Java Map在遍历过程中删除元素
2017-11-27 15:54
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Java中的Map如果在遍历过程中要删除元素,除非通过迭代器自己的remove()方法,否则就会导致抛出ConcurrentModificationException异常。JDK文档中是这么描述的:
The iterators returned by all of this class’s “collection view methods” are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
这么做的原因是为了保证迭代器能够尽快感知到Map的“结构性修改“,从而避免不同视图下不一致现象。
下面代码展示了遍历Map时删除元素的正确方式和错误方式。
The iterators returned by all of this class’s “collection view methods” are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
这么做的原因是为了保证迭代器能够尽快感知到Map的“结构性修改“,从而避免不同视图下不一致现象。
下面代码展示了遍历Map时删除元素的正确方式和错误方式。
import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Set; /** * Created by lh on 15-1-22. */ public class TestMapRemove { public static void main(String[] args){ new TestMapRemove().removeByIterator(); // new TestMapRemove().removeBymap(); } public void removeByIterator(){//正确的删除方式 HashMap<Integer, String> map = new HashMap<Integer, String>(); map.put(1, "one"); map.put(2, "two"); map.put(3, "three"); System.out.println(map); Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator(); while(it.hasNext()){ Map.Entry<Integer, String> entry = it.next(); if(entry.getKey() == 2) it.remove();//使用迭代器的remove()方法删除元素 } System.out.println(map); } public void removeBymap(){//错误的删除方式 HashMap<Integer, String> map = new HashMap<Integer, String>(); map.put(1, "one"); map.put(2, "two"); map.put(3, "three"); System.out.println(map); Set<Map.Entry<Integer, String>> entries = map.entrySet(); for(Map.Entry<Integer, String> entry : entries){ if(entry.getKey() == 2){ map.remove(entry.getKey());//ConcurrentModificationException } } System.out.println(map); } }
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