Java ListIterator 以前的方法不起作用
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ListIterator previous method not working
提问by tcp
package wrap;
import java.util.*;
public class ArrayListDemo {
public static void main(String [] args){
ArrayList<String> a=new ArrayList<String>();
a.add("B");
a.add("C");
a.add("D");
ListIterator<String> i=a.listIterator();
while(i.hasPrevious()){
System.out.println(i.previous());
}
}
}
The program works fine for hasNext() and next() methods but for hasPrevious() and previous() it displays a message as below::
该程序适用于 hasNext() 和 next() 方法，但对于 hasPrevious() 和 previous() 它显示如下消息：
<terminated> ArrayListDemo [Java Application] C:\Program Files (x86)\Java\jre7\bin\javaw.exe (28-Oct-2013 3:20:35 PM)
回答by Jeroen Vannevel
回答by Alexis C.
From the doc :
从文档：
public ListIterator<E> listIterator()
Returns a list iterator over the elements in this list (in proper sequence).
返回此列表中元素的列表迭代器（按适当的顺序）。
and
和
boolean hasPrevious()
Returns true if this list iterator has more elements when traversing the list in the reverse direction.
如果此列表迭代器在反向遍历列表时有更多元素，则返回 true。
Because the iterator is in the first position, hasPrevious()will return false and hence the while loop is not executed.
因为迭代器位于第一个位置，hasPrevious()将返回 false，因此不会执行 while 循环。
a's elements
"B" "C" "D"
^
|
Iterator is in first position so there is no previous element
If you do :
如果你这样做：
ListIterator<String> i=a.listIterator(); <- in first position
i.next(); <- move the iterator to the second position
while(i.hasPrevious()){
System.out.println(i.previous());
}
It will print "B"because you're in the following situation :
它将打印，"B"因为您处于以下情况：
a的元素
"B" "C" "D"
^
|
Iterator is in second position so the previous element is "B"
You could also use the method listIterator(int index). It allows you to place the iterator at the position defined by index.
您也可以使用该方法listIterator(int index)。它允许您将迭代器放置在由 定义的位置index。
If you do :
如果你这样做：
ListIterator<String> i=a.listIterator(a.size());
It will print
它会打印
D
C
B
回答by Prabhakaran Ramaswamy
ListIterator<String> i=a.listIterator();
initially iterator i will point to index of 0
最初的迭代器我会指向 index of 0
You are at index 0so there is no previous element.
你在index 0所以没有前一个元素。
回答by Aniket Thakur
You gave to reach to the end of the list or anywhere in between to traverse back. There is no previous element to element at index 0.
您可以到达列表的末尾或中间的任何位置以返回。索引 0 处的元素没有前一个元素。
Eg
例如
System.out.println("Elements in forward directiton");
while(i.hasNext()){
System.out.println(i.next());
}
System.out.println("Elements in backward directiton");
while(i.hasPrevious()){
System.out.println(i.previous());
}
回答by SudoRahul
Since you get the default ListIteratorfor the list, it starts with the first element, which is why hasPrevious()returns falseand the whileloop is exited. If you want to traverse the list in the reverse order, get the ListIteratorfrom the last index and traverse backwards using the hasPrevious()and previous()methods.
由于您获得ListIterator列表的默认值，它从第一个元素开始，这就是hasPrevious()返回false并while退出循环的原因。如果要以相反的顺序遍历列表，请ListIterator从最后一个索引获取 并使用hasPrevious()和previous()方法向后遍历。
ListIterator<String> i = a.listIterator(a.size()); // Get the list iterator from the last index
while (i.hasPrevious()) {
System.out.println(i.previous());
}
回答by Venu Sam
As said by others, You pointer is still pointing to first element in List so -1 is not possible.
正如其他人所说，你的指针仍然指向 List 中的第一个元素，所以 -1 是不可能的。
List of size n will have 1,2 ....n-1 as index.
大小为 n 的列表将 1,2 ....n-1 作为索引。
n-1 is not -1.
n-1 不是 -1。
THis is the reason why hasPrevious and previous are not working for you.
这就是 hasPrevious 和 previous 不适合您的原因。
Cheers.
干杯。
