Currently there are two different Java APIs:

• org.apache.hadoop.mapred: This is the original API, which is currently deprecated.
• org.apache.hadoop.mapreduce: This is the new API, which we will be using in this tutorial. The only problem is that some library classes have not yet been converted to use the new API and we cannot therefore use them.

When browsing through the documentation, make sure to stay in org.apache.hadoop.mapreduce namespace.

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
• NullWritable – no value

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

The Perl API is always using strings as keys and values. From the Java point of view:

• the type of keys and values produced by Perl API is always Text.
• any type can be used as input to Perl API – if the type is different from Text, a toString method is used to convert the value to string before the value is passed to Perl.

The input formats are the same as in Perl API. Every input format also specifies which types it can provide.

An input format is a subclass of FileInputFormat<K,V>, where K is the type of keys and V is the type of values it can load.

Available input formats:

• TextInputFormat: The type of keys is LongWritable and the type of values is Text.
• KeyValueTextInputFormat: The type of both keys and values is Text.
• SequenceFileInputFormat: Any type of keys and values can be used.

An output format is a subclass of FileOutputFormat<K,V>, where K is the type of keys and V is the type of values it can store.

Available output formats:

• TextOutputFormat: The type of both keys and values is Text.
• SequenceFileOutputFormat: Any type of keys and values can be used.