Welcome to the first lesson in our course on Handling JSON Files with Java. In this lesson, we will explore JSON (JavaScript Object Notation), a popular, lightweight, and easily readable data format.

JSON is widely used in web development for data exchange between a server and a web application, among other applications. Understanding JSON and how to parse it in Java will enable you to work with vast amounts of data more efficiently. Let's dive into JSON and see why it's an integral part of modern programming.

Before we parse JSON, let's understand its structure. JSON is built on two structures: a collection of key-value pairs (often referred to as an object) and an ordered list of values (an array). Each key-value pair consists of a string (the key) followed by a value, which can be a string, number, object, array, or boolean.

Here's our example JSON file, data.json:

JSON
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1{
2    "school": "Greenwood High",
3    "location": {
4        "city": "New York",
5        "state": "NY"
6    },
7    "students": [
8        {"name": "Emma", "age": 15, "grade": "10"},
9        {"name": "Liam", "age": 14, "grade": "9"},
10        {"name": "Olivia", "age": 16, "grade": "11"}
11    ]
12}

• Objects: Encapsulated with curly braces {}, containing key-value pairs. For example, the "location" object.
• Arrays: Defined with square brackets []. For instance, "students" is an array containing multiple student objects.
• Key-Value Pairs: Each entry in the JSON is a key-value pair, such as "city": "New York".

Understanding this structure is crucial when writing code to parse JSON.

Let's start parsing JSON in Java. First, we need to specify the path to our JSON file and read its content.

Java
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1// Path to the JSON file
2Path filePath = Paths.get("data.json");
3
4// Read the entire content of the JSON file into a string
5String json = Files.readString(filePath);

Here, we use Paths.get to create a Path object representing the file's location. Then, we read the entire content of the file into a String using Files.readString.

To work with JSON in Java, we will use the Jackson library. This library provides powerful tools to parse and manipulate JSON data effectively. First, we'll import the necessary classes for working with JSON:

Java
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1import com.fasterxml.jackson.databind.ObjectMapper;
2import com.fasterxml.jackson.databind.JsonNode;

Next, we'll create an instance of the ObjectMapper class to represent the JSON object and prepare for parsing the data.

Java
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1// Create ObjectMapper instance
2ObjectMapper objectMapper = new ObjectMapper();

The ObjectMapper is a core class in the Jackson library, designed to convert JSON content to and from Java objects. It provides functionalities to parse JSON into tree structures and serialize objects into JSON strings.

We will then parse the JSON string into a JsonNode using the readTree method. This method will convert the JSON string into a hierarchical tree structure.

Java
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1// Parse the JSON string into a JsonNode
2JsonNode rootNode = objectMapper.readTree(json);

When we use objectMapper.readTree(json), it takes the JSON string as input and parses it into a JsonNode object, representing the hierarchical structure of the JSON data. This allows us to traverse through the JSON tree, access specific keys and values, and manipulate the JSON content as needed. The JsonNode provides methods to access data by keys, iterate over arrays, and check for the existence of certain nodes. Overall, this parsing process converts the textual JSON data into a structured form that is easy to work with programmatically in Java.

Finally, let's display the parsed data to verify our parsing is successful.

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1// Convert JsonNode to a pretty-printed JSON string
2String prettyJson = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(rootNode);
3
4// Print the formatted JSON string to the console
5System.out.println(prettyJson);

In this code:

• writerWithDefaultPrettyPrinter() creates a writer for the ObjectMapper that is configured to output the JSON in a human-readable format. It adds line breaks and indentation, making the JSON easier to read.

• writeValueAsString(rootNode) takes the JsonNode and serializes it back into a JSON string. The pretty-print formatting is applied during this conversion.

The last step is to print the formatted JSON string to the console, allowing us to visually verify that the JSON has been correctly parsed and retained its structure during processing.

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1{
2  "school": "Greenwood High",
3  "location": {
4    "city": "New York",
5    "state": "NY"
6  },
7  "students": [
8    {
9      "name": "Emma",
10      "age": 15,
11      "grade": "10"
12    },
13    {
14      "name": "Liam",
15      "age": 14,
16      "grade": "9"
17    },
18    {
19      "name": "Olivia",
20      "age": 16,
21      "grade": "11"
22    }
23  ]
24}

In this lesson, we covered the essentials of JSON, understanding its structure, and how to parse it using Java. We explored JSON's components, walked through a real-code example, and discussed practical insights into working with JSON data using the Jackson library.

As you proceed to the practice exercises, focus on reinforcing your understanding of JSON structure. This hands-on experience will prepare you for more advanced topics in subsequent lessons. Happy coding!