Programming

Introduction

public class Main {
    public static void main(String[] args) {
        System.out.print("Hello, world!");
        System.out.print("Another, world!\n");
        System.out.println("Knock, knock");
    }
}

Data Types

int x = 123;
double y = 1.2;
boolean z = true;
char symbol = '@';
String text = "Rangas";

System.out.println(x);
System.out.println(y);
System.out.println(z);
System.out.println(symbol);
System.out.println("Hello " + symbol + text);

Exercise

String x = "Water";
String y = "Cola";
String temp; // String temp = null;

temp = x;
x = y;
y = temp;

System.out.println("x: " + x);
System.out.println("y: " + y);

Input Issue

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		
		System.out.println("What is your name?");
		String name = scanner.nextLine();

		System.out.println("How old are you?");
		int age = scanner.nextInt();

		scanner.nextLine();
        
		System.out.println("What is your fav food?");
		String food = scanner.nextLine();
		
		scanner.close();

		System.out.println("Hello " + name + ".");
		System.out.println("Your are " + age + " year old.");
		System.out.println("Your like " + food + ".");
	}
}

Note

int friends = 10;
//friends = friends + 1; (or) friends++; 

System.out.println(friends);

Dialog

import javax.swing.JOptionPane;

public class Main {
	public static void main(String[] args) {
		String name = JOptionPane.showInputDialog("Enter your name");
		JOptionPane.showMessageDialog(null, "Hello " + name);

		int age = Integer.parseInt(JOptionPane.showInputDialog("Enter your age"));
		JOptionPane.showMessageDialog(null, "Your age is " + age + " year old");

		double height = Double.parseDouble(JOptionPane.showInputDialog("Enter your height"));
		JOptionPane.showMessageDialog(null, "Your height is " + height);
	}
}

Math

double x = 3.14;
double y = -10;
double z = Math.min(x, y); // max
System.out.println(z);
        
double y2 = -10;
double z2 = Math.abs(y2); // without negative sign
System.out.println(z2);
    
double y3 = 10;
double z3 = Math.sqrt(y3); // square root with decimal
System.out.println(z3);
    
double x2 = 3.14;
double z4 = Math.round(x2); // real world calculation
//double z4 = Math.ceil(x2); // up
//double z4 = Math.floor(x2); // down
System.out.println(z4);

Exercise

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		double x, y, z;
		Scanner scanner = new Scanner(System.in);
		
		System.out.print("Enter first number: ");
		x = scanner.nextDouble();
		
		System.out.print("Enter second number: ");
		y = scanner.nextDouble();
		
		z = Math.sqrt((x * x) + (y * y));
		System.out.println(z);
		
		scanner.close();
	}
}

Logical Operator

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		// Logical operator - &&, ||, !
		Scanner scanner = new Scanner(System.in);
		
		System.out.print("q or Q : ");
		String response = scanner.nextLine();
		
		if (response.equals("q") || response.equals("Q")) {
			System.out.println("Bye");
		} else {
			System.out.println("Good");
		}
	}
}

if else, if else if

int age = 17;

if(age >= 75) {
	System.out.println("Ok Boomer.");
} 
else if (age >= 18) {
	System.out.println("You are an adult.");
}
else {
	System.out.println("You are not adult");
}

Exercise

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        String name;
        int age;
        boolean student;
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter your name: ");
        name = scanner.nextLine();

        System.out.print("Enter your age: ");
        age = scanner.nextInt();
        scanner.nextLine();

        System.out.print("Are you student? true/false: ");
        student = scanner.nextBoolean();

        if (name.isEmpty()) {
            System.out.println("Name error.");
        }
        else {
            System.out.println("Hello " + name);
        }

        if (age < 1) {
            System.out.println("Age Error.");
        }
        else if (age <= 16) {
            System.out.println("age 1-16");
        }
        else if (age <= 60) {
            System.out.println("age 17-60");
        }
        else if (age > 60) {
            System.out.println("age 60+");
        }
        else {
            System.out.println("Age type Error");
        }

        if (student) {
            System.out.println("Your are Student.");
        }
        else {
            System.out.println("Your are not Student.");
        }
    }
}

Random

import java.util.Random;

public class Main {
	public static void main(String[] args) {
		Random random = new Random();
		
