Question

首先，我想说我知道其他更容易和更好的循环用于这种工作，但我只是想用Iterator练习一下。所以，我循环遍历一个ArrayList，我想用迭代器添加和删除它中的元素，但是当我运行程序时，屏幕上没有输出。这就是我所拥有的：

public class Orders { 
    ArrayList<String> list = new ArrayList<>();
    Iterator it = list.iterator();

    public void addOrder(String s) {
        while(it.hasNext()){
            String st = (String) it.next();
            if(s.equals(st)){
                System.out.println("The order '"+s+"' already exists.");
            }
            else{
                list.add(s);
                System.out.println("Order '"+s+"' was added.");
            }
        }

    }

    public void deleteOrder(String s) {
        while(it.hasNext()){
            String st = (String) it.next();
            if(s.equals(st)){
                list.remove(s);
                System.out.println("The order '"+s+"' was deleted succesfully.");
            }
            else{
                System.out.println("The order '"+s+"' was not found.");
            }
        }
    }

    public void showList(){
        while(it.hasNext()){
            String st = (String) it.next();
            System.out.println("Order: '"+st+"'. \n");
        }
    }
}



public class RemoveOrder {

  private Orders ord;
  public RemoveOrder( Orders o ) {
    ord = o;
  }
  public void remove() {  
      ord.deleteOrder( "jk123" );
      ord.deleteOrder( "jk125" );
      ord.showList();
  }
}


public class AddOrder {

    private Orders ord;

    public AddOrder( Orders o ) {
        ord = o;
    }

    public void add() {
        ord.addOrder( "jk123" );
        ord.addOrder( "jk124" );
        ord.addOrder( "jk125" );
        ord.addOrder( "jk126" );
        ord.showList();
    }
}


public class IteratorRun implements Runnable {


    Orders ord = new Orders();
    AddOrder ao = new AddOrder(ord);
    RemoveOrder ro = new RemoveOrder(ord);

    public void run() {
        try {
            ao.add();
            ro.remove();
        } catch (Exception e) {
        }
    }
}

主要是：

Thread t1 = new Thread(new IteratorRun());
t1.start();

Answer 1

在addOrder中检查it.hasNext（）。第一次数组列表为空因此it.hasNext将是假的。
如果我们尝试在使用迭代器时为araylist添加值，它会 throw java.util.ConcurrentModificationException

在大多数地方存在类似的代码addOrder / deleteOrder这会导致此异常。

示例：以下代码将抛出ConcurrentModificationException。

 ArrayList al = new ArrayList();
    al.add("test");
    Iterator iter = al.iterator();
    while(iter.hasNext()) {
        System.out.println(iter.next());
        al.add("test");
    }

Answer 2

我已更新订单代码

你需要从list中重新启动迭代器，因为list get 改性
用于检查列表中是否已存在的值的逻辑修改。

 

    import java.util.ArrayList;
    import java.util.Iterator;

    public class Orders { 
        ArrayList list = new ArrayList();
        Iterator it = null;

        public void addOrder(String s) {
            it = list.iterator();
            while(it.hasNext()){
                String st = (String) it.next();
                if(s.equals(st)){
                    System.out.println("The order '"+s+"' already exists.");
                    return;
                }

            }


                list.add(s);
                System.out.println("Order '"+s+"' was added.");    


        }

        public void deleteOrder(String s) {
            it = list.iterator();
            while(it.hasNext()){
                String st = (String) it.next();
                if(s.equals(st)){
                    list.remove(s);
                    System.out.println("The order '"+s+"' was deleted succesfully.");
                    return ;
                }

            }
                System.out.println("The order '"+s+"' was not found.");

        }

        public void showList(){
            it = list.iterator();
            while(it.hasNext()){
                String st = (String) it.next();
                System.out.println("Order: '"+st+"'. \n");
            }
        }
    }

使用Iterator从ArrayList添加，删除和打印

2 个答案: