Wildcard
- Java course
- Intermediate
- Lesson 85/116
- 5 min
Learn wildcard types ? extends T and ? super T.
Wildcard types
For generic types you use the wildcard ?. There are three forms: <?> an unknown type, <? extends T> T and its subtypes (upper bound), <? super T> T and its supertypes (lower bound).
For example static double sum(List<? extends Number> nums) accepts List<Integer>, List<Double> and other lists of Number subtypes. Wildcards are the key to flexible generic collections; they provide covariance and contravariance.
The PECS principle
The PECS principle helps choose wildcards: Producer Extends, Consumer Super. If a list is a producer (you read from it) use ? extends T; if it is a consumer (you write to it) use ? super T.
For example sum(List<? extends Number>) reads numbers, so it is a producer. addNumbers(List<? super Integer>) adds Integers, so it is a consumer. When you need both directions you write a generic method <T> instead.
Practice Write the sum wildcard method
Define static double sum(List<? extends Number> nums). In the body loop with for (Number n : nums) total += n.doubleValue(); and return total;. In main, create a list with List<Integer> ints = Arrays.asList(1, 2, 3); and print System.out.println(sum(ints));.
Starter code
import java.util.List;
import java.util.Arrays;
// Define the sum method: List<? extends Number>
// main: call sum and printlnSample solution
import java.util.List;
import java.util.Arrays;
public class Main {
static double sum(List<? extends Number> nums) {
double total = 0;
for (Number n : nums) {
total += n.doubleValue();
}
return total;
}
public static void main(String[] args) {
List<Integer> ints = Arrays.asList(1, 2, 3);
System.out.println(sum(ints));
}
}Expected output
6.0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does List<? extends Number> accept?
- Only List<Number>
- List<Integer> and List<Double> like Number subtypes
- Only List<Object> Correct answer
- Any List type
Explanation: ? extends Number accepts lists of Number and its subtypes.
2 Match the wildcard concepts
| Term | Meaning |
|---|---|
? extends T |
Producer (covariant) |
? super T |
Consumer (contravariant) |
? |
Unknown type |
extends |
Upper bound keyword |
3 Complete the wildcard parameter
static double sum(List____ nums) {
double total = 0;
return total;
}static double sum(List<? extends Number> nums) {
double total = 0;
return total;
}Explanation: <? extends Number> accepts lists of Number subtypes.