collections - Iterator design pattern examples

最新推荐文章于 2019-02-18 16:55:00 发布

原创最新推荐文章于 2019-02-18 16:55:00 发布 · 232 阅读

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本文探讨了Java中迭代器(Iterator)接口的使用，包括AnIterator从1.2版本开始能够移除由next()产生的最后一个元素，以及1.8版本新增的forEachRemaining方法。通过两个案例展示了不同集合(List, LinkedList, HashSet, TreeSet)的遍历方式和输出结果的差异，强调了迭代器在集合操作中的灵活性和效率。

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An Iterator (since 1.2)can remove the last element produced by next(), which means you must call next() before you call remove().

Iterator interface add new method

 default void forEachRemaining(Consumer<? super E> action)

since 1.8, Performs the given action for each remaining element until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

case 1:

Simple Iterator(design pattern) see here.

case2 : collection display:

// collections/CrossCollectionIteration.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
import typeinfo.pets.*;
import java.util.*;

public class CrossCollectionIteration {
  public static void display(Iterator<Pet> it) {
    while(it.hasNext()) {
      Pet p = it.next();
      System.out.print(p.id() + ":" + p + " ");
    }
    System.out.println();
  }
  public static void main(String[] args) {
    List<Pet> pets = Pets.list(8);
    LinkedList<Pet> petsLL = new LinkedList<>(pets);
    HashSet<Pet> petsHS = new HashSet<>(pets);
    TreeSet<Pet> petsTS = new TreeSet<>(pets);
    display(pets.iterator());
    display(petsLL.iterator());
    display(petsHS.iterator());
    display(petsTS.iterator());
  }
}
/* Output:
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx // 4:Pug is different from 6:Pug
5:Cymric 2:Cymric 7:Manx 1:Manx 3:Mutt 6:Pug 4:Pug 0:Rat // ordered set
*/

case 3: a leaner version example:

// collections/CrossCollectionIteration2.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
import typeinfo.pets.*;
import java.util.*;

public class CrossCollectionIteration2 {
  public static void display(Iterable<Pet> ip) {
    Iterator<Pet> it = ip.iterator(); // diff [1]
    while(it.hasNext()) {
      Pet p = it.next();
      System.out.print(p.id() + ":" + p + " ");
    }
    System.out.println();
  }
  public static void main(String[] args) {
    List<Pet> pets = Pets.list(8);
    LinkedList<Pet> petsLL = new LinkedList<>(pets);
    HashSet<Pet> petsHS = new HashSet<>(pets);
    TreeSet<Pet> petsTS = new TreeSet<>(pets);
    display(pets);
    display(petsLL);
    display(petsHS);
    display(petsTS);
  }
}
/* Output:
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx
0:Rat 1:Manx 2:Cymric 3:Mutt 4:Pug 5:Cymric 6:Pug 7:Manx
5:Cymric 2:Cymric 7:Manx 1:Manx 3:Mutt 6:Pug 4:Pug 0:Rat
*/

[1] All of the classes are Iterable, so now the calls to display() are notably simpler.

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/collections/SimpleIteration.java

3. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Iterator.java

4. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/Pets.java

5. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/LiteralPetCreator.java

6. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/typeinfo/pets/PetCreator.java

7. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/collections/CrossCollectionIteration2.java