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

问题描述

我想将一个键，值对添加到散列表（或任何其他集合）中，但必须维护插入顺序。如何做到这一点？

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?

像我添加1作为键值1作为值，2作为键，2作为值。

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

输出应该按以下顺序排列：

The output should be ordered as:

1:one
2:two

推荐答案

以下是一些重要的 地图 实现：

Here are the characteristic differences of some important Map implementations:

• LinkedHashMap ：具有可预测的迭代顺序[...]键插入地图（插入顺序）。


• HashMap ：不保证地图的顺序 / li>
  
• TreeMap ：根据其键的自然排序，或通过比较器
  
  
  
  
  • ie这是一个 SortedMap
  
  
  • 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

    所以看起来像 LinkedHashMap 是您在这种情况下需要的。

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

    这是一个用来说明差异的代码段;它还显示了一种常见的方法来遍历 Map 的所有条目，以及如何使用接口引用对象，从而允许选择实现的极大灵活性。

    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());
            }
        }
    }
    

    以上代码片段的输出是（如ideone.com上所见）：

    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
    

    
    
    

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