(Sybex) Lambda expression

[Priyanka Leo]

Dears,

I am not able to understand the explanation given in the book for the below code. Can someone please help me understand the same.

Options:
A. ok
B. too high
C. Compiler error on line x1.
D. Compiler error on line x2.
E. Compiler error on a different line.
F. A runtime exception is thrown.


answer:
C. The functional interface takes two int parameters. The code on line x1 attempts to use
them as if one is an Object, resulting in a compiler error making C the correct answer. It
also tries to return String even though the data type for the functional interface method is
boolean. It is tricky to use types in a lambda when they are implicitly specified. Remember
to check the interface for the real type.

public interface Climb {
boolean isTooHigh(int height, int limit);
}
public class Climber {
public static void main(String[] args) {
check((h, l) -> h.toString(), 5); // x1
}
private static void check(Climb climb, int height) {
if (climb.isTooHigh(height, 10)) // x2
System.out.println("too high");
else System.out.println("ok");
} }

Thanks in advance!!

[Junilu Lacar]

Have you tried actually compiling this code? What error did you get?

[Junilu Lacar]

It is tricky to use types in a lambda when they are implicitly specified. Remember
to check the interface for the real type.

If you try to compile the code and see the compiler error message you can start understanding the above statement. What type does the compiler error message mention?

[Jesse Silverman]

Hi Priyanka:

I too find these questions to make me nervous in a real test situation, because you are trying to go so fast.

There are many shortcuts we can take when using lambda expressions or method references, they allow us to type fewer characters, but require us to keep more "in our heads".

Let me ask you this, would you still have been confused if this line:

check((h, l) -> h.toString(), 5); // x1

Was instead written out as this equivalent line?

check((int h, int l) -> { return h.toString(); }, 5); // x1

If I didn't mess anything up, that is what the compiler will actually interpret the line as being.
When it is written out, it seems to simply be the garbage that it is.

The question is checking whether the shortcuts afforded to us in writing lambdas will obscure errors for us.

[Jeanne Boyarsky]

I added "Sybex" to the subject. It's fine to post questions from the book, but we ask you to cite the source.