Java中的AbstractMap entrySet()方法及示例

在Java中，AbstractMap类实现了Map接口，并提供了其它一些方法的默认实现。其中，entrySet()方法返回映射中包含的映射项的Set视图。这个方法返回的Set中的元素类型为Map.Entry，它们代表像这样的一组映射项(key-value)。

public interface Map<K,V> {
    ...
    Set<Map.Entry<K,V>> entrySet();
    ...
}

public interface Set<E> {
    ...
    interface Entry<K,V> {
        K getKey();
        V getValue();
        V setValue(V value);
        boolean equals(Object o);
        int hashCode();
    }
    ...
}

示例1：将Map转换为String

import java.util.*;

public class EntrySetExample {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("name", "Alice");
        map.put("age", "18");
        map.put("gender", "female");

        Set<Map.Entry<String, String>> entries = map.entrySet();
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<String, String> entry : entries) {
            sb.append(entry.getKey() + "=" + entry.getValue() + ",");
        }
        if (sb.length() > 0) {
            sb.deleteCharAt(sb.length() - 1);
        }

        String result = sb.toString();
        System.out.println(result);  // name=Alice,age=18,gender=female
    }
}

示例2：获取Map中最大的value

import java.util.*;

public class EntrySetExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("apple", 10);
        map.put("orange", 20);
        map.put("banana", 30);

        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        int max = Integer.MIN_VALUE;
        for (Map.Entry<String, Integer> entry : entries) {
            if (entry.getValue() > max) {
                max = entry.getValue();
            }
        }

        System.out.println(max);  // 30
    }
}

示例3：从Map中移除满足条件的映射项

import java.util.*;

public class EntrySetExample {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("name", "Alice");
        map.put("age", "18");
        map.put("gender", "female");

        Set<Map.Entry<String, String>> entries = map.entrySet();
        Iterator<Map.Entry<String, String>> iterator = entries.iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, String> entry = iterator.next();
            if (entry.getValue().equals("male")) {
                iterator.remove();
            }
        }

        System.out.println(map);  // {name=Alice, age=18, gender=female}
    }
}