英文:
Collections.emptySet() vs new HashSet<>()
问题
如何验证Set是否为空集或为new HashSet<>()？
尝试了两种方法 -
sizeCollectionUtils.isEmpty()
两者的结果相同。
英文:
How can I validate if the Set is empty set or new HashSet<>() ?
Tried two approaches -
sizeCollectionUtils.isEmpty()
Both result in same value.
答案1
得分: 6
当然，一个新的new HashSet<>()是空的，Set.of()或者Collections.emptySet()也是一样的。重点是：它们都是空集合，我不知道为什么你想要区分这两者之间的差异。
唯一的区别在于，new HashSet<>()现在是空的，但以后可能不为空（它可以被修改；你可以向其中添加元素），而根据规范，Set.of()或者Collections.emptySet()的结果现在是空的，并且以后也会是空的：你不能向它们添加任何内容，在它们上调用.add会导致运行时异常。
这等同于在问：我怎么知道它是否是不可变的。不幸的是，你基本上无法知道，所以问题回归到：为什么你需要知道呢？
英文:
Of course, a fresh new HashSet<>() is empty, and so is Set.of() or Collections.emptySet(). The point is: Both are empty sets, I have no idea why you would want to tell the difference between these two.
The one difference is that new HashSet<>() is empty now but may not be empty later (it can be changed; you can add things to it), whereas as per the spec, the result of Set.of() or Collections.emptySet(), they are empty now and will be empty later: You can't add anything to them, calling .add on them will cause a runtime exception.
That's tantamount to asking: How do I know if it is immutable. You unfortunately basically can't, so that goes right back to: Why would you need to know?
答案2
得分: 1
Collections.emptySet() 返回位于 java.util.Collections 内部的静态类 EmptySet，但 new HashSet<>() 返回一个 java.util.HashSet 类。这两个集合在实例化后都将为空，即大小为 0，但您可以通过调用 .getClass() 来区分这两者，它将返回：
class java.util.Collections$EmptySet
class java.util.HashSet
英文:
Collections.emptySet() returns a static class EmptySetwithin java.util.Collections but new HashSet<>() returns a java.util.HashSet class. Both collections will be empty, i.e., size = 0 after instantiated but you can distinguish those two by calling .getClass() which will return:
class java.util.Collections$EmptySet
class java.util.HashSet
答案3
得分: 1
使用.getClass查看所使用的不同实现。
英文:
Use .getClass to see the different implementation that was used.
答案4
得分: -2
我有一个实用的方法，可以用来执行这个操作 -
Collections.EMPTY_SET.equals(mySet)
感谢大家的回答。
英文:
I got one utility method that can be used to perform the operation -
Collections.EMPTY_SET.equals(mySet)
Thanks all for your answers.
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