Java 简明教程
Java - How to Use Iterator?
通常，您会希望循环访问 collection 中的元素。例如，您可能需要显示每个元素。执行此操作的最简单方法是使用迭代器，该迭代器是实现 Iterator 或 ListIterator 接口的 object 。
Often, you will want to cycle through the elements in a collection. For example, you might want to display each element. The easiest way to do this is to employ an iterator, which is an object that implements either the Iterator or the ListIterator interface.
Iterator 让您可以循环处理集合，获取或移除元素。ListIterator 扩展 Iterator 以允许双向遍历列表并修改元素。
Iterator enables you to cycle through a collection, obtaining or removing elements. ListIterator extends Iterator to allow bidirectional traversal of a list, and the modification of elements.
在您可以通过迭代器访问集合之前，必须获取一个迭代器。每个集合类都提供了一个 iterator( ) 方法，该方法返回一个迭代器，用于集合的开始。通过使用此迭代器对象，您可以一次访问集合中的每个元素。
Before you can access a collection through an iterator, you must obtain one. Each of the collection classes provides an iterator( ) method that returns an iterator to the start of the collection. By using this iterator object, you can access each element in the collection, one element at a time.
一般来说，要使用迭代器循环处理集合的内容，请执行以下步骤：
In general, to use an iterator to cycle through the contents of a collection, follow these steps −
-
Obtain an iterator to the start of the collection by calling the collection’s iterator( ) method.
-
Set up a loop that makes a call to hasNext( ). Have the loop iterate as long as hasNext( ) returns true.
-
Within the loop, obtain each element by calling next( ).
对于实现 List 的集合，您还可以通过调用 ListIterator 来获取一个迭代器。
For collections that implement List, you can also obtain an iterator by calling ListIterator.
The Methods Declared by Iterator
Sr.No. |
Method & Description |
1 |
boolean hasNext( ) Returns true if there are more elements. Otherwise, returns false. |
2 |
Object next( ) Returns the next element. Throws NoSuchElementException if there is not a next element. |
3 |
void remove( ) Removes the current element. Throws IllegalStateException if an attempt is made to call remove( ) that is not preceded by a call to next( ). |
The Methods Declared by ListIterator
Sr.No. |
Method & Description |
1 |
void add(Object obj) Inserts obj into the list in front of the element that will be returned by the next call to next( ). |
2 |
boolean hasNext( ) Returns true if there is a next element. Otherwise, returns false. |
3 |
boolean hasPrevious( ) Returns true if there is a previous element. Otherwise, returns false. |
4 |
Object next( ) Returns the next element. A NoSuchElementException is thrown if there is not a next element. |
5 |
int nextIndex( ) Returns the index of the next element. If there is not a next element, returns the size of the list. |
6 |
Object previous( ) Returns the previous element. A NoSuchElementException is thrown if there is not a previous element. |
7 |
int previousIndex( ) Returns the index of the previous element. If there is not a previous element, returns -1. |
8 |
void remove( ) Removes the current element from the list. An IllegalStateException is thrown if remove( ) is called before next( ) or previous( ) is invoked. |
9 |
void set(Object obj) Assigns obj to the current element. This is the element last returned by a call to either next( ) or previous( ). |
Example 1
以下示例演示了 Iterator 的使用。它使用了一个 ArrayList 对象，但一般原理适用于任何类型的集合。
Here is an example demonstrating Iterator. It uses an ArrayList object, but the general principles apply to any type of collection.
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class IteratorDemo {
public static void main(String args[]) {
// Create an array list
List<String> al = new ArrayList<>();
// add elements to the array list
al.add("C");
al.add("A");
al.add("E");
al.add("B");
al.add("D");
al.add("F");
// Use iterator to display contents of al
System.out.print("Original contents of al: ");
Iterator<String> itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
}
}Example 2
以下示例演示了 ListIterator 的使用。它使用了一个 ArrayList 对象，但一般原理适用于任何类型的集合。
Here is an example demonstrating ListIterator. It uses an ArrayList object, but the general principles apply to any type of collection.
List interface 列表迭代器当然仅对实现了 List interface 的集合可用。
Of course, ListIterator is available only to those collections that implement the List interface.
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
public class IteratorDemo {
public static void main(String args[]) {
// Create an array list
List<String> al = new ArrayList<>();
// add elements to the array list
al.add("C");
al.add("A");
al.add("E");
al.add("B");
al.add("D");
al.add("F");
// Use iterator to display contents of al
System.out.print("Original contents of al: ");
Iterator<String> itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
}
}Example 3
以下示例演示了 ListIterator 如何在迭代时修改列表。它使用了一个 ArrayList 对象，但一般原理适用于任何类型的集合。
Here is an example demonstrating ListIterator to modify the list while iterating. It uses an ArrayList object, but the general principles apply to any type of collection.
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
public class IteratorDemo {
public static void main(String args[]) {
// Create an array list
List<String> al = new ArrayList<>();
// add elements to the array list
al.add("C");
al.add("A");
al.add("E");
al.add("B");
al.add("D");
al.add("F");
// Use iterator to display contents of al
System.out.print("Original contents of al: ");
Iterator<String> itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// Modify objects being iterated
ListIterator<String> litr = al.listIterator();
while(litr.hasNext()) {
Object element = litr.next();
litr.set(element + "+");
}
System.out.print("Modified contents of al: ");
itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// Now, display the list backwards
System.out.print("Modified list backwards: ");
while(litr.hasPrevious()) {
Object element = litr.previous();
System.out.print(element + " ");
}
System.out.println();
}
}