class 是否在 lambda 表达式中被实例化？

Question

我有以下方法调用，我在其中传递了一个 lambda 表达式。 class 是否在此处隐式实例化？

printStudents(
    roster,
    (Student s) -> s.getGender() == Student.Sex.MALE
        && s.getAge() >= 18
        && s.getAge() <= 25
);

方法签名：

printStudents(List<Student> roster, CheckStudent checkstudet)

interface CheckStudent {
    boolean test(Student s);
}

编辑

你们中的一些人建议我重构代码，但出现了同样的问题。

CheckStudent checkStudent = (Student s) -> s.getGender() == Student.Sex.MALE && s.getAge() >= 18 && s.getAge() <= 25;

一个class（我指的不是class Student）是在赋值的右边被实例化了吗？

Answer 1

lambda 表达式的值是对 class 实例的引用。因此，实际上，是的，正在创建 class 的实例。查看文档怎么说：

At run time, evaluation of a lambda expression is similar to evaluation of a class instance creation expression, insofar as normal completion produces a reference to an object.

然而，我们“看不到”的远不止这些。引擎盖下有许多优化运行。例如，根据某些因素，可以再次使用先前创建的对象。这意味着 不需要在每次评估 lambda 表达式时都分配一个新对象 。让我们看看文档：

Evaluation of a lambda expression is distinct from execution of the lambda body. Either a new instance of a class with the properties below is allocated and initialized, or an existing instance of a class with the properties below is referenced.

[...]

These rules are meant to offer flexibility to implementations of the Java programming language, in that:

A new object need not be allocated on every evaluation.

Objects produced by different lambda expressions need not belong to different classes (if the bodies are identical, for example).

Every object produced by evaluation need not belong to the same class (captured local variables might be inlined, for example).

If an "existing instance" is available, it need not have been created at a previous lambda evaluation (it might have been allocated during the enclosing class's initialization, for example).

您可能已经注意到，这是一个复杂的话题。要更深入地了解，请查看 Java® 语言规范，第 “15.27.4. Run-time Evaluation of Lambda Expressions” 章。

class 是否在 lambda 表达式中被实例化？

Is a class being instantiated in a lambda expression?

java

lambda

functional-interface