我正在学习如何将数组转换为json文件，并使用gson导入该数组。

public abstract class Shape {
private Type typeOfShape;

public abstract double countArea();

public abstract double countCircuit();
}

和类Circle、Rectangle、Square，它们扩展了Shape，例如Rectangle

public class Rectangle extends Shape {
private double a;
private double b;

public Rectangle(Type type, int a, int b) {
    super(type);
    this.a = a;
    this.b = b;
}

@Override
public double countArea() {
    return a * b;
}

@Override
public double countCircuit() {
    return a * 2 + b * 2;
}

和2种方法：

public static List<Shape> importShapeListFromJsonWithGson(String path) throws IOException {
    Type listType = new TypeToken<List<Shape>>() {}.getType();
    Gson gson = new GsonBuilder().create();
    Reader reader = new FileReader(path);
    List<Shape> result = gson.fromJson(reader, listType);
    return result;
}

public static void exportShapeListToJsonWithGson(List<Shape> list, String path) throws IOException {
    Writer writer = new FileWriter(path);
    Gson gson = new GsonBuilder().create();
    gson.toJson(list, writer);
    writer.close();
}

但是我在线程“main”中遇到异常java.lang。调用没有参数的公共Shape（）失败。问题是，我使用的是抽象类吗？我尝试从Shape中删除abstract，然后再编译一次，但我只得到了包含形状的列表，没有例如“a”或“b”，只有类型。