Using the JSON tree API

The second approach is to use the tree model. An ObjectMapper instance is used to create a JsonNode instance using the Persons.json file. The fieldNames method returns Iterator allowing us to process each element of the file.

try { 
    ObjectMapper mapper = new ObjectMapper(); 
    JsonNode node = mapper.readTree(new File("Persons.json")); 
    Iterator<String> fieldNames = node.fieldNames(); 
    while (fieldNames.hasNext()) { 
        ... 
        fieldNames.next(); 
    } 
} catch (IOException ex) { 
    // Handle exceptions 
} 

Since the JSON file contains a persons field, we will obtain a JsonNode instance representing the field and then iterate over each of its elements.

JsonNode personsNode = node.get("persons"); 
Iterator<JsonNode> elements = personsNode.iterator(); 
while (elements.hasNext()) { 
    ... 
} 

Each element is processed one at a time. If the element type is a string, we assume that this is the groupname field.

JsonNode element = elements.next(); 
JsonNodeType nodeType = element.getNodeType(); 

if (nodeType == JsonNodeType.STRING) { 
    out.println("Group: " + element.textValue()); 
} 

If the element is an array, we assume it contains a series of persons where each person is processed by the parsePerson method:

if (nodeType == JsonNodeType.ARRAY) { 
    Iterator<JsonNode> fields = element.iterator(); 
    while (fields.hasNext()) { 
        parsePerson(fields.next()); 
    } 
}

The parsePerson method is shown next:

public void parsePerson(JsonNode node) { 
    Iterator<JsonNode> fields = node.iterator(); 
    while(fields.hasNext()) { 
        JsonNode subNode = fields.next(); 
        out.println(subNode.asText()); 
    } 
}

The output follows:

Group: school
Smith
Peter
8475552222
King
Sarah
8475551111
Frost
Nathan
8475553333

There is much more to JSON than we are able to illustrate here. However, this should give you an idea of how this type of data can be handled.