Question

我正在尝试使用removeItem方法从ArrayList中删除字符串项。该项目已被删除，但我仍然得到NullPointerException，但不确定原因。

/**
 * remove item
 */
public void removeItem(String description)
{
   Iterator<Item> i = items.iterator();      
   Item items = i.next();
   if items.getDescription().equals(description) {
    i.remove();
   }
   else {
       System.out.println("Invalid item description");
    }
   System.out.println("Item removed!");
}

Answer 1

阅读此How do I compare strings in Java?

您应该使用.equals来检查两个字符串是否相等。

并且，当您使用迭代器时，您应该使用迭代器的hasNext方法检查是否有任何项目可用。

无论如何，还有其他方法，例如remove！

您的代码将类似于：

public void removeItem(String description)
{
    Iterator<Item> i = items.iterator();

    while (i.hasNext())
    {
        Item items = i.next();

        if (items.getDescription().equals(description))
        {
            i.remove();
            System.out.print("Item removed!");
            return;
        }
    }
}

return因为你应该停止while并退出该方法，如果项目已被删除。

您应该检查items和getDescription是否不是null！

使用您的代码，您将获得

＆＃34;无效的商品说明＆＃34;

无论如何，

。

Answer 2

尝试此操作 - 使用迭代器的hasNext()方法＆amp;使用空检查

Iterator<Item> i = items.iterator();

while (i.hasNext()) {
    Item items = i.next();

    if (items.getDescription() != null && items.getDescription().equals(description)) {
        i.remove();
        System.out.print("Item removed!");
        return;
    }
}

Answer 3

这有很多问题。请参阅我在代码中添加的评论。

/**
 * remove item
 */
public void removeItem(String description)
{
   Iterator<Item> i = items.iterator();  //ok, so the variable items is probably a class field
   Item items = i.next(); //...we just overwrote our class field items with the first item from iter...
   if items.getDescription().equals(description) { //we don't have parenthesis around our if statement...?
    i.remove(); //ok, we removed the item because it was equal
   }
   else {
       System.out.println("Invalid item description"); //it wasn't equal, so we say we got an invalid description
    }
    //aaaand apparently we are done. We only checked the first item in "items"
   System.out.println("Item removed!"); //this is outside of the if statement, so we will print this every time
}

所以，让我们清理这些问题。看起来可能有多个具有相同描述的项目，所以让我们改变方法以反映它。

/**
 * removes all the items with a matching description
 * returns the number of items removed
 */
public int removeItemsByDescription(String description) //make the method name reflect what it does
{
    //do we want to be able to check against a null description? because this changes if that is the case
    if(description == null)
         throw new NullPointerException("description was null");
    if(items == null) //make sure items is not null
         throw new NullPointerException("Items was null");
    Iterator<Item> itemsIter = items.iterator(); //more descriptive iterator name
    Item item; //ok, we name our single item "item" - and we just declare it here, we'll assign it later
    int removedItemNum = 0; //keep track of the total number of items we are removing
    while(itemsIter.hasNext()){//let's go through all the items
        item =  = itemsIter.next(); //get the next item
        if(description.equals(item.getDescription())){ //in order to avoid getting a null pointer exception when the description is null (which is probably perfectly valid), we will use description's .equals method
            itemsIter.remove(); //remove the item
            removedItemNum++; //increment the counter
        }
        //we don't need an else; if it didn't match, we just go on to the next one
    }
    System.out.println("Removed "+removedItemNum+" items");//print out how many we removed
}

然后我们去！完成测试后，您应该将最终的print语句移出它。

就像我在评论中提到的那样，如果您希望能够删除带有空描述的项目（即，您希望将null作为有效参数传递），则此方法会更改。

从ArrayList中删除Item

3 个答案: