Java Map 循环遍历方式与性能对比实例详解

Map<String, String> map = new HashMap<>();
map.put("A", "Apple");
map.put("B", "Banana");
map.put("C", "Cherry");

for (Map.Entry<String, String> entry : map.entrySet()) {
    String key = entry.getKey();
    String value = entry.getValue();
    System.out.println(key + " -> " + value);
}

for (String key : map.keySet()) {
    String value = map.get(key);
    System.out.println(key + " -> " + value);
}

for (String value : map.values()) {
    System.out.println(value);
}

Iterator<Map.Entry<String, String>> iter = map.entrySet().iterator();
while (iter.hasNext()) {
    Map.Entry<String, String> entry = iter.next();
    if ("B".equals(entry.getKey())) {
        iter.remove();  // 安全删除
    }
}

map.forEach((key, value) -> System.out.println(key + " -> " + value));

map.entrySet().stream()
    .filter(e -> e.getKey().startsWith("A"))
    .forEach(e -> System.out.println(e.getKey() + " -> " + e.getValue()));

map.replaceAll((key, value) -> value.toUpperCase());

ConcurrentHashMap<String, String> cmap = new ConcurrentHashMap<>();
// 使用与 HashMap 相同的遍历方式，但弱一致性
cmap.forEach((k, v) -> System.out.println(k));

// 错误示范（会抛异常）
for (String key : map.keySet()) {
    if (condition) map.remove(key);
}
// 正确方式：Iterator 删除
Iterator<Map.Entry<String,String>> it = map.entrySet().iterator();
while(it.hasNext()){
    if(condition) it.remove();
}

public class MapLoopDemo {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("Java", "17");
        map.put("Python", "3.11");
        map.put("Go", "1.20");
        // 1. entrySet + for-each
        System.out.println("=== entrySet ===");
        for (var entry : map.entrySet()) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
        }
        // 2. Java 8 forEach
        System.out.println("=== forEach Lambda ===");
        map.forEach((k, v) -> System.out.println(k + " : " + v));
        // 3. Stream 过滤
        System.out.println("=== Stream filter ===");
        map.entrySet().stream()
            .filter(e -> e.getKey().length() > 3)
            .forEach(e -> System.out.println(e.getKey()));
    }
}