About Question enthuware.ocpjp.v8.2.1859 :

[lucian]

I have two questions:

1. Why can the reduce method from a Stream<Integer> can auto-unbox to a double, when it's returning an Integer ?
double sum = ls.stream().reduce(0, (a, b) -> a + b);

Code: Select all

    java.util.stream.Stream
    T reduce(T identity, BinaryOperator<T> accumulator);
    

2. Why can the sum method in IntStream return a double ?
double sum = ls.stream().mapToInt(x -> x).sum();

Code: Select all

    java.util.stream.IntStream
    int sum();
    

[admin]

1. It is not auto-unboxing to int. It is doing automatic widening of int to double. Remember that you don't need any cast to assign an int to double.

2. Same reason as above.

[RobinDRG]

Hi Paul, I made the following trial, but still don't understand why the compiler gives me an error since the variables sum3 and sum4 outside the lambda expression are not even initialized

Code: Select all

double sum3;ls.stream().peek(x->{sum3=sum3+x;}).forEach(y->{});
System.out.println(sum3);
double sum4;ls.stream().forEach(a->{sum4=sum4+a;});
System.out.println(sum4);

[admin]

What is the error message? It will tell you exactly what is wrong.

[RobinDRG]

I tried to just declare the variable to go one step further and try to understand why the compiler consider the variable sum outside(initialized one single time/only declared) as non-effectively final

Code: Select all

Q4_Streams.java:18: error: variable sum3 might not have been initialized
double sum3;ls.stream().peek(x->{sum3=sum3+x;}).forEach(y->{});
                                      ^
Q4_Streams.java:19: error: variable sum3 might not have been initialized
System.out.println(sum3);
                   ^
Q4_Streams.java:20: error: variable sum4 might not have been initialized
double sum4;ls.stream().forEach(a->{sum4=sum4+a;});
                                         ^
Q4_Streams.java:21: error: variable sum4 might not have been initialized
System.out.println(sum4);
                   ^
Q4_Streams.java:18: error: local variables referenced from a lambda expression must be final or effectively final
double sum3;ls.stream().peek(x->{sum3=sum3+x;}).forEach(y->{});
                                 ^
Q4_Streams.java:20: error: local variables referenced from a lambda expression must be final or effectively final
double sum4;ls.stream().forEach(a->{sum4=sum4+a;});
/code]

[admin]

1. Well, local variables have to be initialized explicitly. In your code, sum3 isn't initialized. What value do you think the JVM will use while computing the expression sum3=sum3+x; for the first time? This is standard OCAJP stuff
2. If you want to use a local variable within a lambda expression, it also has be effectively final. Which means, you can't modify its value. What do you think sum3=sum3+x; is doing?

[RobinDRG]

Okay, many thanks. I thought that effectively-final was referring to variables that didn't change its value outside the lambda expression. I understand it now, you can't reassign the value at all