Comparator.comparing().thenComparing type inference problem

[Ivar_PredixCN]

The problem is type inferencing. Without adding a (Song s) to the first comparison, comparator.comparing doesn't know the type of the input so it defaults to Object.
You can fix this problem 1 of 3 ways:

Use the new Java 8 method reference syntax
Collections.sort(playlist,
Comparator.comparing(Song::getTitle)
.thenComparing(Song::getDuration)
.thenComparing(Song::getArtist)
);
Pull out each comparison step into a local reference
Comparator<Song> byName = (s1, s2) -> s1.getArtist().compareTo(s2.getArtist());
Comparator<Song> byDuration = (s1, s2) -> Integer.compare(s1.getDuration(), s2.getDuration());
Collections.sort(playlist,
byName
.thenComparing(byDuration)
);
Forcing the type returned by the Comparator (note you need both the input type and the comparison key type)
sort(
Comparator.<Song, String>comparing((s) -> s.getTitle())
.thenComparing(p1 -> p1.getDuration())
.thenComparing(p1 -> p1.getArtist())
);
I think the "last" thenComparing syntax error is misleading you. It's actually a type problem with the whole chain, it's just the compiler only marking the end of the chain as a syntax error because that's when the final return type doesn't match I guess.

I'm not sure why List is doing a better inferencing job than Collection since it should do the same capture type but apparently not.