异常：java.util.ConcurrentModificationException

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我是靠谱客的博主专一鼠标，这篇文章主要介绍异常：java.util.ConcurrentModificationException，现在分享给大家，希望可以做个参考。

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1
重现异常：

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复制代码import java.util.ArrayList;
import java.util.Iterator;

class User {
    private String userName, password;
    private int ID;

    User(int p_ID, String p_userName, String p_password) {
        ID = p_ID;
        userName = p_userName;
        password = p_password;
    }
}

public class Test {
    static ArrayList al = new ArrayList();

    public static void main(String[] args) {
        add();
        clear();
    }

    public static void add() {
        for (int i = 0; i < 5; i++) {
            al.add(new User(i, "userName", "password"));
        }
    }

    public static void clear() {
        Iterator it = ((ArrayList) al).iterator();
        while (it.hasNext()) {
            User usr = (User) (it.next());
            int index = al.indexOf(usr);
            al.remove(index);
            // it.remove();
        }
        System.out.println("Cleard");
    }
}
import java.util.ArrayList;
import java.util.Iterator;

class User {
    private String userName, password;
    private int ID;

    User(int p_ID, String p_userName, String p_password) {
        ID = p_ID;
        userName = p_userName;
        password = p_password;
    }
}

public class Test {
    static ArrayList al = new ArrayList();

    public static void main(String[] args) {
        add();
        clear();
    }

    public static void add() {
        for (int i = 0; i < 5; i++) {
            al.add(new User(i, "userName", "password"));
        }
    }

    public static void clear() {
        Iterator it = ((ArrayList) al).iterator();
        while (it.hasNext()) {
            User usr = (User) (it.next());
            int index = al.indexOf(usr);
            al.remove(index);
            // it.remove();
        }
        System.out.println("Cleard");
    }
}

注意到上面代码中注释部分的上面2句，那就是异常出现的根源，我从ArrayList的一个对象al中移去了项目，然而Iterator对象it并不知道al中已经发生了变化，所以再继续遍历的时候会发生错误。正确的写法应该如注释行所示，然后注释行去掉上面的2行。

在Map或者Collection的时候，不要用它们的API直接修改集合的内容，如果要修改可以用Iterator的remove()方法，例如：

    public void setReparation( Reparation reparation ) {
        for (Iterator it = this.reparations.iterator();it.hasNext();){    //reparations为Collection
            Reparation repa = (Reparation)it.next();
            if (repa.getId() == reparation.getId()){
                this.reparations.remove(repa);
                this.reparations.add(reparation);
            }
        }
   }

如上写会在运行期报ConcurrentModificationException，可以如下修改：

    public void setReparation( Reparation reparation ) {
        boolean flag = false;
        for (Iterator it = this.reparations.iterator();it.hasNext();){    //reparations为Collection
            Reparation repa = (Reparation)it.next();
            if (repa.getId() == reparation.getId()){
                it.remove();
                flag = true;
                break;
            }
        }
        if(flag){
          this.reparations.add(reparation);
        }
    }