HashMap

1.HashMap基本使用

package com.lu.day08.map;import java.util.HashMap;
import java.util.Map;
import java.util.Set;public class MapDome {public static void main(String[] args) {HashMap<String , String> map = new HashMap<>();//添加后者修改->如果key不存在則是添加，如果key已經存在就是修改map.put("曹雪芹","紅樓夢");map.put("施耐庵","水滸傳");map.put("吳承恩","西游記");map.put("杜甫","登高");System.out.println(map);//通過key獲取值，如果key不存在則，獲取到的值為nullSystem.out.println(map.get("吳承恩"));System.out.println(map.get("李白"));map.remove("吳承恩");System.out.println("--------------");map.put("杜甫","琵琶行");System.out.println(map);HashMap<Integer, String> map1 = new HashMap<>();map1.put(1,"one");map1.put(2,"two");map1.put(3,"three");map1.put(4,"four");System.out.println(map1.containsKey(7));System.out.println(map1.containsValue("one"));System.out.println(map1.isEmpty());System.out.println(map.size());//獲取鍵集合Set<String> strings = map.keySet();for (String key : strings) {System.out.println(key+"-->"+map.get(key));}System.out.println("-----------------");//entrySet 返回一個存放鍵值對set集合//entry是一個map接口中的內部類->通過put（k，v）放置的鍵值對，實際上存到了entry對象中->對象由HashMap中的內部類中的node實現//實現其中有兩個屬性（key,value）Set<Map.Entry<String, String>> entries = map.entrySet();for (Map.Entry<String, String> entry : entries) {System.out.println(entry.getKey()+"-->"+entry.getValue());}System.out.println("-----------------");//函數式的遍歷方法forEachmap.forEach((k,v)->{System.out.println(k+"-->"+v);});//只能key取value不能反過來}}

TreeMap基本使用

1.TreeMap基本使用

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package com.lu.day08.map;import java.util.Comparator;
import java.util.TreeMap;public class TreeMapDemo {public static void main(String[] args) {//TreeMap只能對key排序Comparator<Integer> c = (o1,o2)->o2-o1;TreeMap<Integer, String> map = new TreeMap<>(c);map.put(1, "a");map.put(2, "b");map.put(3, "c");System.out.println(map);}
}

LinkedHashMap

package com.lu.day08.map;import java.util.LinkedHashMap;
import java.util.LinkedHashSet;public class LinkedHashMapDemo {public static void main(String[] args) {//保證存取有序LinkedHashMap<Integer, String> map = new LinkedHashMap<>();map.put(1, "張三");map.put(2, "李四");map.put(3, "王五");map.put(4, "趙六");map.forEach((k, v) -> {System.out.println(k + ":" + v);});LinkedHashSet<Integer> set = new LinkedHashSet<>();set.add(1);set.add(2);set.add(3);set.forEach(System.out::println);}
}

?集合嵌套

package com.lu.day08.map;import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;public class Test {public static void main(String[] args) {HashMap<String, List<String>> map = new HashMap<>();ArrayList<String> xiaomi = new ArrayList<>();xiaomi.add("小米1");xiaomi.add("小米2");xiaomi.add("小米3");map.put("小米", xiaomi);ArrayList<Map<String,Double>> list = new ArrayList<>();HashMap<String, Double> clothMap = new HashMap<>();clothMap.put("耐克",2300.0);clothMap.put("貴人鳥",7800.0);clothMap.put("凡客",50.0);}
}

Collections

1.集合輔助工具類

package com.lu.day08.collections;import java.util.ArrayList;
import java.util.Collections;public class CollectionsDemo {public static void main(String[] args) {ArrayList<Integer> list = new ArrayList<>();list.add(1);list.add(5);list.add(2);list.add(4);list.add(3);System.out.println(list);// 反轉Collections.reverse(list);System.out.println(list);// 排序Collections.sort(list);System.out.println(list);// 隨機Collections.shuffle(list);System.out.println(list);}
}

可變參數

package com.lu.day08.collections;/*** 可變參數*/
public class VaryParam {public static void main(String[] args) {print(1,2,3,4,5,6,7,8,9);}public static void print(int... a){for (int j : a) {System.out.println(j);}}
}

不可變集合

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package com.lu.day08.collections;import java.util.List;
import java.util.Map;
import java.util.Set;/*** jdk9新增不可變集合*/
public class Test {public static void main(String[] args) {//創建list不可變集合List<Integer> integers = List.of(1, 2, 3);integers.forEach(System.out::println);//創建set不可變集合Set<Integer> integers1 = Set.of(1, 2, 3);System.out.println(integers1);//創建map不可變集合Map<String, Integer> a = Map.of("A", 1, "B", 2, "C", 3);System.out.println(a);}
}

HashMap底層

?面試題

1.所有集合的底層分別是什么