Module java.base Package java.util
Interface Map.Entry<K,V> All Known Implementing Classes: AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry
Enclosing interface: Map<K,V>
public static interface Map.Entry<K,V> A map entry (key-value pair). The Map.entrySet method returns a collection-view of the map, whose elements are of this class. The only way to obtain a reference to a map entry is from the iterator of this collection-view. These Map.Entry objects are valid only for the duration of the iteration; more formally, the behavior of a map entry is undefined if the backing map has been modified after the entry was returned by the iterator, except through the setValue operation on the map entry.
Since: 1.2
See Also: Map.entrySet()
示例:
import java.util.Map; public class IteratorDemo { public static void main(String[] args) { Map.Entry<String, Integer> entry = Map.entry("one", 1); System.out.println("Key=" + entry.getKey()); System.out.println("Value=" + entry.getValue()); System.out.println(entry); System.out.println(entry.getClass().getName()); } }
运行结果:
Key=one Value=1 one=1 java.util.KeyValueHolder
经过分析可以发现:如果想要使用Iterator实现Map集合的输出,则必须按照如下步骤处理:
- 利用Map接口中提供的entrySet()方法将Map集合转为Set集合;
- 利用Set接口中的iterator()方法将Set集合转为Iterator接口实例;
- 利用Iterator进行迭代输出,获取每一组的Map.Entry对象,随后通过getKey()和getValue()方法获取数据。
示例:利用Iterator输出Map集合
import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Set; public class IteratorDemo { public static void main(String[] args) { Map<String, Integer> map = new HashMap<String, Integer>(); map.put("one", 1); map.put("two", 2); map.put("three", 3); Set<Map.Entry<String, Integer>> set = map.entrySet();//将Map集合转为Set集合 Iterator<Map.Entry<String, Integer>> iter = set.iterator();//将Set集合转为Iterator接口实例 while (iter.hasNext()) { Map.Entry<String, Integer> me = iter.next();//获取每一组的Map.Entry对象 System.out.println(me.getKey() + "=" + me.getValue()); } } }
运行结果:
one=1 two=2 three=3
虽然Map集合本身支持迭代输出,但是如果从实际的开发来讲,Map集合最主要的用法在于实现数据的key查找操作,另外需要提醒的是,如果现在不使用Iterator而使用forEach愈发输出,也需要将Map集合转为Set集合。
示例:使用forEach输出Map集合
import java.util.HashMap; import java.util.Map; import java.util.Set; public class IteratorDemo { public static void main(String[] args) { Map<String, Integer> map = new HashMap<String, Integer>(); map.put("one", 1); map.put("two", 2); map.put("three", 3); Set<Map.Entry<String, Integer>> set = map.entrySet();//将Map集合转为Set集合 for(Map.Entry<String, Integer> entry: set) { System.out.println(entry.getKey()+"="+entry.getValue()); } } }
运行结果:
one=1 two=2 three=3