Reading and writing JSON in Java

First, you'll need to get Gson; you can do this by doing a read-only checkout of the repository using SVN over HTTP with SVN by using the following command:

svn checkout http://google-gson.googlecode.com/svn/trunk/google-gson-read-only

Of course, this assumes that you have a Java development kit (http://www.oracle.com/technetwork/java/javase/downloads/index.html) and SVN (TortoiseSVN is a good client for Windows available at http://tortoisesvn.net/downloads.html) installed. Many Java IDEs include support for SVN.

Once you check out the code, follow the instructions that come with it to build the Gson JAR file, and add the JAR file to your project.

To begin, you need to create a com.google.gson.Gson object. This class defines the interface you'll use to convert between JSON and Java:

Gson gson = new com.google.gson.Gson(); 
String json = "{\"call\": \"KF6GPE\", \"type\": \"l\", \"time\":
\"1399371514\", \"lasttime\": \"1418597513\", \"lat\": 37.17667,
\"lng\": -122.14650,\"result\":\"ok\"}";
com.google.gson.JsonObject result = gson.fromJson(json, 
JsonElement.class).getAsJsonObject(); 

The JsonObject class defines the top-level object for containing a JSON object; you use its get and add methods to get and set attributes, like this:

JsonElement result = result.get("result").getAsString();

The Gson library uses the JsonElement class to encapsulate a single JSON value; it has the following methods that let you get the value contained in JsonElement as a plain Java type:

You can also learn about the type in JsonElement using one of the following methods:

You can also convert instances of your classes directly to JSON, writing something like this:

public class SimpleResult {
    public String result;
}

// Elsewhere in your code…
Gson gson = new com.google.gson.Gson(); 
SimpleResult result = new SimpleResult;
result.result = "ok";
String json = gson.toJson(result);	

This defines a class SimpleResult, which we use to create a single instance, and then use the Gson object instance to convert to a string containing the JSON using the Gson method toJson.

Finally, because JsonElement encapsulates a single value, you can also handle simple values expressed in JSON, like this:

Gson gson = new com.google.gson.Gson(); 
String piJson = "3.14";
double result = gson.fromJson(piJson, 
JsonElement.class).getAsDouble(); 

This converts the primitive value 3.14 in JSON to a Java double.

Like the C# example, you can convert directly from JSON to a plain old Java object (POJO) in a type-safe manner. You'll see how to do this in Chapter 7, Using JSON in a Type-safe Manner.

There are other JSON conversion implementations for Java, too. For a complete list, see the list at http://json.org/.