Java 集合 --- 如何遍历Map

• Map没有继承Collection接口
• AbstractMap和AbstractCollection是平级关系
  

Map的基本操作

package map;
import java.util.*;
/*** This program demonstrates the use of a map with key type String and value type Employee.* @version 1.12 2015-06-21* @author Cay Horstmann*/
public class MapTest {public static void main(String[] args) {Map<String, Employee> staff = new HashMap<>();staff.put("144-25-5464", new Employee("Amy Lee"));staff.put("567-24-2546", new Employee("Harry Hacker"));staff.put("157-62-7935", new Employee("Gary Cooper"));staff.put("456-62-5527", new Employee("Francesca Cruz"));// print all entriesSystem.out.println(staff);// remove an entrystaff.remove("567-24-2546");// replace an entrystaff.put("456-62-5527", new Employee("Francesca Miller"));// look up a valueSystem.out.println(staff.get("157-62-7935"));// iterate through all entriesstaff.forEach((k, v) -> System.out.println("key=" + k + ", value=" + v));}
}

如何遍历Map

the set of keys: 将所有的key作为一个set返回

Set<K> keySet()

Set<String> keys = map.keySet();
for (String key : keys) {do something with key
}

the collection of values (which is not a set): 将所有的value作为一个collection返回

Collection<V> values()

 Map<String,String> map=new HashMap<>();map.put("abc","123");map.put("efg","456");// 使用增强型for遍历循环Map集合Collection<String> values = map.values();for (String value : values) {System.out.println(value);}

the set of key/value pairs: 得到每一对key value pair

Set<Map.Entry<K, V>> entrySet()

for (Map.Entry<String, Employee> entry : staff.entrySet()) {String k = entry.getKey();Employee v = entry.getValue();do something with k, v
}

Type of HashMap

HashMap

• Hash table based implementation of the Map interface
• the default load factor (.75)
• 有一个子类为LinkedHashMap, 元素按照加入顺序排序

TreeMap

• A Red-Black tree based NavigableMap implementation.
• 需要根据compareTo排序

EumerateHashMap

• A specialized Map implementation for use with enum type keys.
• All of the keys in an enum map must come from a single enum type that is specified

WeakHashMap

• Hash table based implementation of the Map interface, with weak keys.
• An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use