Java 8 Lambda Expression

Question

I am new learner for Java 8 lambda . I found a statement in a book (Java 8 for Really Impatient ), saying , "It is illegal for a lambda expression to return a value in some branches but not in others. For example, (int x) -> { if (x>= 0) return 1; } is invalid."

Can anyone explain this ? Please provide some examples.

Thanks in advance.

• Sotirios Delimanolis: I don't understand your confusion. You already have an example with an explanation. It's invalid because only one of two branches returns a value.
• C-Otto: No matter if you write a standard method or something like the lambda expression above, the code needs to have some defined outcome. In your example, for a negative input (x) the expression is not returning anything - which is not allowed.
• JB Nizet: This book should be retitled Java 8 for Really patient. Java 8 is out for 3 years now ;-)
• amu61: @C-Otto , Thanks for ur reply . Got it !! . Somehow missed the context
• amu61: @SotiriosDelimanolis , Thanks for ur reply .

Answer 1

I don't get it where you don't get it. Will this compile for example?

 static int test(int x) {
 if(x >= 0) {
 return 1;
 }
 }

Same goes for the lambda expression.

Answer 2

you have to ensure no matter "if" statement is true or false, there is always a return value or there is always not.

In your case:

illegal: (int x) -> { if (x >= 0) return 1; }
legal: (int x) -> { if (x >= 0) return 1; else return 0;}

This is because this method below is illegal

int lambda (int x){
 if (x >= 0)
 return 1;
}