GSON lib: cant parse JSON element's array to String[] of Java object

I import the latest version of GSON lib.
The console of the below java program show "null", but it's okay when i tried printing other attributes of the object. This means that the GSON function can parse the JSON element to Java object but it does not work with the muscle_groups attribute so the String[] return null.
i've tried changing the java datatype, also asked the chat GPT but still not working
I dont know how to fix this...

java object class

import java.util.ArrayList;
import java.util.List;
public class WorkoutExercises {
String name, description, time, level, goal;
int id;
List<String> muscleGroups;
public WorkoutExercises() {
// TODO Auto-generated constructor stub
}
public WorkoutExercises(int id, String name, String description,  List<String> muscleGroups, String tỉme, String level, String goal) {
this.id=id;
this.muscleGroups = muscleGroups;
this.description = description;
this.goal = goal;
this.level = level;
this.name =  name;
this.time = time;
}
public List<String> getMuscleGroups() {
return muscleGroups;
}
public void setMuscleGroups(List<String> muscleGroups) {
this.muscleGroups = muscleGroups;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public String getTime() {
return time;
}
public void setTime(String time) {
this.time = time;
}
public String getLevel() {
return level;
}
public void setLevel(String level) {
this.level = level;
}
public String getGoal() {
return goal;
}
public void setGoal(String goal) {
this.goal = goal;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}	

java program:

import java.io.IOException;
import java.lang.reflect.Type;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Arrays;
import javax.lang.model.util.Elements;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
public class objectToJSON {
public objectToJSON() {
// TODO Auto-generated constructor stub
}
public static void main(String[] args) {
String json = null;
try {
json = new String(Files.readAllBytes(Paths.get("C:\\Users\\data.json")));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Gson gson = new Gson();
Type listType = new TypeToken<ArrayList<WorkoutExercises>>(){}.getType();
ArrayList<WorkoutExercises> wk = gson.fromJson(json, listType);
System.out.println(wk.get(0).muscleGroups);	
}
}

json file with muscle_groups array:

[
{
"id": 1,
"name": "Push-Ups",
"description": "Start in a plank position with your hands slightly wider than shoulder-width apart. Lower your body by bending your elbows until your chest nearly touches the floor. Push yourself back up to the starting position and repeat.",
"muscle_groups": ["Chest", "Shoulders", "Triceps"],
"time": "3 sets of 10 reps",
"level": "Beginner",
"goal": "Strength"
},
{
"id": 2,
"name": "Squats",
"description": "Stand with your feet shoulder-width apart. Lower your body by bending your knees and pushing your hips back. Keep your chest up and your knees in line with your toes. Push through your heels to return to the starting position and repeat.",
"muscle_groups": ["Quadriceps", "Hamstrings", "Glutes"],
"time": "3 sets of 12 reps",
"level": "Beginner",
"goal": "Strength"
},
{
"id": 3,
"name": "Plank",
"description": "Start in a push-up position with your hands directly under your shoulders. Engage your core and hold your body in a straight line from head to heels. Hold the position for the desired duration.",
"muscle_groups": ["Core"],
"time": "30 seconds",
"level": "Beginner",
"goal": "Core Stability"
},
{
"id": 4,
"name": "Lunges",
"description": "Stand with your feet hip-width apart. Take a step forward with your right foot and lower your body until your right knee is bent at a 90-degree angle. Push through your right heel to return to the starting position and repeat on the other side.",
"muscle_groups": ["Quadriceps", "Hamstrings", "Glutes"],
"time": "3 sets of 10 reps per leg",
"level": "Beginner",
"goal": "Lower Body Strength"
}
]

Your JSON file is using muscle_groups as the field name, so your class must be mapped accordingly.<br/>
To do so you can rename your java field object as muscle_groups, or change in the JSON file with muscleGroups.<br/><br/>
You can also force the mapping, like this:

@SerializedName(value = "muscle_groups")
List<String> muscleGroups;

without changing anything else.<br/><br/>
Another way to achieve your goal is using the correct FieldNamingPolicy.