深入剖析Java中Map.Entry的方法与实战应用

interface Map<K,V> {
    interface Entry<K,V> {
        K getKey();
        V getValue();
        V setValue(V value);
        // Java 8 新增方法
        boolean equals(Object o);
        int hashCode();
        // Java 9 静态方法
        static <K,V> Map.Entry<K,V> comparingByKey() {...}
        static <K,V> Map.Entry<K,V> comparingByValue() {...}
    }
}

Map<String, Integer> population = new HashMap<>();
population.put("Beijing", 21_540_000);
population.put("Shanghai", 24_870_000);

// 获取Map.Entry实例
Set<Map.Entry<String, Integer>> entries = population.entrySet();

for (Map.Entry<String, Integer> entry : entries) {
    // 1. getKey() - 获取键
    String city = entry.getKey();
    
    // 2. getValue() - 获取值
    int people = entry.getValue();
    
    // 3. setValue() - 修改值（原映射同步更新）
    if ("Shanghai".equals(city)) {
        entry.setValue(people + 100_000); // 上海新增10万人
    }
    
    System.out.println(city + ": " + entry.getValue());
}

Map.Entry<String, Integer> beijingEntry = Map.entry("Beijing", 21540000);

// 1. 相等性判断
System.out.println(beijingEntry.equals(Map.entry("Beijing", 21540000))); // true

// 2. 哈希码计算
System.out.println(beijingEntry.hashCode()); // 基于键和值的哈希

// 3. 键值比较器（Java 9+）
List<Map.Entry<String, Integer>> cities = new ArrayList<>(entries);

// 按键排序
cities.sort(Map.Entry.comparingByKey()); 
// 按值排序（逆序）
cities.sort(Map.Entry.comparingByValue(Comparator.reverseOrder()));

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    // 处理每个键值对
}

Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<String, Integer> entry = it.next();
    if (entry.getValue() < 1000) {
        it.remove(); // 安全删除
    }
}

Map.Entry<String, Integer> entry = Map.entry("Tokyo", 37_400_000);
// 注意：此方法创建的Entry不可变

class CustomEntry<K, V> implements Map.Entry<K, V> {
    private final K key;
    private V value;

    public CustomEntry(K key, V value) {
        this.key = key;
        this.value = value;
    }

    @Override public K getKey() { return key; }
    @Override public V getValue() { return value; }
    @Override public V setValue(V value) { 
        V old = this.value;
        this.value = value;
        return old;
    }
}

// 使用示例
Map.Entry<String, String> custom = new CustomEntry<>("OS", "Linux");

// 比keySet()+get()更高效，避免重复查找
long total = 0;
for (Map.Entry<String, Integer> entry : population.entrySet()) {
    total += entry.getValue();
}

population.entrySet().removeIf(entry -> 
    entry.getKey().startsWith("A") && entry.getValue() < 1_000_000
);

// 获取键值对视图
Set<Map.Entry<String, Integer>> entrySet = Collections.unmodifiableSet(
    population.entrySet()
);

// 转换为对象数组
Object[] entryArray = population.entrySet().toArray();

// 找出人口最多的三个城市
List<String> topCities = population.entrySet().stream()
    .sorted(Map.Entry.comparingByValue().reversed())
    .limit(3)
    .map(Map.Entry::getKey)
    .collect(Collectors.toList());

Map.Entry<String, Integer> immutableEntry = new AbstractMap.SimpleImmutableEntry<>("London", 8_982_000);
// 尝试修改将抛出UnsupportedOperationException
immutableEntry.setValue(9_000_000); 

Map<String, List<String>> techMap = new HashMap<>();
techMap.put("Java", new ArrayList<>(Arrays.asList("Spring", "Hibernate")));Map.Entry<String, List<String>> entry = techMap.entrySet().iterator().next();


List<String> frameworks = entry.getValue();
frameworks.add("Jakarta EE"); // 修改会影响原Map！

System.out.println(techMap.get("Java")); 
// [Spring, Hibernate, Jakarta EE]

ConcurrentMap<String, AtomicInteger> concurrentMap = new ConcurrentHashMap<>();
concurrentMap.put("Counter", new AtomicInteger(0));

// 原子更新
concurrentMap.entrySet().forEach(entry -> {
    if ("Counter".equals(entry.getKey())) {
        entry.getValue().incrementAndGet();
    }
});

// 错误方式 - 会抛出ConcurrentModificationException
for (Map.Entry<String, Integer> entry : map.entrySet()) {
    if (entry.getValue() < 100) {
        map.remove(entry.getKey()); // 错误！
    }
}

// 正确方案1：使用迭代器的remove()
Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<String, Integer> entry = it.next();
    if (entry.getValue() < 100) {
        it.remove();
    }
}

​​​​​​​// 正确方案2：Java 8+ removeIf()
map.entrySet().removeIf(entry -> entry.getValue() < 100);

Set<Map.Entry<String, Object>> deepCopy = original.entrySet().stream()
    .map(entry -> new AbstractMap.SimpleEntry<>(
        new String(entry.getKey()), 
        deepClone(entry.getValue()) // 自定义深拷贝方法
    ))
    .collect(Collectors.toSet());

public class CustomMap<K, V> implements Iterable<Map.Entry<K, V>> {
    private final Map<K, V> data = new HashMap<>();
    
    public void put(K key, V value) {
        data.put(key, value);
    }
    
    @Override
    public Iterator<Map.Entry<K, V>> iterator() {
        return new Iterator<>() {
            private final Iterator<Map.Entry<K, V>> internal = data.entrySet().iterator();
            
            @Override
            public boolean hasNext() {
                return internal.hasNext();
            }
            
            @Override
            public Map.Entry<K, V> next() {
                Map.Entry<K, V> entry = internal.next();
                return new CustomEntry<>(entry.getKey(), entry.getValue());
            }
        };
    }
    
    // 自定义Entry实现
    private static class CustomEntry<K, V> implements Map.Entry<K, V> {
        // 实现省略
    }
}

// instanceof模式匹配 + Map.Entry
Object obj = Map.entry("Java", 17);

if (obj instanceof Map.Entry<?,?> entry && 
    entry.getKey() instanceof String key && 
    entry.getValue() instanceof Integer value) {
    
    System.out.println(key + " version: " + value);
}

record CityPopulation(String city, int population) {}

​​​​​​​Map<String, Integer> data = Map.of("Paris", 2_161_000, "Rome", 2_873_000);

List<CityPopulation> cityData = data.entrySet().stream()
    .map(entry -> new CityPopulation(entry.getKey(), entry.getValue()))
    .toList();

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

​​​​​​​// 自定义Entry处理
scores.entrySet().forEach(entry -> {
    String grade = entry.getValue() >= 90 ? "A" : "B";
    System.out.println(entry.getKey() + ": " + grade);
});