When to use generic methods and when to use wild-card?

Question

I am reading about generic methods from OracleDocGenericMethod. I am pretty confused about the comparison when it says when to use wild-card and when to use generic methods.

Answer 1

? means unknown

The general rule applies: You can read from it, but not write

given simple pojo Car

class Car {
    void display(){

    }
}

This will compile

private static  void addExtractedAgain1(List cars) {
    T t = cars.get(1);
    t.display();
    cars.add(t);
}

This method won't compile

private static void addExtractedAgain2(List cars) {
    Car car = cars.get(1);
    car.display();
    cars.add(car); // will not compile
}

Another example

List hi = Arrays.asList("Hi", new Exception(), 0);

hi.forEach(o -> {
   o.toString() // it's ok to call Object methods and methods that don't need the contained type
});

hi.add(...) // nothing can be add here won't compile, we need to tell compiler what the data type is but we do not know