Final answer:
The Hadoop MapReduce API contract stipulates that key classes must implement WritableComparable and value classes must implement Writable, along with default constructors and methods for serialization and deserialization. Keys also require a stable hashCode implementation.
Step-by-step explanation:
The Hadoop MapReduce API specifies a contract for the key and value classes used in the MapReduce framework. Essentially, the key and value classes must adhere to several requirements set forth by the framework. For instance, the key must implement the WritableComparable interface, and the value must implement the Writable interface. This ensures that the keys can be compared and sorted during the MapReduce process, and both keys and values can be serialized and deserialized across the network, as MapReduce jobs often run on distributed systems.
Furthermore, it is essential that these classes have a default constructor, as they are instantiated by the framework. They should also be accompanied by methods to serialize (write) and deserialize (readFields) the objects effectively, which is crucial for the proper functioning of the MapReduce job. Lastly, it's important to consider that the key classes should have a consistent and well-defined hashCode() implementation since MapReduce relies heavily on hashing for partitioning the data and ensuring even distribution across the reducers.