		//int x = random.nextInt(); // + & -
		//int x = random.nextInt(6); // 0 to 6
		int x = random.nextInt(6) + 1; // 1 to 6
		
		double y = random.nextDouble(); // between 0 & 1
		
		boolean z = random.nextBoolean();
		
		System.out.println(x);
		System.out.println(y);
		System.out.println(z);
	}
}

Exercise

import java.util.Random;

public class Main {
    public static void main(String[] args) {
        Random random = new Random();
        boolean myBoo;

        myBoo = random.nextBoolean();
        if (myBoo) {
            System.out.println("Hard");
        }
        else {
            System.out.println("Tail");
        }
    }
}

printf()

a method used to format output %[flags][width][.precision][specifier-character]

String name = "DJ Rangas";
char firstLetter = 'R';
int age = 24;
double height = 24.9;
boolean work = true;

System.out.printf("String - %s\n", name);
System.out.printf("Character - %c\n", firstLetter);
System.out.printf("Integer - %d\n", age);
System.out.printf("Double - %f\n", height);
System.out.printf("Boolean - %b\n", work);

System.out.printf("String - %s & Integer - %d\n", name, age);

double num1 = 100.1;
double num2 = 3330000.145;
double num3 = -9900.87;

// 6 decimal auto
System.out.printf("%f\n", num1);
System.out.printf("%f\n", num2);
System.out.printf("%f\n", num3);

// + = output a plus
// , = comma grouping separator
// (= negative numbers are enclosed in ()
// space = display a minus if negative, space if positive

// adjust decimal
System.out.printf("%.2f\n", num1);
System.out.printf("%.2f\n", num2);
System.out.printf("%.2f\n", num3);

System.out.printf("%+.2f\n", num1);
System.out.printf("%+.2f\n", num2);
System.out.printf("%+.2f\n", num3);

System.out.printf("%,.2f\n", num1);
System.out.printf("%,.2f\n", num2);
System.out.printf("%,.2f\n", num3);

System.out.printf("%(.2f\n", num1);
System.out.printf("%(.2f\n", num2);
System.out.printf("%(.2f\n", num3);

System.out.printf("% .2f\n", num1);
System.out.printf("% .2f\n", num2);
System.out.printf("% .2f\n", num3);

// 0 = zero padding
// number = right justified padding
// negative number = left justified padding

int id1 = 1;
int id2 = 23;
int id3 = 456;
int id4 = 7890;

System.out.printf("%4d\n", id1);
System.out.printf("%4d\n", id2);
System.out.printf("%04d\n", id3);
System.out.printf("%-4d\n", id3);
System.out.printf("%d\n", id4);

double principal;
double rate;
int timesCompounded;
int years;
double amount;
Scanner scanner = new Scanner(System.in);

System.out.print("Enter the principal amount: ");
principal = scanner.nextDouble();
System.out.print("Enter the interest rate (in %): ");
rate = scanner.nextDouble() / 100;
System.out.print("Enter the # of times compounded per year: ");
timesCompounded = scanner.nextInt();
System.out.print("Enter the # of years: ");
years = scanner.nextInt();

amount = principal * Math.pow(1 + rate / timesCompounded, timesCompounded * years);

System.out.printf("The amount after %d years is $%,.2f", years, amount);

Netshed if

boolean isStudent = false;
boolean isSenior = true;
double price = 9.99;

if (isStudent) {
    if (isSenior) {
        System.out.println("Student discount 10%");
        System.out.println("Senior discount 20%");
        price *= 0.7;
    }
    else {
        System.out.println("Student discount 10%");
        price *= 0.9;
    }
}
else {
    if (isSenior) {
        System.out.println("Senior discount 20%");
        price *= 0.8;
    }
    else {
        price *= 1;
    }
}
System.out.printf("Balance = %.2f", price);

String

String name = "DJ Rangas";
int intNum = name.length();                 // Start 1
char charNum = name.charAt(0);              // Start 0
int index = name.indexOf("a");              // Start 0
int lastIndex = name.lastIndexOf("a");  // Start 0

System.out.println(intNum);
System.out.println(charNum);
System.out.println(index);
System.out.println(lastIndex);

System.out.println(name.toUpperCase());
System.out.println(name.toLowerCase());

String name2 = "      DJ Rangas       ";
System.out.println(name2.trim());
System.out.println(name2.isEmpty());
System.out.println(name2.contains(" "));
System.out.println(name2.equals("      DJ Rangas       "));
System.out.println(name2.equalsIgnoreCase("      dj Rangas       "));

