Java 返回一个迭代器
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Return an iterator
提问by user1960836
Searching for info about the iterator, I found only examples that showed how to iterate over a collection, and not returning the Iterator, like I want to do.
在搜索有关迭代器的信息时，我只找到了展示如何迭代集合的示例，而不是像我想做的那样返回迭代器。
I am practicing for the exam, so I'm trying out some programming excercises to prepare myself, and this one is about the iterator pattern.
I want to implement the getKnightPositionIterator, . You can see the code below. This code is not mine, I found this.
我正在为考试练习，所以我正在尝试一些编程练习来为自己做准备，这个练习是关于迭代器模式的。我想实现getKnightPositionIterator, . 你可以看到下面的代码。这段代码不是我的，我发现了这个。
package iterator;
import java.util.*;
public class Position {
/** return an iterator that will return all positions
* that a knight may reach from a given starting position.
*/
public static Iterator<Position> getKnightPositionIterator(Position p) {
return null;
}
/** create a position.
* @param r the row
* @param c the column
*/
public Position(int r, int c) {
this.r = r; this.c = c;
}
protected int r;
protected int c;
/** get the row represented by this position.
* @return the row.
*/
public int getRow() { return r; }
/** get the column represented by this position.
* @return the column.
*/
public int getColumn() { return c; }
public boolean equals(Object o) {
if (o.getClass() != Position.class) { return false; }
Position other = (Position) o;
return r==other.r && c==other.c;
}
public int hashCode() {
// works ok for positions up to columns == 479
return 479*r+c;
}
public String toString() {
return "["+r+","+c+"]";
}
}
How ever, I figure that I have to create an Iterator to return, so, so far, this is my attemp.
但是，我认为我必须创建一个 Iterator 才能返回，因此，到目前为止，这是我的尝试。
public static Iterator<Position> getKnightPositionIterator(Position p) {
Iterator<Position> knightPosIter = Position.getKnightPositionIterator(p);
for(Iterator<Position> positions = knightPosIter; positions.hasNext(); ) {
//What should I write here?
}
return knightPosIter;
}
采纳答案by Oleg Sklyar
First, make your class implement Iterableinterface
首先，让你的类实现Iterable接口
public class Position implements Iterable<Position>
and write the public Iterator<Positions> iterator();method as outlined below instead of providing a static method in your example.
并public Iterator<Positions> iterator();按照下面概述的方式编写方法，而不是在您的示例中提供静态方法。
As you actually need to compute a collection of reachable positions in one way or another, you will need a structure to hold it. Any such structure will normally be iterable and, thus, will have an iterator method. So a lazy implementation could look like this:
由于您实际上需要以一种或另一种方式计算一组可到达的位置，因此您将需要一个结构来保存它。任何这样的结构通常都是可迭代的，因此会有一个迭代器方法。所以一个懒惰的实现可能是这样的：
@Override
public Iterator<Position> iterator()
{
// make sure this returns e.g. Collections.unmodifiableList
Collection<Position> positions = computeReachablePositions();
return positions.iterator();
}
In case you have some other structure to compute and store your positions that is not iterable (not advisable), implement an iterator from scratch as follows (an array of positions assumed):
如果您有一些其他结构来计算和存储不可迭代（不可取）的位置，请按如下方式从头开始实现迭代器（假设位置数组）：
@Override
public Iterator<Position> iterator()
{
// must be final to be accessible from the iterator below
final Position[] positions = computeReachablePositions();
return new Iterator<Position>() {
int index = 0;
@Override
public boolean hasNext()
{
return index < positions.length;
}
@Override
public Position next()
{
if (hasNext())
{
Position value = positions[index];
index++;
return value;
}
throw new NoSuchElementException("No more positions available");
}
@Override
public void remove()
{
throw new UnsupportedOperationException("Removals are not supported");
}};
}
