Raw type and Wildcard

Raw type

Raw type means the type is unbounded/unspecified.
The existence of it is for backward compatibility.
But still, it is not recommend to use this raw type.

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List rawList = new ArrayList(2);
        rawList.add(1);
        //warning: [unchecked] unchecked call to add(E) as a member of the raw type List
        rawList.add("Rugal");
        //warning: [unchecked] unchecked call to add(E) as a member of the raw type List
        for (Object elem: rawList)
            System.out.println(elem);
    }
}

You will get warning talking about unchecked type.
Raw type doesn’t provide type check, so you can put different type into a single container without getting compilation error.
But this will cause trouble if you try to cast class that is not compatible.

Again, Raw type is not recommended.

Wildcard

Unbounded wildcard

You can’t assign <A> to <B> even if the A is inherit from B. This is because of type check in compile time.
In order to assign relevant types to their base type, wildcard comes into being.

Assignable by any type

Any container can assign to <?> .

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(2);
        list.add("Bernstein");
        list.add("Rugal");
        List<?> l = list;
        for (Object elem: l)
	        //But ? is only compatible with Object
	        //Refer to following section
            System.out.println(elem);
    }
}

You can even assign raw type container to it.

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List rawList = new ArrayList(2);
        rawList.add(1);
        rawList.add("Rugal");
        List<?> list = rawList;
        for (Object elem: list)
            System.out.println(elem);
    }
}

Type related operations are not compatible

Any operations that type related, including add, class cast etc., are not allowed in unbounded wildcard.

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
       List<?> list = new ArrayList<String>();
       list.add(1); 
       //Main.java:9: error: no suitable method found for add(int)
       for (Object elem: list)
            System.out.print(elem + " ");
    }
}

Bounded wildcard

For this section, please refer to my POST