## org.apache.hadoop.mapred.join
Class Parser

    java.lang.Object
      org.apache.hadoop.mapred.join.Parser

    public class Parser

extends Object

Very simple shift-reduce parser for join expressions. This should be sufficient for the user extension permitted now, but ought to be replaced with a parser generator if more complex grammars are supported. In particular, this "shift-reduce" parser has no states. Each set of formals requires a different internal node type, which is responsible for interpreting the list of tokens it receives. This is sufficient for the current grammar, but it has several annoying properties that might inhibit extension. In particular, parenthesis are always function calls; an algebraic or filter grammar would not only require a node type, but must also work around the internals of this parser. For most other cases, adding classes to the hierarchy- particularly by extending JoinRecordReader and MultiFilterRecordReader- is fairly straightforward. One need only override the relevant method(s) (usually only `CompositeRecordReader.combine(java.lang.Object[], org.apache.hadoop.mapred.join.TupleWritable)`) and include a property to map its value to an identifier in the parser.

Nested Class Summary
`static class` `Parser.Node`
`static class` `Parser.NodeToken`
`static class` `Parser.NumToken`
`static class` `Parser.StrToken`
`static class` `Parser.Token` Tagged-union type for tokens from the join expression.
`static class` `Parser.TType`

Constructor Summary
`Parser()`

Method Summary

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### Parser

    public Parser()

Copyright © 2009 The Apache Software Foundation