In the Getting started guide, we covered an Edgent application where we read from a device's simulated temperature sensor. Yet Edgent supports more operations than simple filtering. Data analysis and streaming require a suite of functionality, the most important components of which will be outlined below.
TStream.map() is arguably the most used method in the Edgent API. Its two main purposes are to perform stateful or stateless operations on a stream's tuples, and to produce a TStream with tuples of a different type from that of the calling stream.
In addition to filtering tuples, TStreams support operations that transform tuples from one Java type to another by invoking the TStream.map() method.
This is useful in cases such as calculating the floating point average of a list of Integers, or tokenizing a Java String into a list of Strings. To demonstrate this, let's say we have a TStream which contains a few lines, each of which contains multiple words:
TStream<String> lines = topology.strings( "this is a line", "this is another line", "there are three lines now", "and now four" );
We then want to print the third word in each line. The best way to do this is to convert each line to a list of Strings by tokenizing them. We can do this in one line of code with the TStream.map() method:
TStream<List<String> > wordsInLine = lines.map(tuple -> Arrays.asList(tuple.split(" ")));
Since each tuple is now a list of strings, the wordsInLine stream is of type List<String>. As you can see, the map() method has the ability to change the type of the TStream. Finally, we can use the wordsInLine stream to print the third word in each line.
wordsInLine.sink(list -> System.out.println(list.get(2)));
As mentioned in the Getting started guide, a TStream can be parameterized to any serializable Java type, including ones created by the user.
In all previous examples, the operations performed on a TStream have been stateless; keeping track of information over multiple invocations of the same operation has not been necessary. What if we want to keep track of the number of Strings sent over a stream? To do this, we need our TStream.map() method to contain a counter as state.
This can be achieved by creating an anonymous Function class, and giving it the required fields.
TStream<String> streamOfStrings = ...; TStream<Integer> counts = streamOfStrings.map(new Function<String, Integer>() { int count = 0; @Override public Integer apply(String arg0) { count = count + 1; return count; } });
The count field will now contain the number of Strings which were sent over streamOfStrings. Although this is a simple example, the anonymous Function passed to TStream.map() can contain any kind of state! This could be a HashMap<K,V>, a running list of tuples, or any serializable Java type. The state will be maintained throughout the entire runtime of your application.