问题描述

根据这些:

1. http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html
2. HashMap、LinkedHashMap和TreeMap的区别
3. java初学者:如何在hashmaps中对key进行排序?

Java 中的 HashMap 应该是未排序的，但它正在根据 Key 进行排序.

The HashMap in Java should be unsorted but it is being sorted with respect to Key.

我遇到了这个问题，因为我需要插入订单数据.所以，我使用 LinkedHashMap 代替.但是我仍然很困惑为什么 HashMap 对其进行排序.

I experienced this as a problem because I needed inserted-order data. So, I used LinkedHashMap instead. But still I am confused why the HashMap sorted it.

谁能解释一下?

我做了一个简单的例子来查看排序.

I did a simple example to view the sort.

public static void main(String[] args) {

        HashMap<Integer, String> newHashMap = new HashMap<Integer, String>();
        newHashMap.put(2, "First");
        newHashMap.put(0, "Second");
        newHashMap.put(3, "Third");
        newHashMap.put(1, "Fourth");

        Iterator<Entry<Integer, String>> iterator = newHashMap.entrySet()
                .iterator();
        while (iterator.hasNext()) {

            Map.Entry<Integer, String> entry = iterator.next();
            System.out.println("Key: " + entry.getKey());
            System.out.println("Value: " + entry.getValue());
            iterator.remove();
        }

    }

结果:

Key: 0
Value: Second
Key: 1
Value: Fourth
Key: 2
Value: First
Key: 3
Value: Third

我尝试使用 Java 的 Random 插入 50 个随机数，但发现一些数据未排序.但是，它仍然能够对大多数整数进行排序.

I tried to insert 50 random numbers using Random of Java and I found some data unsorted. But, it still manages to sort most of the integers.

随机结果:

...
Key: 36
Value: random
Key: 43
Value: random
Key: 47
Value: random
Key: 44
Value: random
Key: 45
Value: random
...

推荐答案

这是巧合(不是真的，而是与哈希算法有关).

It's a coincidence (not really, rather it has to do with the hashing algorithm).

尝试添加

newHashMap.put(-5, "Fifth");

最后一次.

输出将是

Key: 0
Value: Second
Key: 1
Value: Fourth
Key: 2
Value: First
Key: 3
Value: Third
Key: -5
Value: Fifth

javadoc 具体说

The javadoc specifically says

这个类不保证地图的顺序；特别是，它不保证订单会随着时间的推移保持不变.

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

这篇关于HashMap 应该是未排序的，但仍然根据 key 排序的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！