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I have to test a method which takes a Long as argument. I have to test it for any value of the Long and also in case it's null. Does Mockito.anyLong() automatically test both cases? null and any value of the Long? Or randomly picks up a value between any long value and null?
Considering that these are the docs about anyLong():
anyLong
public static long anyLong()
any long, Long or null.
See examples in javadoc for Matchers class
Returns:
0.
Thanks in advance.

It's a matcher, not a test value generator. It's used for saying things like "I expect this stubbed method to be called with any long, Long or null but I don't care about the exact value".

If you have long parameter and you manage to pass null as an argument to mock there will be NPE. If you have Long parameter anyLong() will allow Long and null, long will be autoboxed to Long by compiler. Try this
public class X {
long x(Long arg) {
return 0;
}
public static void main(String[] args) {
X x = mock(X.class);
when(x.x(anyLong())).thenReturn(0L);
System.out.println(x.x(null));
}
}
it will print
0
0