Understanding upper and lower bounds on ? in Java Generics

Question

I am really having a tough time understanding the wild card parameter. I have a few questions regarding that.

1. ? as a type parameter can only

Answer 1

For the first question: you can not define a method with ? as a type parameter either. The following will not compile:

void  foo() {}

? is used for binding to another generics without providing the type parameter. You can write for methods:

void foo(List e) {}

And you can also write for classes:

public class Bar> { }

For the use of super:

public void printAll(MyList){
    // code code code
}

This will not as you say print the list "if it has objects of MyClass". It can have objects of any class that is a subclass of a class that is a parent of MyClass. The compiler does not know at compile time what are the objects that will be in the list anyway.

To get your head around it, consider a simple example with the Number class hierarchy. Float and Integer are children of Number. You can write your method like this:

public void printAll(List){
    // code code code
}

Then you can call that method with a List:

List numbers = new ArrayList<>();
numbers.add(1); // actually only add an Integer
printAll(numbers); // compiles.

This is possible would not be super useful in that case. Where it would be useful for example is when you want to add Float to a collection without wanting it to be only a List, like:

public void addFloat(List list){
    list.add(2.5);
}