Java迭代器示例

朴灿烈づ我的快乐病毒、 2023-02-15 01:27 48阅读 0赞

一些Java `Iterator`和`ListIterator`示例。

## 1.迭代器 ##

1.1从`List`或`Set`获取`Iterator` ，并对其进行循环。

JavaIteratorExample1a.java

package com.mkyong;
    
    import java.util.Arrays;
    import java.util.Iterator;
    import java.util.List;
    
    public class JavaIteratorExample1a {
    
        public static void main(String[] args) {
    
            /*
            Set<Integer> set = new HashSet<>();
            set.add(1);
            set.add(2);
            set.add(3);
            set.add(4);
            set.add(5);
    
            Iterator<Integer> iterator = set.iterator();
            */
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
            Iterator<Integer> iterator = list.iterator();
    
            while (iterator.hasNext()) {
    
                Integer result = iterator.next();
                System.out.println(result);
    
            }
        }
    
    }

输出量

1
    2
    3
    4
    5

1.2 Java 8中，在`Iterator`添加了一个新的`forEachRemaining` ，它提供了`Consumer`参数来让我们定义如何使用该项目。

Iterator.java

default void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (hasNext())
                action.accept(next());
        }

现在，我们可以使用lambda来提供`Consumer`功能接口的实现。

JavaIteratorExample1b.java

package com.mkyong;
    
    import java.util.Arrays;
    import java.util.Iterator;
    import java.util.List;
    
    public class JavaIteratorExample1b {
    
        public static void main(String[] args) {
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
            Iterator<Integer> iterator = list.iterator();
    
            // lambda
            // iterator.forEachRemaining(x -> System.out.println(x));
    
            // Java 8, method reference
            iterator.forEachRemaining(System.out::println);
    
        }
    
    }

输出量

1
    2
    3
    4
    5

1.3 `forEachRemaining`另一个示例。

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
            Iterator<Integer> iterator = list.iterator();
    
            iterator.forEachRemaining(x -> {
                System.out.println(x * 2);
            });

输出量

2
    4
    6
    8
    10

1.4使用`Iterator.remove()`删除元素。

JavaIteratorExample1c.java

package com.mkyong;
    
    import java.util.ArrayList;
    import java.util.Iterator;
    import java.util.List;
    
    public class JavaIteratorExample1c {
    
        public static void main(String[] args) {
    
            // immutable list
            // List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
    
            // mutable list
            List<Integer> list = new ArrayList<>();
            list.add(1);
            list.add(2);
            list.add(3);
            list.add(4);
            list.add(5);
    
            Iterator<Integer> iterator = list.iterator();
            while (iterator.hasNext()) {
                Integer next = iterator.next();
                if (next == 3 || next == 4) {
                    iterator.remove();
                }
            }
    
            list.forEach(System.out::println);
    
        }
    
    }

输出量

1
    2
    5

## 2. ListIterator ##

此`ListIterator`扩展了`Iterator`以提供其他功能，例如

*  在`next()`和`previous()`任一方向遍历列表
 *  在迭代过程中修改列表， `add()`和`set()`
 *  获取迭代器在列表中的当前位置`nextIndex()`和`previousIndex()`

2.1将`List`转换为`ListIterator` 。

JavaIteratorExample2a.java

package com.mkyong;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.ListIterator;
    
    public class JavaIteratorExample2a {
    
        public static void main(String[] args) {
    
            List<String> list = Arrays.asList("a", "b", "c", "d", "e");
    
            // get ListIterator, start from list's index 2
            ListIterator<String> iterator = list.listIterator(2);
            iterator.forEachRemaining(System.out::print);   // cde
    
            System.out.println("");
    
            // get ListIterator, start from list's index 0
            ListIterator<String> iterator2 = list.listIterator();
            iterator2.forEachRemaining(System.out::print);  // abcde
        }
    
    }

输出量

cde
    abcde

2.2沿前后方向遍历列表。

JavaIteratorExample2b.java

package com.mkyong;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.ListIterator;
    
    public class JavaIteratorExample2b {
    
        public static void main(String[] args) {
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
    
            ListIterator<Integer> iterator = list.listIterator();
    
            // loop to the end
            while (iterator.hasNext()) {
                Integer next = iterator.next();
                System.out.println(next);
            }
    
            System.out.println("---");
    
            // now loop back to the begin
            while (iterator.hasPrevious()) {
                Integer previous = iterator.previous();
                System.out.println(previous);
            }
    
