Java泛型列表

Question

Following is an example of Generics with wild card

public static void printListItems(List<object> list) {
        for (Object listItem : list)
            System.out.println(listItem);
    }

In this example we want to print list items of any type but it can't print List<Integer> , List<String> etc. because they are not subtypes of List<Object> . This problem can be solved using unbounded wildcard.

public static void printListItems(List<?> list) {
    for (Object listItem : list)
        System.out.println(listItem);
}

I read this above code in Java tutorial. For the first example , it says it cannot work because List<String> is not sublass of List<Object> .

Then why it is so that in the second example the for loop is working with taking listItem as dataType of Object and iterating through List<String> elements.

Answer 1

You are running into a false contradiction because you are equating the relationship between List<String> and List<Object> to the relationship between String and Object .

In the first example, the compiler will yell at you if you attempt to call printListItem s with a List<String> , because List<String> does not extend List<Object> .

In other words: A list of strings cannot be treated as a list of objects, because (according to the type system) a list of strings is not a list of objects.

In the second example, the compiler will not yell at you, because you are treating the elements of the List , which are String s, as Object s. Unlike the case with the lists, String does extend Object .

In other words: A string can be treated as an object, because (according to the type system) a string is an object.

Answer 2

when the type-declaration of generic class contain wildcard character , its subclass type can be extend in two dimension.

---

Eg Collection<?extends Number>

• its subclass type can be extend in dimension Collection , for List and Set are all subclass type of Collection so List<? extends Number> and Set<? extends Number> List<? extends Number> and Set<? extends Number> List<? extends Number> and Set<? extends Number> are all subclass type of Collection<? extends Number> Collection<? extends Number>
• its subclass type can be extend in dimension Number , because Integer 、Double are all subclass type of Number so Collection<Double> and Collection<Integer> are all subclass of Collection<? extends Number>

---

about your second, as Andreas explained in comment , List is equal to List<? extend Object> and based on the second dimension extend List<String> is **subclass** of List<?>

Answer 3

What you've done is not generic. This is a generic function

public static <T> void printListItems(List<T> list) {
    for (T listItem : list) {
        System.out.println(listItem);
    }
}

Example:

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

public class Main {
    public static <T> void printListItems(List<T> list) {
        for (T listItem : list) {
            System.out.println(listItem);
        }
    }

    // arguments are passed using the text field below this editor
    public static void main(String[] args) {
        List<Integer> l = new ArrayList<>(Arrays.asList(1,2,3,4,5));
        List<String> s = new ArrayList<>(Arrays.asList("Hello", "World"));
        printListItems(l);
        printListItems(s);
    }
}

Output

1
2
3
4
5
Hello
World