Java集合框架怎样利用Map实现键值对数据管理_Java集合框架映射表的基础操作技巧

put(K key, V value)

get(Object key)

null

remove(Object key)

containsKey(Object key)

isEmpty()

size()

keySet()

get()

values()

entrySet()

entrySet()

Set<Map.Entry<K, V>>

Entry

import java.util.HashMap;
import java.util.Map;

public class MapOperationsDemo {
    public static void main(String[] args) {
        // 创建一个HashMap实例
        Map<String, String> userSettings = new HashMap<>();

        // 1. 添加键值对
        userSettings.put("theme", "dark");
        userSettings.put("language", "en_US");
        userSettings.put("notifications", "true");
        System.out.println("初始设置: " + userSettings);

        // 2. 获取值
        String currentTheme = userSettings.get("theme");
        System.out.println("当前主题: " + currentTheme);

        // 3. 更新值 (键已存在，会覆盖)
        userSettings.put("theme", "light");
        System.out.println("更新主题后: " + userSettings.get("theme"));

        // 4. 检查键是否存在
        boolean hasLanguage = userSettings.containsKey("language");
        System.out.println("是否存在语言设置? " + hasLanguage);

        // 5. 移除键值对
        userSettings.remove("notifications");
        System.out.println("移除通知设置后: " + userSettings);

        // 6. 遍历Map (推荐使用entrySet())
        System.out.println("\n遍历用户设置:");
        for (Map.Entry<String, String> entry : userSettings.entrySet()) {
            System.out.println("键: " + entry.getKey() + ", 值: " + entry.getValue());
        }

        // 7. 检查是否为空和获取大小
        System.out.println("Map是否为空? " + userSettings.isEmpty());
        System.out.println("Map中键值对数量: " + userSettings.size());

        // 8. 获取默认值 (Java 8+)
        String defaultFont = userSettings.getOrDefault("font", "Arial");
        System.out.println("字体设置 (如果不存在则为默认): " + defaultFont);

        // 9. putIfAbsent (Java 8+)
        userSettings.putIfAbsent("font", "Roboto"); // 如果font不存在，则添加
        userSettings.putIfAbsent("theme", "blue"); // theme已存在，不会覆盖
        System.out.println("使用putIfAbsent后: " + userSettings);
    }
}

HashMap

LinkedHashMap

TreeMap

ConcurrentHashMap

HashMap

put

get

remove

null

null

HashMap

LinkedHashMap

HashMap

LinkedHashMap

HashMap

TreeMap

TreeMap

Comparator

put

get

remove

TreeMap

null

TreeMap

ConcurrentHashMap

Collections.synchronizedMap

Hashtable

ConcurrentHashMap

HashMap

LinkedHashMap

hashCode()

equals()

User

hashCode()

get()

remove()

hashCode()

equals()

HashMap

HashMap

容量 * 负载因子

HashMap

HashMap

HashMap

null

HashMap

null

null

TreeMap

null

get(key)

null

null

containsKey(key)

getOrDefault(key, defaultValue)

entrySet()

keySet()

values()

get()

entrySet()

forEach()

Map<String, Integer> scores = new HashMap<>();
scores.put("Alice", 90);
scores.put("Bob", 85);
scores.put("Charlie", 92);

// 使用forEach遍历
scores.forEach((name, score) -> System.out.println(name + " 的分数是: " + score));

getOrDefault(Object key, V defaultValue)

if-else

String defaultScore = scores.getOrDefault("David", 0); // 如果David不存在，返回0
System.out.println("David 的分数是: " + defaultScore);

putIfAbsent(K key, V value)

scores.putIfAbsent("Alice", 95); // Alice已存在，不会更新
scores.putIfAbsent("Eve", 88);   // Eve不存在，会添加
System.out.println("更新后 scores: " + scores);

compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

computeIfPresent(...)

computeIfAbsent(...)

computeIfPresent

null

scores.computeIfPresent("Bob", (name, score) -> score + 5); // Bob分数增加5
System.out.println("Bob 分数更新后: " + scores.get("Bob"));

computeIfAbsent

null

scores.computeIfAbsent("Frank", name -> 70); // Frank不存在，添加新分数
System.out.println("Frank 分数: " + scores.get("Frank"));

merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

remappingFunction

Map<String, Integer> itemCounts = new HashMap<>();
itemCounts.put("Apple", 10);

itemCounts.merge("Apple", 5, Integer::sum); // Apple数量增加5
itemCounts.merge("Banana", 3, Integer::sum); // Banana不存在，直接放入3
System.out.println("合并后 itemCounts: " + itemCounts);