java - 为 Java 的 lambda 体提供 this 关键字会出现什么问题？

Question

这个问题已经有答案了:

Lambda this reference in java (2 个回答)

已关闭 3 年前。

考虑以下示例

Consumer<Long> f1 = new Consumer<>() {

    @Override
    public void accept(Long value) {
        if (value < 5) {
            this.accept(value + 1); //this refers to the anonymous context.
        }
    }

}

Consumer<Long> f2 = (value) -> {
    if (value < 5) {
        this.accept(value + 1); //this refers to outer context and the anonymous function is not referable. 
    }
};

可以看出this没有为 lambda f2 提供但针对更明确编写的匿名 Consumer 实现给出 f1 。

为什么会这样呢？提供this的语言设计难度是什么？对于 lambda 体？

Answer 1

Java Language Specification 15.27.2 says :

Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names).

The transparency of this (both explicit and implicit) in the body of a lambda expression - that is, treating it the same as in the surrounding context - allows more flexibility for implementations, and prevents the meaning of unqualified names in the body from being dependent on overload resolution.

Practically speaking, it is unusual for a lambda expression to need to talk about itself (either to call itself recursively or to invoke its other methods), while it is more common to want to use names to refer to things in the enclosing class that would otherwise be shadowed (this, toString()). If it is necessary for a lambda expression to refer to itself (as if via this), a method reference or an anonymous inner class should be used instead.