Question

Use a HashMap to create a reusable class for choosing one of the 13 predefined colors in class Color. The names of the colors should be used as keys, and the predefined Color objects should be used as values. Place this class in a package that can be imported into any Java program. Use your new class in an application that allows the user to select a color and draw a shape in that color.

Short answer

Create a `ColorPicker` class using a HashMap, map color names to Color objects, package it, and use it in a GUI application to draw shapes in user-selected colors.

Step-by-step solution

Understand the Goal

The goal is to create a reusable Java class using a HashMap to map 13 predefined color names to Java's Color objects. This class should be packaged for easy import into any Java application, which can then choose a color to draw shapes.

Import Necessary Packages

In Java, first import the necessary packages, such as `java.awt.Color` for color objects, and `java.util.HashMap` for storing the color mapping.

Initialize the Package and Class

Create a package named, for example, `colors` and within it, create a class named `ColorPicker`. Use the `package` keyword and begin defining the class using `public class ColorPicker`.

Define the HashMap

Inside the `ColorPicker` class, define a private static HashMap that maps strings (color names) to `Color` objects. This map will store the predefined colors.

Populate the HashMap

Initialize the HashMap in a static block or constructor of `ColorPicker`, adding entries for each of the 13 predefined colors. For example: `colorsMap.put("red", Color.RED);` and so on for each color.

Create a Method to Retrieve Colors

Add a public method to the `ColorPicker` class that takes a string as a parameter and returns the `Color` object from the HashMap. Implement error handling to manage invalid keys.

Use the ColorPicker in an Application

In another Java application, import the `colors.ColorPicker` class. Use a GUI framework like Swing to create an interface where a user can select a predefined color by name and draw a shape in that color.

Implement User Interaction

Add event handlers that trigger color retrieval from the `ColorPicker` and redraw the shape with the selected color on the GUI. This helps confirm that the class works as expected.

Key concepts

Java Color class

In Java, the `Color` class is a part of the `java.awt` package and provides predefined constants for a variety of colors. These constants represent the basic colors that programmers commonly use when designing graphical user interfaces. Each color is defined by its RGB (red, green, blue) components, which is a common way to specify colors in digital images and GUIs. By understanding this class, you can easily manipulate color attributes within your applications. For example:

• Color.RED: The color red with RGB components (255, 0, 0).
• Color.BLUE: A blue color with RGB components (0, 0, 255).
• Color.BLACK: Standard black with RGB (0, 0, 0).

These predefined values simplify the task of applying colors in Java applications and ensure consistency across different parts of your GUI. Moreover, you can also create custom colors using the Color constructor, should the need arise.

reusable Java classes

Reusable Java classes allow you to create components that can be used across multiple programs without rewriting the code. This approach promotes efficiency and maintainability. By encapsulating logic within a class, you make it easier to manage changes and updates. If all programs pull from the same class, you only need to update it once, and every dependent application benefits from the new functionality or bug fixes. Let's consider the `ColorPicker` class example:

• It contains a HashMap to store color associations, making it a core component for color selection tasks.
• The logic for retrieving colors is contained within one function, promoting code reuse in any Java application that requires color selection capabilities.

This principle of code reusability extends to other classes as well, where you abstract common patterns into libraries that can be shared across projects.

Java packages

Java packages are used to organize classes into namespaces, similar to how directories are used to group files on your computer. This organization is essential in large projects to

• Prevent naming conflicts - two classes with the same name can co-exist if they're in different packages.
• Provide controlled access - define which packages can access specific classes or data.
• Encourage modular development, making it easier to distribute libraries or frameworks.

Packages in Java are declared at the top of a Java file with the `package` keyword. For instance, `package colors;` might be used for our `ColorPicker` class, grouping color-related classes distinctly. With proper package management, your application's architecture becomes clearer and more maintainable.

GUI application programming

Graphical User Interface (GUI) programming allows developers to create applications with interactive visual elements. In Java, the Swing and JavaFX libraries are commonly used for building GUIs. These libraries allow you to create windows, buttons, text inputs, and other elements that users can interact with. In our exercise, implementing a GUI application allows users to select a color and draw objects, which not only enhances user experience but also presents the functionality of the `ColorPicker` class. Here are some key components in GUI programming:

• **Event Handling**: Captures user actions like clicks or key presses and executes corresponding commands. For example, selecting a color and changing a drawing's color.
• **Layout Management**: Organizes the placement of UI elements, ensuring they appear consistently across different screen sizes.
• **Component Libraries**: Pre-made GUI elements such as buttons, sliders, and panels to simplify development.

By mastering GUI programming in Java, you can build robust, user-friendly applications that effectively leverage components like the `ColorPicker` class for dynamic visual interaction.