java arraylist的问题

不得不说，我犯了错，很基础的。。

遍历list的时候可以删除数组元素吗？ 答案是：简单/增强for循环不可以，list.iterator()这样的方式就可以。

我之前做过类似面试题的，不过忘记了， 不记得是list还是set或者map了 。list？ 貌似也可以吧。

对于set、map也是同理。

如果是删除list倒数第一、倒数第二个元素，不会报错，

private static void listtest() {
          List<String> list = new ArrayList<String>();
          list.add("Java");
          list.add("C");
          list.add("C++");
          list.add("C#");
          try {
              for(String str:list) {
                  System.out.println(str);
 
                  if(str.equalsIgnoreCase("C++")) {
                      list.remove(str);
                  }
              }
          } catch (Exception e) {
              e.printStackTrace();
          }
          System.out.println(list);
    }

打印：

Java
C
C++　　　　-----C#现在的索引为2，被认为已经遍历过了的，被略去！！
[Java, C, C#]------ 删除是成功了的！！！

把c++改成c，则报错

Java
C
java.util.ConcurrentModificationException
[Java, C++, C#]
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:)
    at java.util.ArrayList$Itr.next(ArrayList.java:)
    at Test.listtest(Test.java:)
    at Test.main(Test.java:)

否则：

java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at com.huawei.oms.app.sysmgr.agent.neagent.db.DatabaseMonitor.queryDBInfo(DatabaseMonitor.java:)
    at com.huawei.oms.app.sysmgr.agent.neagent.NeAgentImpl.queryDBInfo(NeAgentImpl.java:)
    at sun.reflect.GeneratedMethodAccessor265.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.Trampoline.invoke(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor114.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
    at com.sun.jmx.mbeanserver.ConvertingMethod.invokeWithOpenReturn(Unknown Source)
    at com.sun.jmx.mbeanserver.ConvertingMethod.invokeWithOpenReturn(Unknown Source)
    at com.sun.jmx.mbeanserver.MXBeanIntrospector.invokeM2(Unknown Source)
    at com.sun.jmx.mbeanserver.MXBeanIntrospector.invokeM2(Unknown Source)
    at com.sun.jmx.mbeanserver.MBeanIntrospector.invokeM(Unknown Source)
    at com.sun.jmx.mbeanserver.PerInterface.invoke(Unknown Source)
    at com.sun.jmx.mbeanserver.MBeanSupport.invoke(Unknown Source)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(Unknown Source)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(Unknown Source)
    at javax.management.remote.rmi.RMIConnectionImpl.doOperation(Unknown Source)
    at javax.management.remote.rmi.RMIConnectionImpl.access$(Unknown Source)
    at javax.management.remote.rmi.RMIConnectionImpl$PrivilegedOperation.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.management.remote.rmi.RMIConnectionImpl.doPrivilegedOperation(Unknown Source)
    at javax.management.remote.rmi.RMIConnectionImpl.invoke(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor176.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
    at sun.rmi.transport.Transport$.run(Unknown Source)
    at sun.rmi.transport.Transport$.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.rmi.transport.Transport.serviceCall(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

如何处理这种情况呢？

用list的iterator就没问题， 这显然跟jdk、jvm本身相关。 —— 参考http://www.cnblogs.com/dongzhouzhou/articles/ConcurrentModificationException.html

Iterator<String> iterator = list.iterator();
          while (iterator.hasNext()) {
            String str = (String) iterator.next();
            System.out.println(str);
              if(str.equalsIgnoreCase("C")) {
                  iterator.remove();
              }
 
          }

同样的，对list做add操作会导致list长度动态增加、增加for和iterator都可以立即检测到。

另外一方面，如果我们对list里面的元素做修改呢？—— 对基础类型元素不会生效，———— 而对引用类型会生效。 当然，估计没人这么用。

private static void listtest() {
          List<Persion> list = new ArrayList<Persion>();  
 
          List<String> list2 = new ArrayList<String>();
          Persion p1 = new Persion("lk1", );
          Persion p2 = new Persion("lk2", );
          Persion p3 = new Persion("lk3", );
          Persion p4 = new Persion("lk4", );
          list.add(p1);
          list.add(p2);
          list.add(p3);
          list.add(p4);  
 
          Iterator<Persion> iterator = list.iterator();
          while (iterator.hasNext()) {
              Persion str = (Persion) iterator.next();
            System.out.println(str.getName());
              if(str.getName().equalsIgnoreCase("lk2")) {
                  str.setName("");
                  //iterator.remove();
              }
          }
 
          System.out.println(list);
    }
 
class Persion {
    String name;
    int age;
    public Persion(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
 
    @Override
    public String toString() {
 
        return "name : " + this.name + "  age : " + this.age;
    }
}