为什么AbstractCollection没有实现size（）？

问题描述

当子类化 AbstractCollection 时，我仍然必须实现 size（）

When sub-classing AbstractCollection, I must still implement size(), even though (I believe) there is a correct (though non-performant) default implementation:

public int size() {
    int count = 0;

    for (Iterator<E> i = iterator(); i.hasNext();) {
        i.next();
        count++
    }

    return count;
}

为什么设计师不包括 size（）？他们试图强制开发人员有意识地考虑这种方法，希望导致开发人员提供的执行效果比默认的更好。

Why did the designers not include a default implementation of size()? Were they trying to force developers to consciously think about this method, hopefully causing the developer to offer an implementation that performs better than the default?

推荐答案

我怀疑你的最后一句话是真正的原因。当子类化一个抽象类时，它有时是诱人的，只覆盖抽象方法。我希望几乎每个实现都有一个更好的实现比只是迭代 - 所以如果你几乎每个人都重写一个方法，这可能是一个好主意，不提供一个基（慢）实现。它只是减少了拧紧的机会：）

I suspect your last sentence is the real reason. When subclassing an abstract class it's sometimes tempting to only override the abstract methods. I would expect almost every implementation to have a better implementation than just iterating - so if you want pretty much everyone to override a method, it's probably a good idea not to provide a base (slow) implementation. It just reduces chances of screwing up :)

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