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The question mark (?), represents the wildcard, stands for unknown type in generics. There may be times when any object can be used when a method can be implemented using functionality provided in the Object class or When the code is independent of the type parameter.
To declare a Unbounded Wildcard parameter, list the ? only.
Example
Following example illustrates how extends is used to specify an unbounded wildcard.
package com.tutorialspoint;
import java.util.Arrays;
import java.util.List;
public class GenericsTester {
public static void printAll(List<?> list) {
for (Object item : list)
System.out.println(item + " ");
}
public static void main(String args[]) {
List<Integer> integerList = Arrays.asList(1, 2, 3);
printAll(integerList);
List<Double> doubleList = Arrays.asList(1.2, 2.3, 3.5);
printAll(doubleList);
}
}
This will produce the following result −
Output
1 2 3 1.2 2.3 3.5
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