Using Objects
Learn the relationship between a Class and an Object. A Class is a template for an Object. An Object is an instances of a Class. Learning can be done in different methods and different order. This article illustrates multiple methods and learnings obtained using different approaches.
Console Based Menu
Java imports allow user input and console output to be displayed in Terminal
Objects Used
- Makes Object from Scanner Class to obtain inputs / menu selection from User
- Use System Class, to call static methods System.out.print and System.out.println to output to console
- User Math Class, to call static method Math.random() to generate random number
Other College Board Topics
- A 2D array is used to store COLORS
- A Control Structure, Switch, is used to process Menu select to code that performs related action
Key PBL Topic
- Menu class when instantiated with new Menu() takes over Frontend experience with user. This is relevant to Draw Lab in this article and how Spring Boot works in Web programming. Objects are intended to encapsulate Frontend, Backend or experience ... Web Site control flow, Database CRUD operations, or a Menu in terminal (this example)
// imports allow you to use code already written by others. It is good to explore and learn libraries. The names around the dots often give you a hint to the originator of the code.
import java.util.Scanner; //library for user input
import java.lang.Math; //library for random numbers
public class Menu {
// Instance Variables
public final String DEFAULT = "\u001B[0m"; // Default Terminal Color
public final String[][] COLORS = { // 2D Array of ANSI Terminal Colors
{"Default",DEFAULT},
{"Red", "\u001B[31m"},
{"Green", "\u001B[32m"},
{"Yellow", "\u001B[33m"},
{"Blue", "\u001B[34m"},
{"Purple", "\u001B[35m"},
{"Cyan", "\u001B[36m"},
{"White", "\u001B[37m"},
};
// 2D column location for data
public final int NAME = 0;
public final int ANSI = 1; // ANSI is the "standard" for terminal codes
// Constructor on this Object takes control of menu events and actions
public Menu() {
Scanner sc = new Scanner(System.in); // using Java Scanner Object
this.print(); // print Menu
boolean quit = false;
while (!quit) {
try { // scan for Input
int choice = sc.nextInt(); // using method from Java Scanner Object
System.out.print("" + choice + ": ");
quit = this.action(choice); // take action
} catch (Exception e) {
sc.nextLine(); // error: clear buffer
System.out.println(e + ": Not a number, try again.");
}
}
sc.close();
}
// Print the menu options to Terminal
private void print() {
//System.out.println commands below is used to present a Menu to the user.
System.out.println("-------------------------\n");
System.out.println("Choose from these choices");
System.out.println("-------------------------\n");
System.out.println("1 - Say Hello");
System.out.println("2 - Output colors");
System.out.println("3 - Loading in color");
System.out.println("0 - Quit");
System.out.println("-------------------------\n");
}
// Private method to perform action and return true if action is to quit/exit
private boolean action(int selection) {
boolean quit = false;
switch (selection) { // Switch or Switch/Case is Control Flow statement and is used to evaluate the user selection
case 0:
System.out.print("Goodbye, World!");
quit = true;
break;
case 1:
System.out.print("Hello, World!");
break;
case 2:
for(int i = 0; i < COLORS.length; i++) // loop through COLORS array
System.out.print(COLORS[i][ANSI] + COLORS[i][NAME]);
break;
case 3:
System.out.print("Loading...");
for (int i = 0; i < 20; i++) { // fixed length loading bar
int random = (int) (Math.random() * COLORS.length); // random logic
try {
Thread.sleep(100); // delay for loading
} catch (Exception e) {
System.out.println(e);
}
System.out.print(COLORS[random][ANSI] + "#");
}
break;
default:
//Prints error message from console
System.out.print("Unexpected choice, try again.");
}
System.out.println(DEFAULT); // make sure to reset color and provide new line
return quit;
}
// Static driver/tester method
static public void main(String[] args) {
new Menu(); // starting Menu object
}
}
Menu.main(null);
Desktop GUI Menu
Swing and AWT imports allow Java to provide a Graphical User Interface on the desktop.
