知乎，让每一次点击都充满意义 —— 欢迎来到知乎，发现问题背后的世界。 @MarkusMeskanen: because the object is immutable, and if you mutated it in place you may not specialise it in a very subclass. ” – Winston Churchill “You are off to fantastic places, currently is your working day. Your mountain is waiting, so get on the way.” – https://propertiesnoidacentral.blogspot.com