RDDs do not really have fields per-se, unless for example your have an RDD of Row objects. You would usually filter on an index:
rdd.filter(x => x(1) == "thing")
(example in scala for clarity, same thing applies to Java)
If you have an RDD of a typed object, the same thing applies, but you can use a getter for example in the lambda / filter function.
If you want field references you would do better wih the dataframes API. Compare for example:
sqlContext.createDataFrame(rdd, schema).where($"fieldname" == "thing")
(see Spark Official Docs - DataFrames for more on the schema)