JAVA Quick Reference

HelloWorld.java

// JAVA QUICK REFERENCE - This is single line comment.

import java.util.Scanner;

/*  This is a multi line comment
    A public class named 'HelloWorld'  */
public class HelloWorld {

    /*  This is a 'public' & 'static' method.
        Static method belongs to class,
        Not to its instance. */
    public static void main(String [] args) {

        // This is a statement
        System.out.println("Hello World!");

        // Built in data types
        int weeks = 5;
        double interestRate = 8.75;
        char initial = 's';
        String day = "Monday";
        Boolean isHoliday = false;

        // Arithmetic operators
        int add = 1 + 2;            //  3
        int subtract = 6 - 3;       //  3
        int multiply = 2 * 3;       //  6
        int divide = 12 / 6;        //  2
        int remainder = 15 % 2;     //  1

        // variables
        int inputX = 1;
        int inputY = 2;

        // Comparison operators
        Boolean equal = inputX == inputY;                   //  false
        Boolean notEqual = inputX != inputY;                //  true
        Boolean lessThan = inputX < inputY;                 //  true
        Boolean lessThanOrEqual = inputX <= inputY;         //  true
        Boolean greaterThan = inputX > inputY;              //  false
        Boolean greaterThanOrEqual = inputX >= inputY;      //  false

        System.out.print("Hello");          // Prints 'Hello'
        System.out.println(day);            // Prints 'Monday' & a newline
        System.out.println();               // Prints a newline

        // Read input from stdin
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        int number = scanner.nextInt();
        double fraction = scanner.nextDouble();

        // Parsing inputs
        int x = Integer.parseInt(input);
        double y = Double.parseDouble(input);
        boolean z = Boolean.parseBoolean(input);
    }
}

public class HelloAgain {

    public static void main(String[] args) {

        int x = 1; int y = 2; int weeks = 5;
        double interest = 8.75;

        if(x > y) {                                 // IF
            System.out.println("X is greater");
        }

        if(x > y) {                                 // IF ELSE
            System.out.println("X is greater");
        }
        else {
            System.out.println("Y is greater");
        }

        if(weeks > 10) {                            // IF ELSE IF
            interest = 8.25;
        }
        else if (weeks > 5) {
            interest = 5.4;
        }
        else {
            interest = 3.5;
        }

        switch (y) {                                // Switch case
            case 0:
                System.out.println("SUN");
                break;
            case 1:
                System.out.println("MON");
                break;
            default:
                System.out.println("Invalid");
                break;
        }

        int num = 0;
        while (num < x) {                           // WHILE Loop
            System.out.println(num);
            num++;
        }

        do {                                        // DO while
            y = y - 1;
        } while (y > 0);

        for(int i = 0; i < 10; i++) {               // FOR Loop
            System.out.println(i);
        }

        // Arrays
        int[] numbers = { 32, 65, 13, 17, 10 };
        int[] names = { "John", "Vivek", "Brad" };

        int sum = 0;
        for (int i = 0; i < numbers.length; i++) {
            sum = sum + numbers[i];
        }

        for (int i = 0; i < numbers.length; i++) {
            numbers[i] = numbers[i] * 2;
        }
    }
}