value?=?entry.getValue();29?????????????System.out.println(key+"..."+value);30?????????}31?????????System.out.println("---------------------");32?????????//使用增強(qiáng)for遍歷Set集合33?????????for(Map.Entry<String, String>?entry:map.entrySet()){34?????????????//4.使用Entry<K,V>中的方法getKey和getValue獲取健和值35?????????????String?key?=?entry.getKey();36?????????????String?value?=?entry.getValue();37?????????????System.out.println(key+"..."+value);38?????????}39?????}HashMap stores custom type key value
HashMap stores custom type key value
Custom type as the value of Map collection
Custom type as Map Keys of collections
Remember: when custom types override hashCode and equals
1. Use HashSet to store custom types
2. Use HashMap collections, and use custom types for keys
Hashtable
Map implementation class Hashtable
The underlying data structure is a hash table, the characteristics are the same as hashMap
Hashtable is a thread-safe collection and runs slowly
HashMap is a thread-unsafe collection and runs fast
The fate of Hashtable is the same as Vector. Starting from JDK1.2, it was replaced by the more advanced HashMap
HashMap allows the storage of null values. null key
hashtable is not allowed to store null values, null key
Hashtable His children, subclass Properties are still active in the development stage
LinkedHashMap collection features
java.util.LinkedHashMap extends HashMap implements Map
LinkedHashMap collection features:
1. Hash table + linked list: doubly linked list, which can guarantee the iteration order
2.Key cannot be repeated
Collections
java.util.Collections: Tool class for operating Collection collections
The methods in the tool class are all static methods and can be used directly through the class name
public static
public static void shuffle(List list) // The storage location of collection elements is shuffled
Variable Parameter
New features that appeared after JDK1.5
Prerequisite for use: The data type of the method parameter is determined, but the number of parameters is uncertain
Using format:
Modifier return value type method name (data type...variable name){
}
...represents that the method can receive multiple parameters of the same data type
The bottom layer of variable parameters can be regarded as It is an array
Notes on variable parameters:
1. A method parameter can only use one variable parameter
2. If the method has multiple parameters , variable parameters must be written at the end of the parameter list
?1?public?static?int?add(int...arr){?2?????????System.out.println(arr);//[I@104c575?3?????????System.out.println(arr.length);?4?????????int?sum?=?0;?5?????????//遍歷可變參數(shù)-->遍歷數(shù)組?6?????????for?(int?i?:?arr)?{?7?????????????sum?+=i;?8?????????}?9?????????10?????????return?sum;11?????}static import
JDK1.5 new feature, static import
Reduce development time Code amount
Standard writing method, can only be used when importing a package
import static java.lang.System.out; At the end, it must be a static member
package cn.itcast.demo05;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
