Day11-Java进阶-HashSet集合LinkedHashSet-Collection工具类Map集合

1. HashSet集合

HashSet-JDK8版本及以后-面试常问

2. LinkedHashSet-Collection工具类

2.1 LinkedHashSet

2.2 Collection工具类

3. Map集合

3.1 Map接口介绍

3.2 Map 集合的遍历方式

3.2.1 三种方式介绍

java">package com.itheima.map;import java.util.HashMap;
import java.util.Map;
import java.util.Set;public class MapDemo3 {public static void main(String[] args) {HashMap<String, String> hm = new HashMap<>();hm.put("张三", "北京");hm.put("李四", "上海");hm.put("王五", "成都");hm.forEach((key, value) -> System.out.println(key + "---" + value));System.out.println("-----------------------------");hm.forEach((k,v) -> System.out.println(k + "---" + v));}private static void method2(HashMap<String, String> hm) {// 1. 获取到所有的键值对对象Set<Map.Entry<String, String>> entrySet = hm.entrySet();// 2. 遍历set集合获取每一个键值对对象for (Map.Entry<String, String> entry : entrySet) {// 3. 通过键值对对象, 获取键和值System.out.println(entry.getKey() + "---" + entry.getValue());}}private static void method1(HashMap<String, String> hm) {// 1. 获取到所有的键Set<String> keySet = hm.keySet();// 2. 遍历set集合, 获取每一个键for (String key : keySet) {// 3. 调用map集合的get方法, 根据键查找对应的值String value = hm.get(key);System.out.println(key + "---" + value);}}
}

3.2.2 Map 集合练习

java">package com.itheima.map;import java.util.TreeMap;
import java.util.function.BiConsumer;public class MapTest1 {public static void main(String[] args) {String info = "aababcabcdabcde";// 1. 准备Map集合, 用于记录每个字符出现次数TreeMap<Character, Integer> tm = new TreeMap<>();// 2. 拆分字符串, 便于得到每个字符char[] charArray = info.toCharArray();// 3. 遍历数组, 获取每个字符for (char c : charArray) {// 4. 判断当前字符, 在集合中是否存在if (!tm.containsKey(c)){tm.put(c, 1);}else {tm.put(c, tm.get(c) + 1);}}// 5. 准备StringBuilderStringBuilder sb = new StringBuilder();tm.forEach((key, value) -> sb.append(key).append("(").append(value).append(")"));System.out.println(sb);}private static void method() {String info = "ababc";// 1. 准备map集合, 用于统计字符的次数TreeMap<Character, Integer> tm = new TreeMap<>();// 2. 拆分字符串char[] charArray = info.toCharArray();// 3. 遍历字符数组, 获取每一个字符for (char c : charArray) {// 4. 判断当前字符在集合中是否存在if (!tm.containsKey(c)) {// 不存在: 说明是第一次出现tm.put(c, 1);} else {// 存在: 说明不是第一次出现, 获取旧值 + 1存回去tm.put(c, tm.get(c) + 1);}}// 5. 准备StringBuilderStringBuilder sb = new StringBuilder();tm.forEach((key, value) -> sb.append(key).append("(").append(value).append(")"));System.out.println(sb);}
}

java">package com.itheima.map;import java.util.*;public class MapTest2 {public static void main(String[] args) {HashMap<String, List<String>> hm = new HashMap<>();ArrayList<String> list1 = new ArrayList<>();Collections.addAll(list1, "南京市", "扬州市", "苏州市", "无锡市", "常州市");ArrayList<String> list2 = new ArrayList<>();Collections.addAll(list2, "武汉市", "孝感市", "十堰市", "宜昌市", "鄂州市");ArrayList<String> list3 = new ArrayList<>();Collections.addAll(list3, "成都市", "绵阳市", "自贡市", "攀枝花市", "泸州市");hm.put("江苏省", list1);hm.put("湖北省", list2);hm.put("四川省", list3);Set<Map.Entry<String, List<String>>> entrySet = hm.entrySet();for (Map.Entry<String, List<String>> entry : entrySet) {// 键: 省份String key = entry.getKey();System.out.print(key + "=");// 值: 多个市名List<String> list = entry.getValue();for (int i = 0; i < list.size() - 1; i++) {System.out.print(list.get(i)+", ");}System.out.println(list.get(list.size() - 1));}}private static void method() {HashMap<String, List<String>> hm = new HashMap<>();ArrayList<String> list1 = new ArrayList<>();Collections.addAll(list1, "南京市", "扬州市", "苏州市", "无锡市", "常州市");ArrayList<String> list2 = new ArrayList<>();Collections.addAll(list2, "武汉市", "孝感市", "十堰市", "宜昌市", "鄂州市");ArrayList<String> list3 = new ArrayList<>();Collections.addAll(list3, "成都市", "绵阳市", "自贡市", "攀枝花市", "泸州市");hm.put("江苏省", list1);hm.put("湖北省", list2);hm.put("四川省", list3);Set<Map.Entry<String, List<String>>> entrySet = hm.entrySet();for (Map.Entry<String, List<String>> entry : entrySet) {// 键: 省份名称String key = entry.getKey();System.out.print(key + "=");// 值: 多个市名List<String> list = entry.getValue();for (int i = 0; i < list.size() - 1; i++) {System.out.print(list.get(i) + ", ");}System.out.println(list.get(list.size() - 1));}}
}