java 8 findFirst vs map on optional

Question

Given this code:

class Foo {
  Integer attr;
  public Integer getAttr() {return attr;}
}

List<Foo> list = new ArrayList<>();
list.add(new Foo());

list.stream().map(Foo::getAttr).findAny().orElse(null);  //A
list.stream().findAny().map(Foo::getAttr).orElse(null);  //B

Line A throws

java.lang.NullPointerException: null

while line B returns null.

What is the cause of this behaviour? Both findAny() and map() return Optional<T>.

Answer 1

list.stream().map(Foo::getAttr).findAny().orElse(null);

Java doc for streams says that Stream: "returns a stream consisting of the results of applying the given function to the elements of this stream", and findAny() "could return aNullPointerException - if the element selected is null". In your class Foo, Integer(not int) by default is set to null because is declared but not initialized. see Primitives see default values and Object initialization in Java

Initialization is different for: A) Class Members (Objects and primitives) B) Local Variables

Answer 2

First, your two code snippet map are different operations:

//            v--- stream intermediate operation
list.stream().map(Foo::getAttr).findAny().orElse(null);  //A
//                      v---- a Optional utility method 
list.stream().findAny().map(Foo::getAttr).orElse(null);  //B

and the NullPointerException occurs in Stream#findAny operation, since it can't accept a null value. due to it uses Optional.of rather than Optional.ofNullable. and the documentation of Stream#findAny is already asserts:

Throws:

NullPointerException - if the element selected is null

so if you want your A code snippet works fine you must filter all null values before calling Stream#findAny, for example:

//when no elements in stream, `findAny` will be return a empty by Optional.empty()
//                                                       v   
list.stream().map(Foo::getAttr).filter(Objects::nonNull).findAny().orElse(null);//A