Gson NumberFormatException for empty string

GsonEmptyStringDemo.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonSyntaxException;
import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import org.apache.commons.lang.math.NumberUtils;

import java.io.IOException;

public class GsonEmptyStringDemo {

    /*
    *
    *  Gson throws NumberFormatException when the json string contains an empty string value and the key belongs to a numeric field.
    *
    *  "" -> int                                                        : NumberFormatException
    *  "" -> Integer                                                    : NumberFormatException
    *  "" -> int or Integer using TypeAdapter handles empty string      : Works
    *  "" -> String                                                     : Works
    *
    * */
    public static void main(String[] args) {

        /*Test test = new Test();
        test.setA(1);
        test.setB(2);
        Gson gson = new Gson();
        String json = gson.toJson(test);
        System.out.println(json);*/

        deserializeEmptyStringToPrimitive();
        deserializeEmptyStringToInteger();
        deserializeEmptyStringToString();
        deserializeEmptyStringWithTypeAdapter();

    }

    private static void deserializeEmptyStringToPrimitive() {
        System.out.println("Try to map empty String to int");
        try {
            Gson gson =
                    new GsonBuilder()
                            .create();

            Test1 t = gson.fromJson("{\"a\":\"\",\"b\":2.0}", Test1.class);
            System.out.println(t);
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("----------------------------------------");
    }

    private static void deserializeEmptyStringToInteger() {
        System.out.println("Try to map empty String to Integer");
        try {
            Gson gson =
                    new GsonBuilder()
                            .create();

            Test2 t = gson.fromJson("{\"a\":\"\",\"b\":2.0}", Test2.class);
            System.out.println(t);
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("----------------------------------------");
    }

    private static void deserializeEmptyStringToString() {
        System.out.println("Using String type to handle Empty String");
        try {
            Gson gson =
                    new GsonBuilder()
                            .create();

            Test t = gson.fromJson("{\"a\":\"\",\"b\":2.0}", Test.class);
            System.out.println(t);
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("----------------------------------------");
    }

    private static void deserializeEmptyStringWithTypeAdapter() {
        System.out.println("Using TypeAdapter to handle Empty String");
        try {
            Gson gson =
                    new GsonBuilder()
                            .registerTypeAdapter(int.class, new EmptyStringToNumberTypeAdapter())
                            .registerTypeAdapter(Integer.class, new EmptyStringToNumberTypeAdapter())
                            .registerTypeAdapter(double.class, new EmptyStringToNumberTypeAdapter())
                            .registerTypeAdapter(Double.class, new EmptyStringToNumberTypeAdapter())
                            .create();
            Test1 t = gson.fromJson("{\"a\":\"\",\"b\":\"\"}", Test1.class);
            System.out.println(t);
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("----------------------------------------");
    }


    public static class Test {

        //String solves the problem
        private String a;
        private double b;

        public int getA() {
            if(a == null || a.equals("")){
                return 0;
            }

            return Integer.valueOf(a);
        }

        public void setA(String a) {
            this.a = a;
        }

        public double getB() {
            return b;
        }

        public void setB(double b) {
            this.b = b;
        }

        @Override
        public String toString() {
            return "Test{" +
                    "a=" + a +
                    ", b=" + b +
                    '}';
        }
    }

    public static class Test2 {

        private Integer a;
        private double b;

        //Integer: empty string still causes NumberFormatException
        public Integer getA() {
            return a;
        }

        public void setA(Integer a) {
            this.a = a;
        }

        public double getB() {
            return b;
        }

        public void setB(double b) {
            this.b = b;
        }

        @Override
        public String toString() {
            return "Test{" +
                    "a=" + a +
                    ", b=" + b +
                    '}';
        }
    }

    public static class Test1 {

        private int a;
        private double b;

        //int: empty string causes NumberFormatException
        public int getA() {
            return a;
        }

        public void setA(int a) {
            this.a = a;
        }

        public double getB() {
            return b;
        }

        public void setB(double b) {
            this.b = b;
        }

        @Override
        public String toString() {
            return "Test{" +
                    "a=" + a +
                    ", b=" + b +
                    '}';
        }
    }

    public static class EmptyStringToNumberTypeAdapter extends TypeAdapter<Number> {
        @Override
        public void write(JsonWriter jsonWriter, Number number) throws IOException {
            if (number == null) {
                jsonWriter.nullValue();
                return;
            }
            jsonWriter.value(number);
        }

        @Override
        public Number read(JsonReader jsonReader) throws IOException {
            if (jsonReader.peek() == JsonToken.NULL) {
                jsonReader.nextNull();
                return null;
            }

            try {
                String value = jsonReader.nextString();
                if ("".equals(value)) {
                    return 0;
                }
                return NumberUtils.createNumber(value);
            } catch (NumberFormatException e) {
                throw new JsonSyntaxException(e);
            }
        }
    }
}