⌛⌛ JSON

JSON is a system independent file format to store data and to transfer data between systems. If you are not yet familiar with JSON, you can earn more, for example, from Wikipedia.

JSON data consists of following types of data items:

• Number. In this task type double.

• Truth value. In this task type boolean.

• String. In this task type String.

• Null. In this task the null reference.

• Array. An ordered list that may contain zero or more JSON data items.

• Object. A dictionary type structure that may contain zero or more key–item pairs. The keys are strings.

In this task, you will familiarise yourself with JSON by implementing a class hierarchy for JSON data items. The readily provided Node.java contains an abstract class Node that acts as the base class for JSON data items. This class is available at the remote material repository (round7/json/Node.java). The class Node provides functions isValue, isArray and isObject for inspecting the Node type and function printJson for the printing of the data items.

You need to implement concrete subclasses ValueNode, ArrayNode and ObjectNode that inherit Node. Place your code into files named ArrayNode.java, ObjectNode.java and ValueNode.java in the directory round7/json of your local repository.

You do not need to create a Maven project for this task. If you do, copy your classes into the round7/json directory of your local repository and make a commit before submitting your answer.

Implement the classes as follows:

• ValueNode stores a number, truth value, string or null. Public members:

  • Default constructor ValueNode() that initialises the object by storing the null in the object.

  • Constructors ValueNode(double value), ValueNode(boolean value) and ValueNode(String value) initialize the object to store the value of corresponding type. It is assumed that null is not passed as a parameter value to the ValueNode(String value) constructor.

  • Member functions isNumber(), isBoolean(), isString() and isNull() that return truth values indicating whether the stored value is of the corresponding type or not. For example, isString returns true, if and only if the stored value is a string.

  • Member functions double getNumber(), boolean getBoolean(), String getString() and Object getNull() that return the corresponding value. For example getBoolean returns the value stored by the object as a boolean, and it is the responsibility of the caller to first call isBoolean to check that the value really can be returned as a truth value.

• The ArrayNode class stores Node objects. Properties and public members:

  • Implements the interface Iterable<Node>.

  • A default constructor that initializes an empty array that does not contain any Node objects yet.

  • Member function void add(Node node) that adds Node to the end of the array.

  • Member function int size() that returns the number of stored Node objects.

• ObjectNode that stores key–value pairs, where keys are strings and values are Node objects. Properties and public members:

  • Implements the interface Iterable<String>. Iterates over the keys of the stored key–value pairs in natural sorted order of String.

  • A default constructor that initializes an empty JSON object that does not have any key-value pairs yet.

  • Member function Node get(String key) that returns the Node object corresponding to key or null, if it does not exits. Please, note that the “plain” null is here an error code, while a ValueNode containing the null value is a valid return value.

  • Member function void set(String key, Node node) that adds a key–value pair described by the parameters. A possible earlier value for the key will be overwritten.

  • Member function int size() that returns the number of stored Node objects.

The implementations of ArrayNode and ObjectNode can be simple, if you use the Java containers in a wisely.

Testing

You may test your implementation by using the test program given in the file JsonTest.java and the example output given in the files output1.txt, output2.txt, output3.txt and output4.txt. Place these files to the same directory with your code, compile the test program, for example, as javac *.java, and run the tests as java JsonTest 1, java JsonTest 2, java JsonTest 3, and java JsonTest 4. The expected outputs of these four tests are given in the files output1.txt, output2.txt, output3.txt and output4.txt.

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