The Java API differs from the Perl API in one important aspect: the keys and values are types.

The type of a value must be a subclass of Writable, which provides methods for serializing and deserializing values.

The type of a key must be a subclass of WritableComparable, which provides both Writable and Comparable interface.

Here is a list of frequently used types:

• Text – UTF-8 encoded string
• BytesWritable – sequence of arbitrary bytes
• IntWritable – 32-bit integer
• LongWritable – 64-bit integer
• FloatWritable – 64-bit floating number
• DoubleWritable – 64-bit floating number

For more complicated types like variable-length encoded integers, dictionaries, bloom filters, etc., see Writable.