如何使用传入的通用对象列表并使用我的 Java 方法返回通用 object？

Question

I am trying to make my application more efficient by using Java generics, but I don't have a lot of generics experience. I am trying to do this:

My Two Model Objects:

public class Person {
    private String firstName;
    private String lastName;
    private int age;
    // ...

    // Getters and Setters Here
}

public class Car {
    private String make;
    private String model;
    private int year;
    // ...

    // Getters and Setters Here
}

My Result Objects:

private class PersonResult {
    private boolean success;
    private String notes;
    private int score;

    // Getters and Setters Here
}

private class CarResult {
    private boolean success;
    private String notes;
    private int score;
    private double price;

    // Getters and Setters Here
}

My Main Class:

public class MyClass{

    public mainMethod(){

        List<Person> listOfPersons = new ArrayList<>();
        List<Car> listOfCars = new ArrayList<>();

        listOfPersons.add(// a Person object);
        // ...

        listOfCars.add(// a Car object);
        // ...

        PersonResult = resultMethod(listOfPersons);
        CarResult = resultMethod(listOfCars);
    }

    private <E,T> E resultMethod(List<T> listOfData)
    {
        Engine engine = EngineFactory.getEngine();

        // if(listOfData.contains(Person.class))
        if (listOfData instanceof List<Person>)
        {
            PersonResult personResult = new PersonResult();
            perosnResult = engine.getPersonResult(listOfData);
            return personResult;
        }
        // else if(listOfData.contains(Car.class))
        else if (listOfData instanceof List<Car>)
        {
            CarResult carResult = new CarResult();
            carResult engine.getCarResult(listOfData);
            return carResult;
        }

        return null;
    }
}

However my IDE is complaining about:

1 Illegal generic type for instanceof

2 (for the return value) Incompatible types: Required: E - Found PersonResult

As you can see, I also tried listOfData.contains(Person.class) , that took care of error #1, but not the return error.

Answer 1

You cannot test for generic parameters by instanceof . instanceof will be evaluated at runtime. But the compiler removes the generic types. They are not present at runtime. For example List<Integer> will be the same as List at runtime.

You can use a simple implementation without generics here:

private PersonResult resultMethodPerson(List<Person> listOfData) {
  return EngineFactory.getEngine().getPersonResult(listOfData);
}

private CarResult resultMethodCar(List<Car> listOfData) {
  return EngineFactory.getEngine().getCarResult(listOfData);
}