可变的hashmap键是一种危险的做法吗？ [英] Are mutable hashmap keys a dangerous practice?

问题描述

使用可变对象作为Hashmap键是不好的做法吗？如果您尝试使用已修改足以更改其哈希码的密钥从Hashmap中检索值，会发生什么情况？

例如，给定

  class Key 
 {
 int a; //可变字段
 int b; //可变字段
 
 public int hashcode（）
 return foo（a，b）; 
} 
  

包含代码

  HashMap< Key，Value> map = new HashMap< Key，Value>（）; 
 
钥匙1 =新钥匙（0，0）; 
 map.put（key1，value1）; 
 
 key1.setA（5）; 
 key1.setB（10）; 
  

如果我们现在调用 map.get（key1）？这是安全还是可取的？或者是依赖于语言的行为？

解决方案

许多备受尊敬的开发人员已经注意到这一点，如Brian Goetz和Josh Bloch那么：

lockquote
如果一个对象的hashCode（）值可以根据它的状态改变，那么当使用这样的方法时，我们
必须小心对象作为基于散列的
集合中的键，以确保当
被用作散列键时，我们不允许它们的状态发生更改。所有基于散列的集合都假定
，在集合中它被用作
键时，对象的散列值不会更改。如果密钥的哈希码在
在集合中时发生更改，则可能会出现一些不可预知和令人困惑的后果
。这在实际中通常不是问题 - 使用像List这样的可变对象作为
HashMap中的键是不常见的做法。

Is it bad practice to use mutable objects as Hashmap keys? What happens when you try to retrieve a value from a Hashmap using a key that has been modified enough to change its hashcode?

For example, given

class Key
{
    int a; //mutable field
    int b; //mutable field

    public int hashcode()
        return foo(a, b);
}

with code

HashMap<Key, Value> map = new HashMap<Key, Value>();

Key key1 = new Key(0, 0);
map.put(key1, value1);

key1.setA(5);
key1.setB(10);

What happens if we now call map.get(key1)? Is this safe or advisable? Or is the behavior dependent on the language?

解决方案

It has been noted by many well respected developers such as Brian Goetz and Josh Bloch that :

If an object’s hashCode() value can change based on its state, then we must be careful when using such objects as keys in hash-based collections to ensure that we don’t allow their state to change when they are being used as hash keys. All hash-based collections assume that an object’s hash value does not change while it is in use as a key in the collection. If a key’s hash code were to change while it was in a collection, some unpredictable and confusing consequences could follow. This is usually not a problem in practice — it is not common practice to use a mutable object like a List as a key in a HashMap.

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