模拟java8 stream.map

Question

I am using mockito how can I mock stream.map() call.

I tried something like this which return null.

@Mock Stream<String> stringStream;
@Mock Stream<String> upperCaseStream;

when(stringStream.map(String::toUpperCase)).thenReturn(upperCaseStream);
syso(stringStream.map(String::toUpperCase));

This prints null.

I am looking for a correct way of mocking that would return 'upperCaseStream' in output.

Answer 1

Keep in mind that the identity of method references or lambdas is not guaranteed to be the same even if the calls look identical. Check this out

I don't know much about Mockito, but I believe it checks if there exists a value that a when registered, this might invlove a call to equals somewhere which is not guranteed to work here. For example:

Function<String, String> fn1 = String::toUpperCase;
Function<String, String> fn2 = String::toUpperCase;
System.out.println(fn1.equals(fn2)); // false on my machine

You can simply store the reference in a variable and use it later on

Function<String, String> toUpperCase = String::toUpperCase;
when(stringStream.map(toUpperCase)).thenReturn(upperCaseStream);

System.out.println(stringStream.map(toUpperCase));

Btw, I really don't understand why would ever need to mock Stream<String> where you can simply do Stream.of("foo","bar")