Java中遍历ConcurrentHashMap的四种方式

                   package other;

import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * Java中遍历Map的四种方式,这里使用的是ConcurrentHashMap,
 * 读者可以替换为HashMap
 */
public class IteratorMap {
	public static void main(String[] args) {
		Map<String, String> map = new ConcurrentHashMap<String, String>();
		init(map);
		
		//方式一：在for-each循环中使用entries来遍历
		System.out.println("方式一：在for-each循环中使用entries来遍历");
		for(Map.Entry<String, String> entry: map.entrySet()) {
			 System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
		}
		
		//方法二：在for-each循环中遍历keys或values,这种方式适用于需要值或者键的情况,方法二比方法一快了10%
		System.out.println("方法二：在for-each循环中遍历keys或values,这种方式适用于需要值或者键的情况");
		//遍历键
		for(String key : map.keySet()) {
			System.out.println("key = " + key);
		}
		
		//遍历值
		for(String value : map.values()) {
			System.out.println("value = " + value);
		}
		
		//方法三：使用Iterator遍历,使用并发集合不会报异常,性能类似于方法二
		//使用泛型
		Iterator<Map.Entry<String, String>> entries = map.entrySet().iterator();  
		System.out.println("使用Iterator遍历,并且使用泛型:");
		while (entries.hasNext()) {  
		  
		    Map.Entry<String, String> entry = entries.next();  
		  
		    System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());  
		    
		    //注意这里操作了集合,下面的的遍历不会再打印0   if("0".equals(entry.getKey())) {
		    	map.remove(entry.getKey());
		    }   }  
		
		//不使用泛型
		Iterator entrys = map.entrySet().iterator();  
		System.out.println("使用Iterator遍历,并且不使用泛型");
		while (entrys.hasNext()) {  
		  
		    Map.Entry entry = (Map.Entry) entrys.next();  
		  
		    String key = (String)entry.getKey();  
		  
		    String value = (String)entry.getValue();  
		  
		    System.out.println("Key = " + key + ", Value = " + value);  
		  
		}  
		
		//方式四：通过键找值遍历,该方法效率相当低,不建议使用
		System.out.println("方式四：通过键找值遍历");
		for (String key : map.keySet()) {  
			  
		    String value = map.get(key);  
		  
		    System.out.println("Key = " + key + ", Value = " + value);  
		  
		}  
	}

	/**
	 * 初始化Map
	 * @param map
	 */
	private static void init(Map<String, String> map) {
		if(map == null) {
			throw new RuntimeException("参数为空,无法执行初始化");
		}
		for(int i = 0; i < 10; i ++) {
			map.put(String.valueOf(i), String.valueOf(i));
		}
	}
	
}  

方式一：在for-each循环中使用entries来遍历
Key = 0, Value = 0
Key = 1, Value = 1
Key = 2, Value = 2
Key = 3, Value = 3
Key = 4, Value = 4
Key = 5, Value = 5
Key = 6, Value = 6
Key = 7, Value = 7
Key = 8, Value = 8
Key = 9, Value = 9
方法二：在for-each循环中遍历keys或values,这种方式适用于需要值或者键的情况
key = 0
key = 1
key = 2
key = 3
key = 4
key = 5
key = 6
key = 7
key = 8
key = 9
value = 0
value = 1
value = 2
value = 3
value = 4
value = 5
value = 6
value = 7
value = 8
value = 9
使用Iterator遍历,并且使用泛型:
Key = 0, Value = 0
Key = 1, Value = 1
Key = 2, Value = 2
Key = 3, Value = 3
Key = 4, Value = 4
Key = 5, Value = 5
Key = 6, Value = 6
Key = 7, Value = 7
Key = 8, Value = 8
Key = 9, Value = 9
使用Iterator遍历,并且不使用泛型
Key = 1, Value = 1
Key = 2, Value = 2
Key = 3, Value = 3
Key = 4, Value = 4
Key = 5, Value = 5
Key = 6, Value = 6
Key = 7, Value = 7
Key = 8, Value = 8
Key = 9, Value = 9
方式四：通过键找值遍历
Key = 1, Value = 1
Key = 2, Value = 2
Key = 3, Value = 3
Key = 4, Value = 4
Key = 5, Value = 5
Key = 6, Value = 6
Key = 7, Value = 7
Key = 8, Value = 8
Key = 9, Value = 9