如何使用集合维护插入顺序 [英] How to Maintain order of insertion using collections

问题描述

我想将一个键，值对添加到hashtable（或任何其他集合），但必须维护插入顺序。

像我将加1作为键，1作为值，2作为键，2作为值。 b
$ b

输出应按如下排序：

  1：one 
 2：两个
  

解决方案

href =http://java.sun.com/javase/6/docs/api/java/util/Map.html =nofollow noreferrer> 地图 实施：

• LinkedHashMap ：可预测的迭代顺序[...]


• HashMap ：不保证地图


• TreeMap ：根据其键的自然排序或者通过 Comparator
  
  
  
  
  • ie它是 SortedMap
  
  

$ c> LinkedHashMap 是这种情况下你需要的。

这里是一个片断来说明差异;它还显示了对 Map 的所有条目进行迭代的常用方法，以及如何使用接口引用对象，可以极大地灵活选择实现。

  import java.util。*; 
 public class MapExample {
 public static void main（String [] args）{
 populateThenDump（new HashMap< String，Integer>（））; 
 populateThenDump（new TreeMap< String，Integer>（））; 
 populateThenDump（new LinkedHashMap< String，Integer>（））; 
} 
 static void populateThenDump（Map< String，Integer> map）{
 System.out.println（map.getClass（）。getName（））; 
 
 map.put（Zero，0）; 
 map.put（One，1）; 
 map.put（Two，2）; 
 map.put（Three，3）; 
 map.put（Four，4）; 
 
 for（Map.Entry< String，Integer> entry：map.entrySet（））{
 System.out.println（entry.getKey（）+=>+ getValue（））; 
} 
} 
} 
  

上述代码片段的输出是（在ideone.com 上显示）：

  java.util.HashMap //无序，结果可能不同
 Three => 3 
 Zero => 0 
 One => 1 
 Four => 4 
二=> 2 
 java.util.TreeMap //按字符串键排序lexicographically 
 Four => 4 
 One => 1 
 Three => 3 
二=> 2 
 Zero => 0 
 java.util.LinkedHashMap //插入顺序
 Zero => 0 
 One => 1 
二=> 2 
 Three => 3 
 Four => 4 
  

相关问题

• 迭代地图


• 审核和删除地图
  
  
  
  
  • 如果您想在迭代时修改地图，则需要使用其 Iterator 。
  
  

类似问题

• 如何保持元素在哈希表中的顺序


• LinkedHashMap中的entrySet（）是否也保证顺序？


• 实现的Java类映射并保持插入顺序？


• 在Java中执行有序列表映射

I want to add a key,value pair into a hashtable (or any other collection) but have to maintain insertion order. How can I do this?

Like I'll add 1 as key "one" as value, 2 as key and "two" as value.

The output should be ordered as:

1:one
2:two

解决方案

Here are the characteristic differences of some important Map implementations:

• LinkedHashMap: "with predictable iteration order [...] which is normally the order in which keys were inserted into the map (insertion-order)."
• HashMap: "makes no guarantees as to the order of the map"
• TreeMap: "is sorted according to the natural ordering of its keys, or by a Comparator"
  • i.e. it's a SortedMap

So it looks like LinkedHashMap is what you need in this case.

Here's a snippet to illustrate the differences; it also shows a common way to iterate over all entries of a Map, and how using an interface to refer to objects allow great flexibility of choice of implementation.

import java.util.*;
public class MapExample {
    public static void main(String[] args) {
        populateThenDump(new HashMap<String,Integer>());
        populateThenDump(new TreeMap<String,Integer>());
        populateThenDump(new LinkedHashMap<String,Integer>());
    }
    static void populateThenDump(Map<String,Integer> map) {
        System.out.println(map.getClass().getName());

        map.put("Zero",  0);
        map.put("One",   1);
        map.put("Two",   2);
        map.put("Three", 3);
        map.put("Four",  4);

        for (Map.Entry<String,Integer> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " => " + entry.getValue());
        }
    }
}

The output of the above snippet is (as seen on ideone.com):

java.util.HashMap          // unordered, results may vary
Three => 3
Zero => 0
One => 1
Four => 4
Two => 2
java.util.TreeMap          // ordered by String keys lexicographically
Four => 4
One => 1
Three => 3
Two => 2
Zero => 0
java.util.LinkedHashMap    // insertion order
Zero => 0
One => 1
Two => 2
Three => 3
Four => 4

Related questions

• Iterate Over Map
• iterating over and removing from a map
  • If you want to modify the map while iterating, you'd need to use its Iterator.

Similar questions

• How to keep the order of elements in hashtable
• Does entrySet() in a LinkedHashMap also guarantee order?
• Java Class that implements Map and keeps insertion order?
• Ordered List Map implementation in Java

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