lambda 表达式如何初始化参数？

Question

I'm confused with lamda expression.

JavaDStream<ConsumerRecord<String, String>> rsvpsWithGuestsStream =
        meetupStream.filter(f -> !f.value().contains("\"guests\":0"));

rsvpsWithGuestsStream.foreachRDD((JavaRDD<ConsumerRecord<String, String>> r) -> {        
    MongoSpark.save(
            r.map(
                e -> Document.parse(e.value())
            )
    );            
});

Here is a foreachRDD method void foreachRDD(VoidFunction<R> foreachFunc) , It accepts a functional interface.

And in code, JavaRDD<ConsumerRecord<String, String>> r passed as a argument which is internally used by its call method.

I want to know Does lambda expression initialize r on its own ? Becasue it can call map only if its initialized. And In code I cant see anywhere its already created.

Can anyone help me to understand this ?

Answer 1

A lambda expression does not initialize anything. It is just an anonymous function which does something with its arguments - assuming they have been already initialized. I am not familiar with your use case (guess it is Spark) but it looks like r is just one of the elements in the stream, and each such element is passed in to your lambda. Below is a similar but much simpler example:

Stream.of("a", "b", "c").forEach(el -> System.out.println(el));

Here el is each of the elements "a", "b" and "c", passed in to the lambda which simply prints it. Just like a function, a lambda doesn't know anything about its arguments and whether they are initialiazed - this is up to the caller, in the above cases - the forEach methods.

Answer 2

Here is a quote from Java spec:

15.27.1. Lambda Parameters

When the lambda expression is invoked (via a method invocation expression (§15.12)), the values of the actual argument expressions initialize newly created parameter variables, each of the declared or inferred type, before execution of the lambda body. The Identifier that appears in the LambdaParameter or directly in the LambdaParameterList or LambdaParameters may be used as a simple name in the lambda body to refer to the formal parameter.

https://docs.oracle.com/javase/specs/jls/se15/html/jls-15.html#jls-15.27.1