Deadly simple JSON serializing of java pojos

TinyJsonMapper.java

package core;

import java.lang.reflect.Field;
import java.util.Collection;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

public class TinyJsonMapper {

      public String convertJson(Object object){
        StringBuilder builder = new StringBuilder();
        try {
            if(object instanceof Collection){
                convertCollection(builder,(Collection)object);
            }
            else{
                convertObject(builder,object);
            }

        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

        return  builder.toString();
    }
    
    interface  Converter {
        StringBuilder convert(StringBuilder sb, Object object);
    }
    public static Map<Class,Converter> converterMap  = new HashMap<>();

    static {
        converterMap.put(Number.class,(sb,ob)->{
            sb.append(ob);
            return sb;
        });

        converterMap.put(String.class,(sb,ob)->{
            String str = (String) ob;

            str =  str.replaceAll("\"","\\\\\"");

            sb.append("\"").
                    append(str)
                    .append("\"");
            return sb;
        });

        converterMap.put(Date.class,(sb, ob)->{
            sb.append(((Date)ob).getTime());
            return sb;
        });
    }

    public  boolean convertRegisteredTypes(StringBuilder sb, Object ob){

        if(ob == null){
            sb.append("null");
            return true;
        }
        Converter con =  null;
        if(ob instanceof  Number){
            con = converterMap.get(Number.class);
        }
        else if(ob instanceof  String ){
            con = converterMap.get(String.class);

        }
        else if(ob instanceof  Date){
            con = converterMap.get(Date.class);
        }
        else{
            con = converterMap.get(ob.getClass());
        }

        if(con!=null){
            con.convert(sb,ob);
            return true;
        }
        return false;
    }


    public StringBuilder convertCollection(StringBuilder builder,Collection collection) throws IllegalAccessException {
        builder.append("[");
        for(Object colObj : (Collection)collection){
            convert(builder,colObj);
            builder.append(",");
        }
        builder.deleteCharAt(builder.length()-1);
        builder.append("]");
        return  builder;
    }

    public StringBuilder convertObject(StringBuilder builder , Object object) throws IllegalAccessException {
        builder.append("{");
        converToJson(builder,object);
        builder.append("}");
        return builder;
    }

    public StringBuilder convert(StringBuilder builder,Object o) throws IllegalAccessException {
        boolean registeredTypeConverted = convertRegisteredTypes(builder,o);
        if(!registeredTypeConverted){
            if(o instanceof Collection){
                convertCollection(builder,(Collection) o);
            }
            else {
                convertObject(builder,o);
            }
        }
        return builder;
    }

    protected  StringBuilder converToJson(StringBuilder builder,Object object) throws IllegalAccessException {
        Field[] fields = object.getClass().getDeclaredFields();

        for(int i = 0;i<fields.length;i++){
            Field f = fields[i];
            builder.append("\""+f.getName()+"\"").append(":");
            f.setAccessible(true);
            Object o = f.get(object);

            convert(builder,o);

            if(i!=fields.length-1){
                builder.append(",");
            }
        }

        return builder;
    }
}