Other College Board Topics
- A 1D array is used to store MENUS
- A Control Structure, if-else if-else, is used to process Menu selection to code that performs related action
Using Objects
- Javax Swing UI (JFrame)
- Timer with a TimerTask to allow action to repeatedly occur without halting thread.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Timer;
import java.util.TimerTask;
// Graphical-User-Interface for Desktop in Java using Java Swing.
public class MenuJFrame extends JFrame implements ActionListener {
private JFrame frame;
private JMenuBar menubar;
private JMenu menu;
private JLabel message = new JLabel("Click on Menu to select an action.");
public final String[] MENUS = { // 1D Array of Menu Choices
"Hello", "Colors", "Loading bar",
};
// Statics to assist with timer and messaging, single copy (no instance)
private static int delay = 20;
private static int step = 1;
private static String hashes = "";
// Constructor enables the Frame instance, the object "this.frame"
public MenuJFrame(String title) {
// Initializing Key Objects
frame = new JFrame(title);
menubar = new JMenuBar();
menu = new JMenu("Menu");
// Initializing Menu objects and adding actions
for (String mx : MENUS) {
JMenuItem m = new JMenuItem(mx);
m.addActionListener(this);
menu.add(m);
}
// Adding / Connecting Objects
menubar.add(menu);
frame.setJMenuBar(menubar);
frame.add(message);
// Sets JFrame close operation to Class variable JFrame.EXIT_ON_CLOSE
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// set the size of window based on objects
frame.setSize(300,200);
// makes the frame object visible according to properties previously set
frame.setVisible(true); // flow of control shifts to frame object
}
// event from user selecting a menu option
public void actionPerformed(ActionEvent e) {
// local variable to ActinEvent
String selection = e.getActionCommand(); // menu selection
String msg; // local variable to create response from action
final String[] COLORS = {"Red", "Green", "Blue"}; // add more colors here
final String start_msg = "<html>"; // html building
final String end_msg = "</html>";
final String hash = "#";
// run code based on the menuItem that was selected
if ( selection.equals(MENUS[0]) ) { // Hello Action
msg = "Hello, World";
message.setText(msg);
} else if ( selection.equals(MENUS[1]) ) { // Color Action
msg = start_msg + "<p>" + selection + "</p>";
for (String color : COLORS) {
msg += "<font color=" + color + ">" + color + " </font>";
}
msg += end_msg;
message.setText(msg);
} else { // Loading Bar Action
String loading = "<p>Loading</p>";
// Code to run on a Timer
Timer timer = new Timer();
TimerTask task = new TimerTask() {
public void run() { // Method for TimerTask
// Static and Local variables used to manage message building
int random = (int) (Math.random() * COLORS.length); // random logic
MenuJFrame.hashes += "<font color=" + COLORS[random] + ">" + hash + "</font>";
String msg = start_msg + loading + hashes + end_msg;
message.setText(msg);
// Shutdown timer and reset data
if(MenuJFrame.step++ > MenuJFrame.delay) {
MenuJFrame.step = 1; MenuJFrame.hashes="";
timer.cancel();
}
};
};
// Schedule task and interval
timer.schedule(task, 200, 200);
message.setText(start_msg + loading + hash + end_msg); // prime/initial display
}
}
// Driver turn over control the GUI
public static void main(String[] args) {
// Activates an instance of MenuJFrame class, which makes a JFrame object
new MenuJFrame("Menu");
}
}
MenuJFrame.main(null);
Code.org Object Oriented Programming - The Neighborhood
Students learn fundamental Java concepts as they navigate and interact with The Neighborhood with Painter objects and create new types of Painters to expand the capabilities of their programs.
College Board Topics
- Inheritance (extends) is introduced early in this activity
- Conditional statements, While loops, and Boolean expressions
Using Objects
- Code.org Painter class and objects
Activity Page Start: Code.org CSA1
Hacks
College Board and CTE competences.
- Documentation / Analysis. Describe with Markdown Cell(s) and triple backtick code fragments that answers to the following from your work...
- Explain where a Class is defined
- Explain where an instances of a Class is defined
- Explain where an object is Calling a Method
- Explain where an object is Mutating data
- Describe Console, GUI differences, or Code.org differences.
- Exploration work
Build a new Console or GUI lab for your own solution of interest, this is great opportunity to visualize data from Math, Physics, or other class using Math library. Look to make these changes in your work
- Make constructors of different signatures.
- Use Wrapper Classes of Integer and Double for defining object instance or static data in the Class
- Use the Math Class in performing calculations
- Code.org learning Follow the Neighborhood lessons and work through the OOP Unit.