Recap - Generic Container

Part of the Object Oriented Programming section of Coddy's Java journey — lesson 58 of 87.

Challenge

Easy

Let's build a versatile storage system that brings together everything you've learned about generics! You'll create a Container class that can store, retrieve, and process elements of any type, along with utility methods that demonstrate wildcards in action.

You'll organize your code across three files:

Container.java: Create a generic class Container<T> that manages a collection of items. Your container should use an ArrayList<T> internally to store elements. Include methods to add(T item) an element, get(int index) to retrieve an element at a specific position, size() to return the count of items, and isEmpty() to check if the container has no elements. Also add a getAll() method that returns the internal list.
ContainerUtils.java: Create a utility class with static generic methods that work with your containers:
printAll(Container<?> container) - Uses an unbounded wildcard to print all elements in any container, one per line.

countItems(Container<? extends Number> container) - Uses an upper bounded wildcard to calculate the sum of all numeric values in the container, returning the result as a double.

addDefaults(Container<? super Integer> container) - Uses a lower bounded wildcard to add the integers 1, 2, and 3 to any container that can hold integers.

<T> T getLastOrDefault(Container<T> container, T defaultValue) - A generic method that returns the last element in the container, or the default value if the container is empty.
Main.java: Bring your generic container system together! You'll receive three inputs: a word (String), an integer, and a double.
First, create a Container<String> and add the word to it twice. Print String container: then use printAll to display its contents.

Next, create a Container<Double> and add the double value to it. Print a blank line, then Number sum: [sum] using countItems.

Then create a Container<Number> and call addDefaults on it. Print a blank line, then After adding defaults: and use printAll to show the contents.

Finally, create an empty Container<Integer> and use getLastOrDefault with your integer input as the default. Print a blank line, then Last or default: [result].

You will receive three inputs in order: a word (String), an integer, and a double.

This challenge combines generic classes, generic methods, bounded type parameters, and all three wildcard types. Notice how each piece serves a different purpose: the generic class provides type-safe storage, bounded wildcards enable flexible reading and writing, and generic methods offer reusable operations across different container types!

Try it yourself

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // Read inputs
        String word = scanner.nextLine();
        int intValue = Integer.parseInt(scanner.nextLine());
        double doubleValue = Double.parseDouble(scanner.nextLine());
        
        // TODO: Create a Container<String> and add the word twice
        // Print "String container:" then use ContainerUtils.printAll()
        
        // TODO: Create a Container<Double> and add the double value
        // Print a blank line, then "Number sum: [sum]" using ContainerUtils.countItems()
        
        // TODO: Create a Container<Number> and call ContainerUtils.addDefaults() on it
        // Print a blank line, then "After adding defaults:" and use ContainerUtils.printAll()
        
        // TODO: Create an empty Container<Integer>
        // Use ContainerUtils.getLastOrDefault() with intValue as default
        // Print a blank line, then "Last or default: [result]"
    }
}