Github user StephanEwen commented on a diff in the pull request:
https://github.com/apache/incubator-flink/pull/87#discussion_r15678326
--- Diff: docs/java_api_guide.md ---
@@ -435,6 +493,85 @@ Defining Keys
Functions
---------
+You can define a user-defined function and pass it to the DataSet
+transformations in several ways:
+
+#### Implementing an interface
+
+The most basic way is to implement one of the provided interfaces:
+
+{% highlight java %}
+class MyMapFunction implements MapFunction<String, Integer> {
+ public Integer map(String value) { return Integer.parseInt(value); }
+});
+data.map (new MyMapFunction());
+{% endhighlight %}
+
+#### Anonymous classes
+
+You can pass a function as an anonmymous class:
+{% highlight java %}
+data.map(new MapFunction<String, Integer> () {
+ public Integer map(String value) { return Integer.parseInt(value); }
+});
+{% endhighlight %}
+
+#### Java 8 Lambdas
+
+***Warning: Lambdas are currently only supported for filter and reduce
+ transformations***
+
+{% highlight java %}
+DataSet<String> data = // [...]
+data.filter(s -> s.startsWith("http://";));
+{% endhighlight %}
+
+{% highlight java %}
+DataSet<Integer> data = // [...]
+data.reduce((i1,i2) -> i1 + i2);
+{% endhighlight %}
+
+#### Rich functions
+
+All transformations that take as argument a user-defined function can
+instead take as argument a *rich* function. For example, instead of
+{% highlight java %}
+class MyMapFunction implements MapFunction<String, Integer> {
+ public Integer map(String value) { return Integer.parseInt(value); }
+});
+{% endhighlight %}
+you can write
+{% highlight java %}
+class MyMapFunction extends RichMapFunction<String, Integer> {
+ public Integer map(String value) { return Integer.parseInt(value); }
+});
+{% endhighlight %}
+and pass the function as usual to a `map` transformation:
+{% highlight java %}
+data.map(new MyMapFunction());
+{% endhighlight %}
+
+Rich functions can also be defined as an anonymous class:
+{% highlight java %}
+data.map (new RichMapFunction<String, Integer>() {
+ public Integer map(String value) { return Integer.parseInt(value); }
+});
+{% endhighlight %}
+
+Rich functions provide, in addition to the user-defined function (map,
+reduce, etc), four methods: `open`, `close`, `getRuntimeContext`,
+`getIterationRuntimeContext`, and `setRuntimeContext`. These are
--- End diff --
I would skip the `getIterationRuntimeContext()` method. Then it becomes
actually four methods, as mentioned in the text.
It would be nice to regular RuntimeContext and the IterationRuntimeContext.
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