如何保持插入顺序 [英] How to Maintain order of insertion

问题描述

我想将一个键，值对添加到哈希表(或任何其他集合)中，但必须保持插入顺序.我该怎么办?

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?

就像我将1作为键"one"添加为值，将2作为键并添加"2"作为值一样.

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

输出应为:

1:one
2:two

推荐答案

以下是一些重要的

Here are the characteristic differences of some important Map implementations:

• LinkedHashMap :与可预测的迭代顺序，通常是将键插入到映射中的顺序(插入顺序)."
• HashMap :不保证地图的顺序"
• TreeMap :是根据其键的自然顺序或Comparator进行排序"
  • 即这是 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

    在这种情况下，您似乎需要LinkedHashMap.

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

    以下是说明差异的代码段；它还显示了遍历Map的所有条目的常用方法，以及如何使用接口引用对象使实现的选择具有极大的灵活性.

    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());
            }
        }
    }
    

    以上代码段的输出为(在ideone.com上看到的):

    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
    

    相关问题

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      • 如果要在迭代时修改地图，则需要使用其

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