Basic JSON parser (super incomplete)

Parser.java

package mx.letmethink;

import java.util.HashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        String text = "{}";
        // System.out.println(new JsonParser(text).parse());
        // System.out.println(new JsonParser("{\"key\": \"value\", \"num\": 1234}").parse());
        System.out.println(new JsonParser("{\"key\":{\"a\":{\"b\":\"{12345}678\"}}}").parse());
    }
}

class JsonParser {
    private static final char CLOSING_BRACE = '}';
    private static final char COMMA = ',';
    private final char[] chars;
    int position;
    private static final JsonException UNEXPECTED_END_OF_TEXT = new JsonException("Unexpected end of text");


    public JsonParser(String text) {
        this.chars = text.toCharArray();
    }

    public JsonObject parse() {
        return readObject();
    }

    public JsonObject readObject() {
        JsonObject object = new JsonObject();
        readOpeningBrace();
        while (true) {
            if (peek() == CLOSING_BRACE) {
                break;
            }
            String key = readKey();
            readColon();
            if (Character.isDigit(peek())) {
                JsonNumber jsonNumber = readNumber();
                object.put(key, jsonNumber);
            } else if (peek() == '"') {
                object.put(key, readString());
            } else {
                object.put(key, readObject());
            }

            if (peek() == CLOSING_BRACE) {
                break;
            }

            readComma();
        }
        readClosingBrace();
        return object;
    }

    private JsonNumber readNumber() {
        strip();
        long value = 0;
        while (position < chars.length && Character.isDigit(chars[position])) {
            value = value * 10 + chars[position++] - '0';
        }
        if (position == chars.length) {
            throw UNEXPECTED_END_OF_TEXT;
        }
        return new JsonNumber(value);
    }

    private JsonString readString() {
        strip();
        if (chars[position] != '"') {
            throw new JsonException("Expected opening '\"'");
        }
        position++;
        String value = "";
        while (position < chars.length && chars[position] != '"') {
            value += chars[position++];
        }
        if (position == chars.length) {
            throw UNEXPECTED_END_OF_TEXT;
        }
        if (chars[position] != '"') {
            throw new JsonException("Expected closing '\"'");
        }
        position++;
        return new JsonString(value);
    }

    private void readOpeningBrace() {
        strip();
        if (chars[position] != '{') {
            throw new JsonException("Expected '{', found " + chars[position]);
        }
        position++;
    }

    private void readClosingBrace() {
        strip();
        if (chars[position] != CLOSING_BRACE) {
            throw new JsonException("Expected '}'");
        }
        position++;
    }

    private char peek() {
        strip();
        return chars[position];
    }

    private String readKey() {
        strip();
        if (chars[position] != '"') {
            throw new JsonException("Expected opening \"");
        }
        position++;
        String key = "";
        while (position < chars.length && chars[position] != '"') {
            key += chars[position++];
        }
        if (position == chars.length) {
            throw UNEXPECTED_END_OF_TEXT;
        }
        if (chars[position] != '"') {
            throw new JsonException("Expected closing \"");
        }
        position++;
        return key;
    }

    private void readColon() {
        strip();
        if (chars[position] != ':') {
            throw new JsonException("Expected ':'");
        }
        position++;
    }

    private void readComma() {
        strip();
        if (chars[position] != COMMA) {
            throw new JsonException("Expected ','");
        }
        position++;
    }



    private void strip() {
        while (position < chars.length && chars[position] == ' ') {
            position++;
        }
        if (position == chars.length) {
            throw UNEXPECTED_END_OF_TEXT;
        }
    }

}


class JsonObject {
    private Map<String, JsonObject> map;

    public JsonObject() {
        map = new HashMap<>();
    }

    public void put(String key, JsonObject object) {
        map.put(key, object);
    }

    @Override
    public String toString() {
        return toString("");
    }

    public String toString(String indent) {
        String str = "{\n";
        int totalEntries = map.entrySet().size();
        int count = 0;
        for (Map.Entry<String, JsonObject> entry : map.entrySet()) {
            str += indent + "    " + "\"" + entry.getKey() + "\": " + entry.getValue().toString(indent + "    ");
            count++;
            if (count < totalEntries) {
                str += ", ";
            }
        }
        str += "\n" + indent + "}";
        return str;
    }
}

class JsonNumber extends JsonObject {
    private final long value;

    public JsonNumber(long value) {
        this.value = value;
    }

    public String toString(String indent) {
        return toString();
    }

    @Override
    public String toString() {
        return value + "";
    }
}

class JsonString extends JsonObject {
    private final String value;
    public JsonString(String value) {
        this.value = value;
    }

    public String toString(String indent) {
        return toString();
    }

    @Override
    public String toString() {
        return "\"" + value.replace("\"", "\\") + "\"";
    }
}

class JsonException extends RuntimeException {
    public JsonException(String message) {
        super(message);
    }
}