Arraylist remove元素

àì夳堔傛蜴生んèń 2022-05-30 01:35 182阅读 0赞

### Arraylist remove元素 ###

用Arraylist 的remove方法删除元素时出现如下异常：

三月 01, 2018 2:41:59 下午 org.apache.catalina.core.StandardWrapperValve invoke
    严重: Servlet.service() for servlet [user] in context with path [/jq] threw exception
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at com.zjht.action.Add.delete(Add.java:151)
    	at com.zjht.action.Add.doGet(Add.java:32)
    	at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    	at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:522)
    	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1095)
    	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:672)
    	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1502)
    	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1458)
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    	at java.lang.Thread.run(Thread.java:748)

我使用for-each 循环，如下代码：

for (User user : userList) {
    	if(user.getUserid() == userid){
    	userList.remove(user);
    	}
    }

从异常堆栈可以看出，是ArrayList的迭代器报出的异常，说明通过元素遍历集合时，实际上是使用迭代器进行访问的。

可为什么会产生这个异常呢?打断点单步调试进去发现，是这行代码抛出的异常：

final void checkForComodification() { 
         
        if (modCount != expectedModCount) 
         
        throw new ConcurrentModificationException(); 
         
        }

modCount是集合修改的次数，当remove元素的时候就会加1，初始值为集合的大小。迭代器每次取得下一个元素的时候，都会进行判断，比较集合修改的次数和期望修改的次数是否一样，如果不一样，则抛出异常。查看集合的remove方法：

private void fastRemove(int index) { 
         
        modCount++; 
         
        int numMoved = size - index - 1; 
         
        if (numMoved > 0) 
         
        System.arraycopy(elementData, index+1, elementData, index, 
         
        numMoved); 
         
        elementData[--size] = null; // clear to let GC do its work 
         
        }

可以看到，删除元素时modCount已经加一，但是expectModCount并没有增加。所以在使用迭代器遍历下一个元素的时候，会抛出异常。那怎么解决这个问题呢?

其实使用迭代器自身的删除方法就没有问题了

Iterator<User> iterator = userList.iterator();
    	while(iterator.hasNext()){
    		if(iterator.next().getUserid() == userid){
    			iterator.remove();
    			}
    		}

查看迭代器自身的删除方法，果不其然，每次删除之后都会修改expectedModCount为modCount。这样的话就不会抛出异常

public void remove() { 
         
        if (lastRet < 0) 
         
        throw new IllegalStateException(); 
         
        checkForComodification(); 
         
        try { 
         
        ArrayList.this.remove(lastRet); 
         
        cursor = lastRet; 
         
        lastRet = -1; 
         
        expectedModCount = modCount; 
         
        } catch (IndexOutOfBoundsException ex) { 
         
        throw new ConcurrentModificationException(); 
         
        } 
         
        }

建议以后操作集合类的元素时，尽量使用迭代器。

可是还有一个地方不明白，modCount和expectedModCount这两个变量究竟是干什么用的?

为什么集合在进行操作时需要修改它?为什么迭代器在获取下一个元素的时候需要判断它们是否一样?

它们存在总是有道理的吧