CS 151 - Lecture 16

Cover page image

Cay S. Horstmann

Lab

Work with a buddy
One of you codes, the other types up the answers.
One of you submits the code in the lab16 subdirectory of your personal repo, the other submits a file report.txt in the lab16 subdirectory of your personal repo.
Put the name of the coding buddy into the report

Preparation

The first time, it takes a very long time to fetch all dependencies—do what's on this slide before the lab.
Download the Play Framework starter project from https://example.lightbend.com/v1/download/play-java
Uncompress in your repo directory and rename the resulting directory to lab16:
```
cd cs151
unzip ~/Downloads/play-java.zip
mv play-java lab16
```
Run
```
cd lab16
./sbt run
```
Point your browser to localhost:9000. You should see a welcome screen. Hit Ctrl+C when done.
With any text editor, open the file project/plugins.sbt and add the line
```
addSbtPlugin("com.typesafe.sbteclipse" % "sbteclipse-plugin" % "5.1.0")
```
after the other addSbtPlugin commands
Run
```
./sbt eclipse
```
in the lab16 directory
In Eclipse, import the project in the lab16 directory. It will be called play-java in Eclipse.

A Clicker Web App

Run ./sbt run in a terminal window.
We want to build a clicker app where students can respond to a clicker question. We won't build a web UI on the server—that's so 1990s. Instead, we'll send commands to the server such as this one:
```
http://localhost:9000/select/lesson16q1/C
```
To do that, you need to
- Make an entry in the conf/routes file
- Provide a handler method
Add this method to the HomeController class:
```
public Result select(String problem, String choice) {
   return ok("You selected " + choice + " for " + problem + "\n").as("text/plain");
}
```
Save the file. Point your browser to http://localhost:9000/select/lesson16q1/C. What happens?
Look at the section “The routes file syntax” in this page. Then fix up conf/routes so that your method gets called. What did you do?
What output do you get now for http://localhost:9000/select/lesson16q1/C

Add this field to HomeController:

private static HashMap<String, HashMap<String, Integer>> results = new HashMap<>();

In the select method, add this code before the return statement:

if (results.get(problem) == null) 
   results.put(problem, new HashMap<>());
if (results.get(problem).get(choice) == null) 
   results.get(problem).put(choice,  0);
results.get(problem).put(choice, 
   results.get(problem).get(choice) + 1);

What does the code do?

Add another method public Result view(String problem) that yields the map of counts for the given problem. Just turn the map results.get(problem) into a string. What is your method?

Add a route to it. Point your browser to

http://localhost:9000/select/lesson16q1/C
http://localhost:9000/select/lesson16q1/A
http://localhost:9000/select/lesson16q1/B
http://localhost:9000/select/lesson16q1/C
http://localhost:9000/view/lesson16q1

What happens?

A Web Framework

Discuss the following questions with your buddy and provide answers in your report.
What makes this a framework?
What did you need to know about web programming to use Play?
Did you need to extend framework classes? Which?
What other mechanisms other than extending classes did the framework use to discover what you wanted to achieve with your code? (Reflection, annotations, etc.)
Is this an example of “inversion of control”?