为什么 AbstractCollection 不实现 equals()?

问题描述

你知道吗:

地图m1 = new HashMap<对象，对象>();地图<对象，对象>m2 = new HashMap<对象，对象>();System.out.println("m1.equals(m2) = "+m1.equals(m2));System.out.println("m1.keySet().equals(m2.keySet()) = "+m1.keySet().equals(m2.keySet()));System.out.println("m1.entrySet().equals(m2.entrySet()) = "+m1.entrySet().equals(m2.entrySet()));System.out.println("m1.values().equals(m2.values()) = "+m1.values().equals(m2.values()));

会输出:

m1.equals(m2) = truem1.keySet().equals(m2.keySet()) = truem1.entrySet().equals(m2.entrySet()) = truem1.values().equals(m2.values()) = false

这是因为 AbstractCollection(HashMap$Values 继承自)没有覆盖 #equals().p>

你知道为什么会这样吗?

根据Collection#equals()，Collection 没有通用的 equals() 方法，因此 AbstractCollection 无法提供.

请注意，HashMap$Values 既不是 Set 也不是 List，因此很困惑，从某种意义上说，它不支持 equals().

Did you know that :

Map<Object,Object> m1 = new HashMap<Object, Object>();
Map<Object,Object> m2 = new HashMap<Object, Object>();
System.out.println("m1.equals(m2) = "+m1.equals(m2));
System.out.println("m1.keySet().equals(m2.keySet()) = "
            +m1.keySet().equals(m2.keySet()));
System.out.println("m1.entrySet().equals(m2.entrySet()) = "
            +m1.entrySet().equals(m2.entrySet()));
System.out.println("m1.values().equals(m2.values()) = "
            +m1.values().equals(m2.values()));

would output :

m1.equals(m2) = true
m1.keySet().equals(m2.keySet()) = true
m1.entrySet().equals(m2.entrySet()) = true
m1.values().equals(m2.values()) = false

This is caused by the fact that AbstractCollection (which HashMap$Values inherits from) does not overrides #equals().

Do you have an idea why this is so ?

Per the contract of Collection#equals(), there is no general-purpose equals() methods for Collections, and thus AbstractCollection cannot provide one.

Note that HashMap$Values is neither a Set nor a List, thus the quandary and in a sense the reason it does not support equals().

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