System.out.println(name.replace("a", "u"));

.substring

// A method used to extract a portion of a string
//.substring(start, end)
//.substring(start)

String email = "djrangas21@gmail.com";
String username = email.substring(0, 10);
String domain = email.substring(11);
String usernameAdv = email.substring(0, email.indexOf("@"));
String domainAdv = email.substring(email.indexOf("@") + 1);

System.out.println(username);
System.out.println(domain);
System.out.println(usernameAdv);
System.out.println(domainAdv);

Exercise

String email;
Scanner scanner = new Scanner(System.in);
System.out.print("Enter your Email: ");
email = scanner.nextLine();

if (email.contains("@")) {
    System.out.println("name - " + email.substring(0, email.indexOf("@")));
    System.out.println("domain - " + email.substring(email.indexOf("@") + 1));
}
else {
    System.out.println("Email error");
}

Exercise

Scanner scanner = new Scanner(System.in);
double weightLb, weightKg;
int choice;

System.out.println("Weight Conversion Program");
System.out.println("1: Convert lbs to kgs");
System.out.println("2: Convert kgs to lbs");
System.out.print("Choose an option: ");

choice = scanner.nextInt();

if (choice == 1) {
    System.out.print("Enter the weight in lbs: ");
    weightLb = scanner.nextDouble();
    weightKg = weightLb * 0.453592;
    System.out.printf("kgs: %.2f", weightKg);
}
else if (choice == 2) {
    System.out.print("Enter the weight in kgs: ");
    weightKg = scanner.nextDouble();
    weightLb = weightKg / 0.453592;
    System.out.printf("lbs: %.2f", weightLb);
}
else {
    System.out.println("Error");
}

ternary operator ?

// Return 1 of 2 values if a condition is true
// variable = (condition) ? ifTrue : ifFalse;
int Score = 50;
String passOrFail = (Score > 60) ? "Pass" : "Fail";
System.out.println(passOrFail);

Exercise

Scanner scanner = new Scanner(System.in);
double temp, newtemp;
String unit;

System.out.print("Enter a temperature: ");
temp = scanner.nextDouble();

scanner.nextLine();

System.out.print("Unit C or F: ");
unit = scanner.nextLine().toUpperCase();

newtemp = (unit.equals("C")) ? (temp - 32) * 5 / 9 : (temp * 5 / 9) + 32;

System.out.printf("%.2f° %s", newtemp, unit);

switch

String day = "Friday";

switch (day) {
    case "Sunday" -> System.out.println("Today is Sunday");
    case "Monday" -> System.out.println("Today is Monday");
    case "Tueday" -> System.out.println("Today is Tueday");
    case "Wednesday" -> System.out.println("Today is Wednesday");
    case "Thursday" -> System.out.println("Today is Thursday");
    case "Friday", "Saturday" -> System.out.println("Today is Friday or Saturday");
    default -> System.out.println("Today is not Luckyday");
}

String day = "Gym";

switch (day) {
    case "Sunday":
        System.out.println("Today is Sunday");
        break; // should test without break

    case "Monday":
        System.out.println("Today is Monday");
        break;

    case "Tueday":
        System.out.println("Today is Tueday");
        break;

    case "Wednesday":
        System.out.println("Today is Wednesday");
        break;

    case "Thursday":
        System.out.println("Today is Thursday");
        break;

    case "Friday":
        System.out.println("Today is Friday");
        break;

    case "Saturday":
        System.out.println("Today is Saturday");
        break;

    default:
        System.out.println("Today is not Luckyday");
}

Last updated 7 months ago

hashtagIntroduction

hashtagData Types

hashtagExercise

hashtagInput Issue

hashtagNote

hashtagDialog

hashtagMath

hashtagExercise

hashtagLogical Operator

hashtagif else, if else if

hashtagExercise

hashtagRandom

hashtagExercise

hashtagprintf()

hashtagNetshed if

hashtagString

hashtag.substring

hashtagExercise

hashtagExercise

hashtagternary operator ?

hashtagExercise

hashtagswitch

Introduction

Data Types

Exercise

Input Issue

Note

Dialog

Math

Exercise

Logical Operator

if else, if else if

Exercise

Random

Exercise

printf()

Netshed if

String

.substring

Exercise

Exercise

ternary operator ?

Exercise

switch