Java 8 Generics - Generate a list (jdk 8) that contains using a method

Question

I will try to simplify my compilation problem.
I have a Car Object and two successors called Honda & Toyota.
I have another Object called CarContainer which is defined as follows:

public class CarContainer<T> {} // doesn't do anyhthing

I want to create a list of car containers of many types of cars objects.
In this example it will hold an array of that type of cars.

List<CarContainer<? extends Car[]>> obs = new ArrayList<>();

This works fine -

CarContainer<Honda[]> hondaCarContainer = new CarContainer<>();
CarContainer<Toyota[]> toyotaCarContainer = new CarContainer<>();
Collections.addAll(obs, hondaCarContainer, toyotaCarContainer);

But when I try to generate the container using a method

private static <T> CarContainer<T[]> getCarContainer(T car) {
    return null;
}

and calling

Collections.addAll( obs, 
                    getCarContainer(new Honda()), 
                    getCarContainer(new Toyota()));

My code doesn't compile.

(though this does

Collections.addAll(obs, getCarContainer(new Honda()));

and unbelivably this also

    CarContainer<Honda[]> carContainer = getCarContainer(new Honda());
    Collections.addAll(obs, carContainer, getCarContainer(new Toyota()));

which is the same. )

How is it solvable?

This kind of problem is discussed [here](http://stackoverflow.com/q/22509488/2711488) — Holger, Feb 02 '15 at 13:51

score 1 · Answer 1 · answered Feb 02 '15 at 11:49

I can compile and run your code without any problem, (win7 jdk7):

public class MyTest {

public static void main(String[] args) {
    new MyTest();
}

public MyTest() {
    List<CarContainer<? extends Car[]>> obs = new ArrayList<>();

    Collections.addAll( obs,
            getCarContainer(new Honda()),
            getCarContainer(new Toyota()));

    System.out.println(obs.size());
}

private <T> CarContainer<T[]> getCarContainer(T car) {
    return null;
}

class Car{}

class Honda extends Car {}

class Toyota extends Car {}

class CarContainer<T> {}
}

It outputs "2" in console.

You are right - pointing it to jdk 7 instead of 8 works. Updating the question. — Bick, Feb 02 '15 at 12:08

score 1 · Answer 2 · answered Feb 02 '15 at 12:25

This will compile.

public class MyTest {

public static void main(String[] args) {
    new MyTest();
}

public MyTest() {
    List<CarContainer<? extends Car[]>> obs = new ArrayList<>();
    CarContainer<Honda[]> c = getCarContainer(new Honda());
    CarContainer<Toyota[]> c2 = getCarContainer(new Toyota());

    Collections.addAll(obs,
            c, c2);

    System.out.println(obs.size());
}

private <T> CarContainer<T[]> getCarContainer(T car) {
    return null;
}

class Car{}

class Honda extends Car {}

class Toyota extends Car {}

class CarContainer<T> {}

}

It seems that JDK developers changed type inference rules to support lambdas.

http://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html

score 0 · Answer 3 · edited May 23 '17 at 11:49

0

cm123h's answer compiles and runs fine for me in eclipse Luna using jdk8_20.

I might change your getCarContainer method to

private <T extends Car> CarContainer<T[]> getCarContainer(Class<T> car) {
    return null;
}

as there's no reason to instantiate an object just to get its type at compile-time, but that's being nitpicky.

edited May 23 '17 at 11:49

Community

1
1

answered Feb 03 '15 at 01:35

Steve K

4,863
2
32
41

Java 8 Generics - Generate a list (jdk 8) that contains using a method

3 Answers3