如何解决当我们在LinkedList中只有一个元素时,它不应该给我们带来一些错误,因为我们试图使用previous访问上一个索引
我包括了很多SoP语句,只是为了查看代码中发生了什么。
此行时
addInOrder(placesToVisit,"Sydney");
被执行,将不会执行while
方法中的addInOrder()
循环,因为LinkedList
中没有元素,并且hasNext()
将返回false,“ Sydney”将插入到索引0。
现在何时
addInOrder(placesToVisit,"Melbourne");
执行后,控件将以while
方法进入addInOrder()
循环。由于必须先在“悉尼”之前插入“墨尔本”才能按字母顺序排列列表,所以compareTo()
将返回一个大于0的值。现在,在此块中,我们使用了previous()
方法来移动光标在“ Sydney(index = 0)”之前,但是在索引0(它的null或-1)之前,我也已经打印了previousIndex()
值为-1的值,仍然可以使用。
以下是输出:
nextIndex: 0
previousIndex: -1
此打印在进入while循环之前
Now visiting Sydney
=============================
nextIndex: 0
previousIndex: -1
nextIndex: 0
nextIndex: 1
nextIndex = 1,表示currentIndex = 0,怎么将previousIndex也设为0;
previousIndex: 0
counter 1
6
comparision > 0
nextIndex: 1
previousIndex() 0
nextIndex = 1,即currentIndex = 0,那么previousIndex怎么也为0?应该是-1
Now visiting Melbourne
Now visiting Sydney
=============================
Now visiting Melbourne
Now visiting Sydney
=============================
请解释为什么它起作用。当我们在索引0上使用previous()并尝试插入某些内容时,它应该给出一些错误或某些内容。不是吗?
public class Launch {
static int counter;
public static void main(String[] args) {
LinkedList<String> placesToVisit = new LinkedList<String>();
addInOrder(placesToVisit,"Sydney");
printList(placesToVisit);
addInOrder(placesToVisit,"Melbourne");
printList(placesToVisit);
printList(placesToVisit);
}
private static void printList(LinkedList<String> linkedList)
{
Iterator<String> i = linkedList.iterator();
while(i.hasNext())
{
System.out.println("Now visiting " + i.next());
}
System.out.println("=============================");
}
private static boolean addInOrder(LinkedList<String> linkedList,String newCity)
{
ListIterator<String> stringListIterator = linkedList.listIterator();
System.out.println("nextIndex: " + stringListIterator.nextIndex());
System.out.println("previousIndex: " + stringListIterator.previousIndex());
while(stringListIterator.hasNext())
{
System.out.println("nextIndex: " + stringListIterator.nextIndex());
int comparison = stringListIterator.next().compareTo(newCity);
System.out.println("nextIndex: " + stringListIterator.nextIndex());
System.out.println("Here nextIndex = 1,that means,currentIndex = 0,then how can be the previousIndex also 0 ? Is should be -1");
System.out.println("previousIndex: " + stringListIterator.previousIndex());
counter++;
System.out.println("counter "+ +counter);
System.out.println(comparison);
if(comparison == 0)
{
// equal,do not add
System.out.println(newCity + " is already in the list. So discarded.");
System.out.println("comarision = 0");
System.out.println();
return false;
}
else if(comparison > 0)
{
//move control to previous position and add the new item
System.out.println("comarision > 0");
System.out.println("nextIndex: " + stringListIterator.nextIndex());
System.out.println("previousIndex() "+stringListIterator.previousIndex());
System.out.println("Here nextIndex = 1,then how can be the previousIndex also 0 ? Is should be -1");
stringListIterator.previous();
stringListIterator.add(newCity);
System.out.println();
return true;
}
else if(comparison < 0)
{
// Do nothing,let it go ahead
System.out.println("comarision < 0");
System.out.println();
}
}
stringListIterator.add(newCity);
System.out.println("This printed before going into while loop");
return true;
}
}
解决方法
When we use previous() on index 0 and try to insert something,it should give some error or something.
我们知道,在链表概念中,指针是指向列表中节点或元素的东西。
但是在Iterator中,存在游标的概念,与指针不同,它存在于节点/元素之间。
在这里
nextIndex():给出光标的下一个索引值。
previousIndex():给出光标的宝贵索引值。
您的情况;
当nextIndex:0和previousIndex:-1时,光标位于索引0之前。 当nextIndex:1和previousIndex:0时,光标位于第一个元素和第二个元素之间。
与其他人类似。
不要在游标和指针之间混淆。
示例:
List<String> list = new ArrayList<>();
list.add("Berlin");
list.add("Sydney");
list.add("London");
list.add("Italy");
// Get the list iterator
ListIterator<String> iterator = list.listIterator();
while (iterator.hasNext()) {
int ii = iterator.previousIndex();
int index = iterator.nextIndex();
String element = iterator.next();
System.out.println("PreviousIndex="+ii+",NextIndex=" + index + ",Element=" + element);
}
输出:
PreviousIndex=-1,NextIndex=0,Element=Berlin
PreviousIndex=0,NextIndex=1,Element=Sydney
PreviousIndex=1,NextIndex=2,Element=London
PreviousIndex=2,NextIndex=3,Element=Italy
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