Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://www.geeksforgeeks.org/java/wildcards-in-java/ below:

Wildcards in Java - GeeksforGeeks

Last Updated : 06 Aug, 2025

In Java Generics, wildcards are used when you don’t know the exact type. They let you write flexible and reusable code. The wildcard is represented by a ? (question mark). Wildcards are mainly used in method parameters to accept different generic types safely.

These wildcards can be used when you want to relax the restrictions on a variable. For example, say you want to write a method that works on List < Integer >, List < Double > and List < Number >, you can do this using an upper bounded wildcard. 

To declare an upper-bounded wildcard, use the wildcard character ('?'), followed by the extends keyword, followed by its upper bound. 

public static void add(List<? extends Number> list)

Implementation:

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

class WildcardDemo {
    public static void main(String[] args)
    {
        // Upper Bounded Integer List
        List<Integer> list1 = Arrays.asList(4, 5, 6, 7);

        System.out.println("Total sum is:" + sum(list1));

        // Double list
        List<Double> list2 = Arrays.asList(4.1, 5.1, 6.1);

        System.out.print("Total sum is:" + sum(list2));
    }

    private static double sum(List<? extends Number> list)
    {
        double sum = 0.0;
        for (Number i : list) {
            sum += i.doubleValue();
        }

        return sum;
    }
}

Total sum is:22.0
Total sum is:15.299999999999999

Explanation: In the above program, list1 holds Integer values and list2 holds Double values. Both are passed to the sum method, which uses a wildcard <? extends Number>. This means it can accept any list of a type that is a subclass of Number, like Integer or Double.

It is expressed using the wildcard character ('?'), followed by the super keyword, followed by its lower bound: <? super A>. 

Syntax: Collectiontype <? super A>

Implementation: 

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

class WildcardDemo {
    public static void main(String[] args)
    {
        // Lower Bounded Integer List
        List<Integer> list1 = Arrays.asList(4, 5, 6, 7);

        // Integer list object is being passed
        printOnlyIntegerClassorSuperClass(list1);

        // Number list
        List<Number> list2 = Arrays.asList(4, 5, 6, 7);

        // Integer list object is being passed
        printOnlyIntegerClassorSuperClass(list2);
    }

    public static void printOnlyIntegerClassorSuperClass(
        List<? super Integer> list)
    {
        System.out.println(list);
    }
}

[4, 5, 6, 7]
[4, 5, 6, 7]

Explanation: Here, the method printOnlyIntegerClassorSuperClass accepts only Integer or its superclasses (like Number). If you try to pass a list of Double, it gives a compile-time error because Double is not a superclass of Integer.

Note: Use extend wildcard when you want to get values out of a structure and super wildcard when you put values in a structure. Don’t use wildcard when you get and put values in a structure. You can specify an upper bound for a wildcard or you can specify a lower bound, but you cannot specify both.

This wildcard type is specified using the wildcard character (?), for example, List. This is called a list of unknown types. These are useful in the following cases -

• When writing a method that can be employed using functionality provided in Object class.
• When the code is using methods in the generic class that doesn't depend on the type parameter

Implementation:

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

class unboundedwildcardemo {
    public static void main(String[] args)
    {

        // Integer List
        List<Integer> list1 = Arrays.asList(1, 2, 3);

        // Double list
        List<Double> list2 = Arrays.asList(1.1, 2.2, 3.3);

        printlist(list1);

        printlist(list2);
    }

    private static void printlist(List<?> list)
    {

        System.out.println(list);
    }
}

[1, 2, 3]
[1.1, 2.2, 3.3]

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4