Groovy 在 GStringImpl 上使用 equals() 和 == 的不同结果 [英] Groovy different results on using equals() and == on a GStringImpl

问题描述

根据 Groovy 文档，== 只是一个聪明的"equals() 因为它还负责避免 NullPointerException:

According to the Groovy docs, the == is just a "clever" equals() as it also takes care of avoiding NullPointerException:

Java的==其实就是Groovy的is()方法，而Groovy的==是一个巧妙的equals()代码>!

Java’s == is actually Groovy’s is() method, and Groovy’s == is a clever equals()!

[...]

但是要进行通常的 equals() 比较，您应该更喜欢 Groovy 的 ==，因为它还可以避免 NullPointerException，与左侧或右侧是否 null 无关.

But to do the usual equals() comparison, you should prefer Groovy’s ==, as it also takes care of avoiding NullPointerException, independently of whether the left or right is null or not.

因此，如果对象不为空，== 和 equals() 应该返回相同的值.但是，我在执行以下脚本时得到了意想不到的结果:

So, the == and equals() should return the same value if the objects are not null. However, I'm getting unexpected results on executing the following script:

println "${'test'}" == 'test'
println "${'test'}".equals('test')

我得到的输出是:

true
false

这是一个与 GStringImpl 相关的已知错误还是我遗漏的东西?

Is this a known bug related to GStringImpl or something that I'm missing?

推荐答案

好问题，上面代码令人惊讶的地方是

Nice question, the surprising thing about the code above is that

println "${'test'}".equals('test')

返回 false.另一行代码返回预期结果，所以让我们忘记这一点.

returns false. The other line of code returns the expected result, so let's forget about that.

"${'test'}".equals('test')

equals 被调用的对象是 GStringImpl 类型，而 'test' 是 String 类型，因此它们不被视为相等.

The object that equals is called on is of type GStringImpl whereas 'test' is of type String, so they are not considered equal.

显然，equals 的 GStringImpl 实现可以这样编写，这样当它传递一个 String 时，它包含与 this，它返回真.表面上看，这似乎是一个合理的做法.

Obviously the GStringImpl implementation of equals could have been written such that when it is passed a String that contain the same characters as this, it returns true. Prima facie, this seems like a reasonable thing to do.

我猜它不是这样写的原因是因为它违反了 equals 约定，其中规定:

I'm guessing that the reason it wasn't written this way is because it would violate the equals contract, which states that:

它是对称的:对于任何非空引用值 x 和 y，当且仅当 y.equals(x) 返回 true 时，x.equals(y) 才应返回 true.

It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

String.equals(Object other)的实现在传递一个GSStringImpl时总是会返回false，所以如果GStringImpl.equals(Object other)code>在传递任何String时返回true，这将违反对称要求.

The implementation of String.equals(Object other) will always return false when passed a GSStringImpl, so if GStringImpl.equals(Object other) returns true when passed any String, it would be in violation of the symmetric requirement.

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