Java – Gson.toJson( )”Exception java.lang.IllegalArgumentException: class java.text.DecimalFormat declares multiple JSON fields named maximumIntegerDigits”

I’m trying to make a JSON file to use as a database. My class is:
1. Sheep: Include some information about each sheep
2. Farm: Includes a series of sheep
3.DB: Here I’m trying to parse the Farm object to JSON

I’ve tried adding “transient” to all fields of date type without success.

To give you a better idea of what I’m trying to do: this is a farm manager where users can store data about his sheep

public class Sheep implements Serializable {

@SerializedName("localId")
    @Expose
    private int localId;
    @SerializedName("globalId")
    @Expose
    private int globalId;
    @SerializedName("birthDate")
    @Expose
    private transient Date birthDate = null;
    @SerializedName("sellingDate")
    @Expose
    private transient Date sellingDate = null;
    @SerializedName("deathDate")
    @Expose
    private transient Date deathDate = null;
    @SerializedName("lastpregnancyDate")
    @Expose
    private transient Date lastpregnancyDate = null;
    @SerializedName("lastDoctorCheck")
    @Expose
    private transient Date lastDoctorCheck = null;
    @SerializedName("alive")
    @Expose
    private boolean alive;
    @SerializedName("sold")
    @Expose
    private boolean sold;
    @SerializedName("pregnant")
    @Expose
    private boolean pregnant;
    @SerializedName("mother")
    @Expose
    private Sheep mother = null;
    @SerializedName("father")
    @Expose
    private Sheep father = null;
    @SerializedName("allDoctorChecks")
    @Expose
    private transient List<Date> allDoctorChecks = null;
    @SerializedName("allpregnancies")
    @Expose
    private transient List<Date> allpregnancies = null;
    @SerializedName("children")
    @Expose
    private List<Sheep> children = null;
    @SerializedName("allHusbands")
    @Expose
    private List<Sheep> allHusbands = null;
    private SimpleDateFormat sdf = new SimpleDateFormat("dd.mm.yyyy");
}

public class Farm implements Serializable {

@SerializedName("allSheeps")
    @Expose
    private List<Sheep> allSheeps = null;
}

public class DataBase {
    private static Farm farm;

public static void main(String[] args){
        ...

Gson g = new GsonBuilder().setPrettyPrinting().create();
**/*40*/  String strGsn = g.toJson(farm);/*this is line 40*/** 

FileWriter writer = null;
        try{
            writer = new FileWriter("farm.json");
            writer.write(strGsn);
        }catch (Exception e){
            System.out.println(e.fillInStackTrace());
        }finally {
            if(writer != null)
                try{
                    writer.close();
                }catch (Exception e){
                    System.out.println(e.fillInStackTrace());
                }
        }

}

}

"C:\Program Files\Java\jdk-12.0.1\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.1.2\lib\idea_rt.jar=53834:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.1.2\bin" -Dfile.encoding=UTF-8 -classpath C:\Users\saleh\Desktop\farm\out\production\farm; C:\Users\saleh\.m2\repository\com\google\code\gson\gson\2.8.5\gson-2.8.5.jar com.company.DB.DataBase
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.google.gson.internal.reflect.UnsafeReflectionAccessor (file:/C:/Users/saleh/.m2/repository/com/google/code/gson/gson/2.8.5/ gson-2.8.5.jar) to field java.text.SimpleDateFormat.serialVersionOnStream
WARNING: Please consider reporting this to the maintainers of com.google.gson.internal.reflect.UnsafeReflectionAccessor
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Exception in thread "main" java.lang.IllegalArgumentException: class java.text.DecimalFormat declares multiple JSON fields named maximumIntegerDigits
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:172)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
    at com.google.gson.Gson.getAdapter(Gson.java:458)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:56)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:127)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:245)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:127)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:245)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:97)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:61)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:127)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:245)
    at com.google.gson.Gson.toJson(Gson.java:704)
    at com.google.gson.Gson.toJson(Gson.java:683)
    at com.google.gson.Gson.toJson(Gson.java:638)
    at com.google.gson.Gson.toJson(Gson.java:618)
    at com.company.DB.DataBase.main(DataBase.java:40)

Exceptions

java.lang.IllegalArgumentException: class java.text.DecimalFormat declares multiple JSON fields named maximumIntegerDigits

This is because Sheep's field sdf is being serialized. There is a question as to why SimpleDateFormat cannot be serialized to json, but to solve this problem, since it is @Expose annotations to control which field is serialized, change the line
Gson g = new GsonBuilder().setPrettyPrinting().create();
to
Gson g = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().setPrettyPrinting().create();
To avoid SDF being serialized.