可扩展集合类（遍历的时间执行插入操作）

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * 可扩展集合类
 */
public class AccessingMap {

    private static void userMap(final Map<String, Integer> scores) {
        scores.put("Joes", 90);
        scores.put("Mike", 85);
        
        try {
            for(final String key : scores.keySet()) {
                System.out.println(key + " " + scores.get(key));
                scores.put("Tom", 99);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    
    public static void main(String[] args) {
        System.out.println("Using HashMap");
        userMap(new HashMap<String, Integer>());//
        
        System.out.println("Using synchronized HashMap");
        userMap(Collections.synchronizedMap(new HashMap<String, Integer>()));//实现了线程安全的特性
        
        System.out.println("Using ConcurrentHashMap");
        userMap(new ConcurrentHashMap<String, Integer>());//保证了线程安全的同时也兼顾了并发访问的性能，将整个数据集合分成了若干个块
    }
}