Is a class being instantiated in a lambda expression?

Question

I have the following method invocation, in which I am passing a lambda expression. Is a class being instantiated implicitly here?

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

Method signature:

printStudents(List<Student> roster, CheckStudent checkstudet)

interface CheckStudent {
    boolean test(Student s);
}

Edit

Some of you suggested me to refactor the code, but the same question arises.

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

Is a class (I am not referring to class Student ) being instantiated on the right side of the assignment?

@BoristheSpider i mean a class the implements CheckStudent - it is a functional interface — The Scientific Method, Oct 18 '18 at 12:13
Of course, it might be clearer if you can refactor `(Student s) -> s.getGender() == Student.Sex.MALE && s.getAge() >= 18 && s.getAge() <= 25` by introducing a local variable of class `CheckStudent` similar to `CheckStudent checkStudent = (Student s) -> s.getGender() == Student.Sex.MALE && s.getAge() >= 18 && s.getAge() <= 25;` — Smutje, Oct 18 '18 at 12:18
This should answer your question https://download.java.net/java/early_access/jdk11/docs/api/java.base/java/lang/invoke/LambdaMetafactory.html — Boris the Spider, Oct 18 '18 at 12:33

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

The value of a lambda expression is a reference to an instance of a class. So, in practical terms, yes, an instance of a class is being created. See what the docs say:

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.

However, there is more to that than we can "see". There are many optimizations running under the hood. Depending on certain factors, a previously created object can, for example, be used again. This means that a new object need not be allocated on every evaluation of a lambda expression. Let's take a look at the docs:

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).

As you might have noticed, this is a complex topic. For a deeper understanding, take a look at the The Java® Language Specification, chapter “15.27.4. Run-time Evaluation of Lambda Expressions”.

Is a class being instantiated in a lambda expression?

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