Vtyjkyuk

This is from the Stream interface from Oracle's implementation of JDK 8:

public interface Stream<T> extends BaseStream<T, Stream<T>> 
 Stream<T> sorted();
 

and it is very easy to blow this up at run time and no warning will be generated at compile time. Here is an example:

class Foo 
 public static void main(String args) 
 Arrays.asList(new Foo(), new Foo()).stream().sorted().forEach(f -> );
 

which will compile just fine but will throw an exception at run time:

Exception in thread "main" java.lang.ClassCastException: Foo cannot be cast to java.lang.Comparable

What could be the reason that the sorted method was not defined where the compiler could actually catch such problems? Maybe I am wrong but isn't it this simple:

interface Stream<T> 
 <C extends Comparable<T>> void sorted(C c);

?

Obviously the guys implementing this (who are light years ahead of me as far as programming and engineering is considered) must have a very good reason that I am unable to see, but what is that reason?

Essentially, you're asking if a method can require the class' type parameter be more strict for only said method. This is not possible in Java. There is no way to tell the compiler, "hey, this one method requires that the type parameter match more specific bounds than defined at the class level". Such a feature may be useful but I'd also expect confusing and/or complicated.

There's also no way to work around this with the way generics is currently implemented. For instance, you were wondering why something like the following wouldn't work:

public interface Stream<T> 

 <C extends Comparable<? super T>> Stream<T> sorted(Class<C> clazz);

The problem is there's no guarantee that Class<C> is assignable from Class<T>. What if you had the following:

public class Foo implements Comparable<Foo> /* implementation */ 

public class Bar extends Foo 

public class Other extends Foo 

You could basically tell the Stream that it's a Stream of Bar elements but sort as if it was a Stream of Other elements:

Stream<Bar> stream = barCollection.stream().sorted(Other.class);

And, if casting is done via the given Class, you'd still have ClassCastExceptions at runtime. If the Class wasn't utilized for casting then the argument is useless since it adds no type-safety.

You also can't do something like:

public interface Stream<T> 

 <C extends T & Comparable<? super T>> Stream<T> sorted(Class<C> clazz);

Because <C extends T & ...> is a compile-time error; type parameters cannot be followed by other bounds. Even if this was allowed I'm not sure if it'd be effective.

I'd also like to point out the Stream.sorted(Comparator<? super T>) isn't type-safe because of Stream's method signature. It's type-safe because the Comparator enforces the correct types. In other words, it's the Comparator that makes sure the elements can be compared, not the Stream.

How would you implement that? sorted is a intermediate operation (can be called anywhere between other intermediate operations), meaning you can start with a stream that is not Comparable, but call sorted on one that is Comparable:

Arrays.asList(new Foo(), new Foo())
 .stream()
 .map(Foo::getName) // name is a String for example
 .sorted()
 .forEach(f -> );

The thing that you are proposing takes an argument as input, but Stream::sorted does not, so you can't do that. The overload version accepts a Comparator - meaning you can sort something by a property, but still return Stream<T>. I think that this is quite easy to understand if you would try to write your minimal skeleton of a Stream interface/implementation.

The documentation for Stream#sorted explains it perfectly:

Returns a stream consisting of the elements of this stream, sorted according to natural order. If the elements of this stream are not Comparable, a java.lang.ClassCastException may be thrown when the terminal operation is executed.

You're using the overloaded method that accepts no arguments (not the one that accepts a Comparator), and Foo does not implement Comparable.

If you're asking why the method doesn't throw a compiler error if the contents of the Stream do not implement Comparable, it would be because T is not forced to extend Comparable, and T cannot be changed without a call to Stream#map; it seems to only be a convenience method so no explicit Comparator needs to be provided when the elements already implement Comparable.

For it to be type-safe, T would have to extend Comparable, but that would be ridiculous, as it would prevent a stream from containing any objects that aren't Comparable.