Continuing OOP Java Part 1

Continuing OOP Java Part 1 from University of Helsinki’s MOOC (Massive Open Online Courses). This post will cover week 1 #20-23 exercises. Last exercise will be used in NetBeans, because that exercise rely on component that is exclusively installed in NetBeans.

Using .equals

Every programming languages have some similarities and differences. I would say that Python and JavaScript are closer to each other than to Java. Java do have some resemblances with both of these languages. This .equals is different from these. In Java, String cannot be compared using equality operator ==. An example in Exercise #20 shows that.

package usingScanner;

import java.util.Scanner;

public class week1_ex19_21 {
	public static void main(String[] args) {
		Scanner reader = new Scanner(System.in);

		String username1 = "alex";
		String password1 = "mightyduck";

		String username2 = "emily";
		String password2 = "cat";


		System.out.print("What is your username?: ");
		String username = reader.nextLine();
		System.out.print("What is your password?: ");
		String password = reader.nextLine();


		if (username.equals(username1) && password.equals(password1)) {
			System.out.println("You are now logged in!");
		} else if (username.equals(username2) && password.equals(password2)) {
			System.out.println("You are now logged in!");
		} else {
			System.out.println("Your username or password is invalid.");
		}

In that exercise, all variables created are strings. Usernames and passwords are created, and then using conditional statements to verify whether usernames and passwords match or not. && is one of operators like in JavaScript, and and for Python; short for “true AND true” condition. username.equals(username1) && password.equals(password1) is a condition saying that username1 and password1 TOGETHER is true. Java’s .equals will bite us in the butt someday later if we forget about it and for using equality operator instead. NOTE: This is not the proper way to store username and password, just a practice.

Leap Years

Exercise #21 is pretty neat and it’s about using &&. It wants us to check if the year is a leap year or not. The rules for a year to be a leap year:

• divisible by 4
• if divisible by 100 then it has to be divisible by 400 too

There are four different years to test: 2011, 2012, 1800, 2000.

  System.out.print("Enter the year: ");
  int year = Integer.parseInt(reader.nextLine());

  if (year % 4 == 0 && year % 100 == 0 && year % 400 == 0) {
    System.out.println("It is a leap year!");
  } else if (year % 4 == 0 && year % 100 != 0) {
    System.out.println("It is a leap year!");
  } else {
    System.out.println("It is NOT a leap year.");
  }

It took me awhile to solve this. I tried three different codings and they did not work. It’s mostly about logic processes. I think this exercise is great way of showing how condition is very strict and inflexible. The first if statement says if year is divisible by 4, 100, and 400 then it is a leap year. It doesn’t worry about year that is divisible by 4 alone. Any year that is divisible by 4 will be considered not a leap year. So add else if year is divisible by 4 alone and not by 100, or 400 for that matter, then it is a leap year. Important, let computer read that as a second command so that way it will look for a year that’s divisible by both 4, and 100 first. Else means everything else, therefore is not a leap year.

while(true)

This Exercise #23 uses NetBeans because it contains a component exclusively. It has a Graph.java that automatically opens up a graph based on data we write in our java file. Mine is week1-ex23.java, name yours whatever. In this exercise, while(true) loop is used. If it is used in a wrong way, could trigger an infinite loop. We don’t want that because it will crash our computer. The way to end a while(true) loop is to have a break;

It wants us to create a program that asks us to:

• input a floating point number as often as we want to
• numbers inputed need to be between -30 and 40 degree
• if it is below -30 and above 40, then have program to ignore it and not include it into the Graph

while(true) will repeat endlessly until break; is triggered, so we will need to make a trigger.

while(true) {
    System.out.print("Enter a temperature between -30 and 40: ");
    double value = Double.parseDouble(reader.nextLine());


    if ((value > -31 && value < 41)) {
        Graph.addNumber(value);
        System.out.print("Add another value? (y/n): ");
        String finish = reader.nextLine();
        if (finish.equals("n")) {
            System.out.println("The graph session ended.");
            break;
        }
    } else {
      System.out.println("Value is outside the range");
    }
}

The program above begins with asking an user to enter a temperature. It will loop continuously. If the value input is greater than -31 and less than 41, then it is added onto the graph. The question will pop out asking if another value is to be entered. y will have loop keep rolling. If n is entered, then break; is triggered. I made a mistake again… I used an equality operator! I was stuck on this exercise for awhile, and oh right! Use an .equals! Lastly… if value is outside the range, it will be not included into the graph, and will notify the user to try again and this time in the range.