Java 7+/Android JSON parser/generator, MIT (c) 2020-2024 @miktim

Release notes:

- Java SE 7+/Android RFC 8259 compliant package
(see: https://datatracker.ietf.org/doc/rfc8259/?include_text=1 );
- no external dependencies;
- “JSON” means text in JSON format. JSON text exchanged between systems MUST be encoded using UTF-8 (default charset);
- “Json” means the Java representation of a JSON object.

package org.miktim.json;

Class JSON.

The class contains static methods for parsing/generating text in JSON format.
- JSON parser converts JSON text to Java objects:
Json object, String, Number, Boolean, null, Object[ ] array of listed types;
- when the names within an object are not unique, parser stores the last value only;
- in addition to listed types, the generator converts Java Lists to JSON arrays and Java Maps to JSON objects. The null key is converted to a “null” member name. Other Java objects are converted to string representation.

 Methods:

static Object fromJSON(String jsonText) throws IOException, ParseException
Parse JSON text

static Object fromJSON(InputStream in, String charsetName) throws IOException, ParseException
Parse text in JSON format from a stream with the specified encoding

static String toJSON(Object obj) throws IOException;
Generate JSON text as single line

static String toJSON(Object obj, int space) throws IOException
Generate the text in JSON format with the specified number of spaces in the indentation

static <T>T toJSON(T obj, OutputStream out, int space, String charsetName) throws IOException
Generate the text in JSON format into a stream with the specified indentation and encoding

Methods for converting objects supported by JSON to a Java primitive or an array of Java primitives.
- sample must be initialized;
- casting numbers may involve rounding or truncation;
- casting null to a Java primitive returns corresponding initial value;
- casting null to an array returns an empty array;
- casting null to String returns “null”;
- casting null to other Java objects returns null;
- casting to null returns null.

static <T> T cast(Object obj, T sample) throws ClassCastException
Cast Java object by sample

static <T> T cast(Object obj, Class <T> cls) throws ClassCastException
Cast Java object by class

 Example:

/*
 * Parse and cast JSON array
 */
int[] ints;
// float numbers truncated to integers
ints = JSON.cast(JSON.fromJSON("[1.2, 3.4, 5.6]"), int[].class);
System.out.println(JSON.toJSON(ints, 2));
/* console output:
[
  1,
  3,
  5
]
*/

Class Json extends HashMap <String, Object>

This class is a Java representation of a JSON object. Json member types:
Json object, String, Number, Boolean, null, Object[ ] array of listed types.

Put, set, get notes:
- Json object setters accept any Java object, all Java primitives and primitive arrays;
- RFC 8259 does not recommend using Java BigDecimal and BigInteger as Json member values;
- AVOID RECURSION!;
- put, set methods cast Java primitives to the corresponding objects.
Java objects and arrays are stored “as is” (as reference). For example:
float -> Float, int[ ][ ] -> int[ ][ ], String[ ] -> String[ ]
- after JSON text parsing or normalization, they are stored as:
Number, Object[ ]{Object[ ]{Number,…}, Object[ ]{Number,…}}, Object[ ] {String,…};
- getters return null if the member does not exist.

 Constructors:

Json(Object… members) throws IndexOutOfBoundsException
Members is a name,value pairs

Json(String jsonText) throws IOException, ParseException
Create Json object from String

Json(InputStream inStream) throws IOException, ParseException
Create Json object from UTF-8 encoded stream.

 Example:

/*
 * Create Json object from name/value pairs
 */
Json j = new Json("number", 1, "string", "qwerty", "boolean", true);
System.out.println(j.toJSON());
/* console output: 
{"number": 1, "string": "qwerty", "boolean": true}
*/
// create Json object from String
j = new Json("{ \"number\": 1, \"string\": \"qwerty\", \"boolean\": true }"); 

 Methods:

String[ ] listNames()
Returns list of member names

boolean exists(String memberName, int… indices)
Returns true if there is a member or an element of the member array

Object put(String memberName, Object value);
inherited

Json set(Object memberName, Object value);
Create or replace member. Returns this.

