Java 如何保持插入顺序

How to Maintain order of insertion

提问by sarah

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?

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。我怎样才能做到这一点？

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

就像我将添加 1 作为键“一”作为值，添加 2 作为键和“二”作为值。

The output should be:

输出应该是：

1:one
2:two

采纳答案by polygenelubricants

Here are the characteristic differences of some important Mapimplementations:

以下是一些重要Map实现的特性差异：

• 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

• LinkedHashMap：“具有可预测的迭代顺序 [...]，这通常是将键插入映射的顺序（插入顺序）。”
• HashMap：“不保证地图的顺序”
• TreeMap: "根据其键的自然顺序排序，或按Comparator"
  • 即它是一个 SortedMap

So it looks like LinkedHashMapis what you need in this case.

所以LinkedHashMap在这种情况下，它看起来就是你所需要的。

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.

这是一个说明差异的片段；它还展示了迭代 a 的所有条目的常用方法Map，以及如何使用接口来引用对象允许实现选择的极大灵活性。

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):

上面代码片段的输出是（如在 ideone.com 上看到的）：

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

相关问题

• 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.

• 迭代地图
• 迭代并从地图中删除
  • 如果你想在迭代时修改地图，你需要使用它的Iterator.

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• Does entrySet() in a LinkedHashMap also guarantee order?
• Java Class that implements Map and keeps insertion order?
• Ordered List Map implementation in Java

• 如何保持哈希表中元素的顺序
• LinkedHashMap 中的 entrySet() 是否也保证顺序？
• 实现 Map 并保持插入顺序的 Java 类？
• Java中的有序列表映射实现

回答by Peter ?tibrany

For hash table, use LinkedHashMapclass.

对于哈希表，使用LinkedHashMap类。

回答by Pedantic

You are searching for the LinkedHashMapclass.

您正在寻找LinkedHashMap班级。