...

However, in practice, most processors will produce keys or values or both of a predictable type, and authors would benefit from the parity-check that type safety provides. This can be achieved by adding generic parameters to ProcessorContext, and also adding "output type" bounds to the Processor interface itself. This does not  restrict the range of use cases at all, though, since heterogeneous-output Processors, which wish to forward variable types, can still bound the "output types" to `<?,?>`.

Some examples of use cases that would benefit from this change:

• Any processor can benefit. Imagine a pure user of the ProcessorAPI who has very complex processing logic. I have seen several processorimplementation that are hundreds of lines long and call `context.forward` in many different locations and branches. In such an implementation, it would be very easy to have a bug in a rarely used branch that forwards the wrong kind of value. This would structurally prevent that from happening.
• Also, anyone who heavily uses the ProcessorAPI would likely have developed helper methods to wire together processors, just as we have in the DSL implementation. This change would enable them to ensure at compile time that they are actually wiring together compatible types. This was actually my original motivation, since I found it very difficult and time consuming to follow the Streams DSL internal builders.

Proposed Changes

The high-level approach is to bound the output  types of Processor, which we can enforce by bounding the types that ProcessorContext accepts for forward.

...

public interface ProcessorContext<K, V> {
  ...
- <K,            V>            void forward(final K  key, final V  value);
+ <K1 extends K, V1 extends V> void forward(final K1 key, final V1 value);
. <K1 extends K, V1 extends V> void forward(final K1 key, final V1 value, final To to);

// the following are deprecated. We'll update the generic bounds, and keep them deprecated
. @Deprecated
. <K1 extends K, V1 extends V> void forward(final K1 key, final V1 value, final int childIndex);
. @Deprecated
. <K1 extends K, V1 extends V> void forward(final K1 key, final V1 value, final String childName);
}

...