Object get(String memberName)
inherited

Object remove(String memberName);
inherited

Object get(String memberName, int… indices) throws IndexOutOfBoundsException
Returns null, value or array element

Json getJson(String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException
Cast member or array element to Json object

String getString(String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException Cast member or array element to String

Number getNumber(String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException
Cast member or array element to Number

Boolean getBoolean(String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException
Cast member or array element to Boolean

Object[ ] getArray(String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException
Cast member or array element to Object array

<T> T castMember(T sample, String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException
Casting Json member value or array element by sample. See notes for a JSON.cast methods

<T> T castMember(Class <T> cls, String memberName, int… indices) throws ClassCastException, IndexOutOfBoundsException
Casting Json member value or array element by Class. See notes for a JSON.cast methods

Json normalize() throws IOException, ParseException
Returns normalized Json object

String toString()
overridden. Returns JSON text as single line

String toJSON()
Stringify Json object as single line

String toJSON(String memberName, int… indices)
Stringify member value or array element as single line

Json toJSON(OutputStream outStream) throws IOException
OutStream is UTF-8 encoded single line JSON text. Returns this

 Example:

/*
 * Another way to create a Json object and cast member
 */
Json j = (new Json()).set("personId", 1234).set("firstName","John")
  .set("phones", new String[]{"123-4567","890-1234"});
String[] phones;
phones = j.castMember(String[].class, "phones" );
System.out.println(JSON.toJSON(phones));
/* console output:
["123-4567", "890-1234"]
*/

Abstract Class JsonObject

Java object extender. Unload/load fields of a Java object to/from a Json object.
- visibility of object fields as from the object constructor;
- Java transient fields are ignored;
- Java final fields are unloaded, but not initialized at load;
- the accessible fields of the object MUST be initialized;
- see Json set/get/cast rules for Java object fields in the notes for JSON object;
- arrays of custom objects and collections MUST be managed using replacer/reviewer;
- it is recommended to create a default constructor

 Constants:

protected static final transient Object IGNORED
Returned from the replacer/reviver methods to skip the field

 Methods:

Json toJson() throws IllegalArgumentException, IllegalAccessException;
Returns a Json object from this object

<T> T fromJson(Json jsonObj) throws IllegalArgumentException, IllegalAccessException;
Loads Json to this object. Returns this object.

Json toJson(Object targetObj) throws IllegalArgumentException, IllegalAccessException;
Returns a Json object from the target object

<T> T fromJson(T targetObj, Json json) throws IllegalArgumentException, IllegalAccessException;
Loads Json to target object. Returns target object.

protected Object replacer(String name, Object value);
Applies on unloading:
- name is object field class and name delimitet with dot (.), value is object field value;
- first call with the target object class name and the target object as the value;
- returns Json-supported object or IGNORED

protected Object reviver(String name, Object value);
Applies on loading:
- name is object field class and name delimited with dot (.), value is Json-supported object;
- first call with the target object class name and the Json object as value;
- returns a value that is compatible with the object field or IGNORED

protected <T> T getTarget( );
Get target object. Accessible from replacer/reviver

String toString( );
Overridden. Generate JSON text as a single line

static boolean isClassName(String name);

protected <T> T castMember( T sample, String memberName, Json jsonObj );
Returns the sample if Json member does not exists or is null

 Example:

/*
 * Create JsonObject and serialize Java Map
 */
public static class Person extends JsonObject {
  int personId = 0;
  String firstName = "";
  String lastName = "";
  boolean married = false;
  HashMap<String, String> phones = new HashMap<>();

// default constructor
  public Person() {
  }

  @Override
  protected Object replacer(String name, Object value) {
    if(name.endsWith(".phones")) {
// unload phones Map
      Json json = new Json();
      for (Map.Entry<String, String> entry : phones.entrySet()) {
        json.set(entry.getKey(), entry.getValue());
      }
      return json;
    }
    return value;
  }

  @Override
  protected Object reviver(String name, Object value) {
    if (name.endsWith(".phones")) {
// load phones Map
      phones.clear(); // fill existing Map
      Json json = (Json) value;
      for (String key : json.listNames()) {
        phones.put(key, json.getString(key));
      }
      return IGNORED; // phones already loaded
    }
    return value;
  }
}

public static void main(String[] args)
  throws IllegalArgumentException, IllegalAccessException,
         IOException, ParseException {

// create and fill Person object
  Person person = new Person();
  person.personId = 12345;
  person.firstName = "John";
  person.lastName = "Doe";
  person.phones.put("home", "123-4567");
  person.phones.put("work", "789-0123");
// unload person to string
  String s = person.toJson().toJSON();
  System.out.println(s);
/* console output:
{"personId": 12345, "firstName": "John", "lastName": "Doe", "married": false, "phones": {"work": "789-0123", "home": "123-4567"}}
*/
// load person from string
  person = (new Person()).fromJson(new Json(s));
}

See usage here:
./test/json/JsonTest.java
./test/json/JsonCastTest.java
./test/json/JsonObjectTest.java