What kind of lambda expression syntax is this in Java 8?

Question

This is from one question about lambda expression. I'm baffled with the syntax in line check((h, l) -> h > l, 0);:

The check() function requires a Climb object and an int. The line above does not provide any Climb object. And what does h > l, 0 mean?

interface Climb {
  boolean isTooHigh(int height, int limit);
}

class Climber {
  public static void main(String[] args) {
    check((h, l) -> h > l, 0);
  }
  private static void check(Climb climb, int height) {
    if (climb.isTooHigh(height, 1)) 
      System.out.println("too high");
    else 
      System.out.println("ok");
  }
}

Alexis C. · Accepted Answer · 2015-11-23 09:04:12Z

5

Your Climb interface respects the contract of a functional interface, that is, an interface with a single abstract method.

Hence, an instance of Climb can be implemented using a lambda expression, that is, an expression that takes two ints as parameters and returns a boolean in this case.

(h, l) -> h > l is the lambda expression that implements it. h and l are the parameters (int) and it will return whether h > l (so the result is indeed a boolean). So for instance you could write:

Climb climb = (h, l) -> h > l;
System.out.println(climb.isTooHigh(0, 2)); //false because 0 > 2 is not true

answered Nov 23, 2015 at 9:04

Alexis C.

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Comments

Codor · Accepted Answer · 2015-11-23 08:51:54Z

2

Apparently, (h, l) -> h > l is a lambda expression where the result is of a boolean type and the 0 is the second argument of check, which is unrelated to the lambda expression itself; the 0 does not belong to the lambda expression as such.

answered Nov 23, 2015 at 8:51

Codor

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2 Comments

Max Over a year ago

How can a boolean be cast to a Climb object in the call to check()? I don't understand.

Sam Over a year ago

@FukuzawaYukio sound like you aren't familiar with lambda expressions implementing functional interfaces. Go learn about it :).

JSQuareD · Accepted Answer · 2015-11-23 08:53:52Z

1

(h, l) -> h > l is the climb object. It is a lambda that returns true when the first argument (h) is greater than the second (l). 0 is the int, and the comma separates the arguments.

answered Nov 23, 2015 at 8:53

JSQuareD

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3 Comments

Max Over a year ago

How come (h,l) -> h>l is the Climb object? I can only see that it's a boolean and a boolean cannot be cast to a Climb object.

Mark Rotteveel Over a year ago

@FukuzawaYukio the lambda (h, l) -> h > l can be (and is) interpreted as the funtional interface Climb as it matches boolean isTooHigh(int height, int limit);

JSQuareD Over a year ago

It's a lambda with a boolean as its return type. In the end, that's all a Climb object is, a function from two ints to a boolean. Java apparently has special syntactic sugar to convert such lambdas to anonymous implementations of an interface.

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