        }
    
    }

输出量

1
    2
    3
    4
    5
    ---
    5
    4
    3
    2
    1

2.3使用`ListIterator.set()`更新元素

JavaIteratorExample2c.java

package com.mkyong;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.ListIterator;
    
    public class JavaIteratorExample2c {
    
        public static void main(String[] args) {
    
            List<Integer> list = new ArrayList<>();
            list.add(1);
            list.add(2);
            list.add(3);
            list.add(4);
            list.add(5);
    
            ListIterator<Integer> iterator = list.listIterator();
    
            while (iterator.hasNext()) {
                Integer next = iterator.next();
                if (next == 3) {
                    iterator.set(33);
                }
                if (next == 4) {
                    iterator.set(44);
                }
            }
    
            list.forEach(x -> System.out.println(x));
        }
    
    }

输出量

1
    2
    33  // updated, was 3
    44  // updated, was 4
    5

2.4使用`ListIterator.add()`添加元素

JavaIteratorExample2d.java

package com.mkyong;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.ListIterator;
    
    public class JavaIteratorExample2d {
    
        public static void main(String[] args) {
    
            List<Integer> list = new ArrayList<>();
            list.add(1);
            list.add(2);
            list.add(3);
            list.add(4);
            list.add(5);
    
            ListIterator<Integer> iterator = list.listIterator();
    
            while (iterator.hasNext()) {
                Integer next = iterator.next();
                if (next == 5) {
                    iterator.add(6);
                }
            }
    
            list.forEach(x -> System.out.println(x));
        }
    
    }

输出量

1
    2
    3
    4
    5
    6  // new!

2.4获取迭代器的当前位置， `previousIndex()`和`nextIndex()`

JavaIteratorExample2c.java

package com.mkyong;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.ListIterator;
    
    public class JavaIteratorExample2c {
    
        public static void main(String[] args) {
    
            List<Integer> list = new ArrayList<>();
            list.add(1);
            list.add(2);
            list.add(3);
            list.add(4);
            list.add(5);
    
            ListIterator<Integer> iterator = list.listIterator();
    
            while (iterator.hasNext()) {
    
                // cursor index, iterator not move yet!
                // init = -1 , add 0
                if (iterator.previousIndex() < 0) {
                    iterator.add(0);
                }
    
                // move the iterator
                iterator.next();
    
                // if this is last index
                if (iterator.nextIndex() >= list.size()) {
                    iterator.add(6);
                }
    
            }
    
            list.forEach(x -> System.out.println(x));
        }
    
    }

输出量

0
    1
    2
    3
    4
    5
    6

## 3.将迭代器转换为流 ##

遵循步骤`Iterator -> Spliterators -> Stream -> List`

JavaIteratorExample3a.java

package com.mkyong;
    
    import java.util.*;
    import java.util.stream.Collectors;
    import java.util.stream.StreamSupport;
    
    public class JavaIteratorExample3a {
    
        public static void main(String[] args) {
    
            List<Integer> list = new ArrayList<>();
            list.add(1);
            list.add(2);
            list.add(3);
            list.add(4);
            list.add(5);
    
            Iterator<Integer> iterator = list.iterator();
    
            // Iterator -> Spliterators -> Stream -> List
            List<Integer> result = StreamSupport.stream(
                    Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false)
                    .map(x -> x * 2)
                    .collect(Collectors.toList());
    
            result.forEach(x -> System.out.println(x));
            System.out.println(result.getClass());
    
        }
    
    }

输出量

2
    4
    6
    8
    10
    class java.util.ArrayList

## 参考文献 ##

*  [迭代器JavaDoc][JavaDoc]
 *  [ListIterator JavaDoc][]
 *  [ArrayList JavaDoc][]

标签： [迭代器][Link 1] [Java][] [循环][Link 2] [流][Link 3]

> 翻译自: [https://mkyong.com/java/java-iterator-examples/][https_mkyong.com_java_java-iterator-examples]

[JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
[ListIterator JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html
[ArrayList JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html
[Link 1]: https://mkyong.com/tag/iterator/
[Java]: https://mkyong.com/tag/java/
[Link 2]: https://mkyong.com/tag/loop/
[Link 3]: https://mkyong.com/tag/stream/
[https_mkyong.com_java_java-iterator-examples]: https://mkyong.com/java/java-iterator-examples/