??1?/*??2??*?帶排序功能的斗地主案例:??3??*?????1.準(zhǔn)備牌??4??*?????2.洗牌??5??*?????3.發(fā)牌??6??*?????4.排序??7??*?????5.看牌??8??*/??9?public?class?DouDiZhu?{?10?????public?static?void?main(String[]?args)?{?11?????????//1.準(zhǔn)備牌?12?????????//創(chuàng)建存儲序號和拍面值的Map集合?13?????????HashMap<Integer,String>?poker?=?new?HashMap<Integer, String>();?14?????????//創(chuàng)建存儲序號的List集合?15?????????ArrayList<Integer>?pokerNumber?=?new?ArrayList<Integer>();?16?????????//創(chuàng)建序號的數(shù)組?17?????????String[]?numbers?=?{"2","A","K","Q","J","10","9","8","7","6","5","4","3"};?18?????????//創(chuàng)建花色數(shù)組?19?????????String[]?colors?=?{"?","?","?","?"};?20?????????//先把大王和小王存儲到集合中?21?????????int?index?=?0;?22?????????poker.put(index,?"大王");?23?????????pokerNumber.add(index);?24?????????index++;?25?????????poker.put(index,?"小王");?26?????????pokerNumber.add(index);?27?????????index++;?28??????????29?????????//使用循環(huán)嵌套遍歷兩個數(shù)組,組成52張牌?30?????????for?(String?number?:?numbers)?{?31?????????????for?(String?color?:?colors)?{?32?????????????????//把組合包的牌添加到集合中?33?????????????????poker.put(index,?color+number);?34?????????????????pokerNumber.add(index);?35?????????????????index++;?36?????????????}?37?????????}?38?????????//System.out.println(poker);?39?????????//System.out.println(pokerNumber);?40??????????41?????????//2.洗牌:洗的是牌的序號?42?????????//使用Collections中的方法shuffle?43?????????Collections.shuffle(pokerNumber);?44?????????//System.out.println(pokerNumber);?45??????????46?????????/*?47??????????*?3.發(fā)牌:發(fā)的也是牌的序號?48??????????*?????a.定義4個集合存儲3個玩家和1個底牌?49??????????*?????b.遍歷存儲序號的List集合?50??????????*?????c.使用list集合的索引%進(jìn)行判斷進(jìn)行發(fā)牌?51??????????*?????注意:先判斷底牌?52??????????*/?53?????????//a.定義4個集合存儲3個玩家和1個底牌?54?????????ArrayList<Integer>?player01?=?new?ArrayList<Integer>();?55?????????ArrayList<Integer>?player02?=?new?ArrayList<Integer>();?56?????????ArrayList<Integer>?player03?=?new?ArrayList<Integer>();?57?????????ArrayList<Integer>?diPai?=?new?ArrayList<Integer>();?58??????????59?????????//b.遍歷存儲序號的List集合?60?????????for(int?i=0;?i<pokerNumber.size(); i++){ 61 //定義變量,接收排的序號 62 int number = pokerNumber.get(i); 63 //c.使用list集合的索引%進(jìn)行判斷進(jìn)行發(fā)牌 64 if(i>=51){?65?????????????????//存儲底牌?66?????????????????diPai.add(number);?67?????????????}else?if(i%3==0){?68?????????????????//給玩家1發(fā)牌?69?????????????????player01.add(number);?70?????????????}else?if(i%3==1){?71?????????????????//給玩家2發(fā)牌?72?????????????????player02.add(number);?73?????????????}else?if(i%3==2){?74?????????????????//給玩家3發(fā)牌?75?????????????????player03.add(number);?76?????????????}?77?????????}?78?????????/*System.out.println(player01);?79?????????System.out.println(player02);?80?????????System.out.println(player03);?81?????????System.out.println(diPai);*/?82??????????83?????????//4.排序?84?????????//使用Collections中的方法sort?85?????????Collections.sort(player01);?86?????????Collections.sort(player02);?87?????????Collections.sort(player03);?88?????????Collections.sort(diPai);?89??????????90?????????/*System.out.println(player01);?91?????????System.out.println(player02);?92?????????System.out.println(player03);?93?????????System.out.println(diPai);*/?94??????????95?????????/*?96??????????*?5.看牌?97??????????*/?98?????????//調(diào)用看牌方法?99?????????lookPoker("劉德華",player01,?poker);100?????????lookPoker("周潤發(fā)",player02,?poker);101?????????lookPoker("周星馳",player03,?poker);102?????????lookPoker("底牌",diPai,?poker);103?????}104?????105?????/*106??????*?定義一個看牌的方法107??????*?返回值類型:void108??????*?方法名:lookPoker109??????*?參數(shù)列表:玩家和底牌的集合,存儲排的Map集合110??????*?使用查表法看牌:111??????*?????遍歷List集合,獲取Map集合key,使用key去Map集合中查找value112??????*/113?????public?static?void?lookPoker(String?name,ArrayList<Integer>?list,HashMap<Integer,String>?map){114?????????System.out.print(name+":");115?????????//遍歷List集合,獲取Map集合key116?????????for?(Integer?key?:?list)?{117?????????????//使用key去Map集合中查找value118?????????????String?value?=?map.get(key);119?????????????System.out.print(value+"?");120?????????}121?????????System.out.println();//換行122?????}123?}
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