Java Essentials

Java JDK, JRE and JVM

• JVM (Java Virtual Machine) is an abstract machine that enables your computer to run a Java program.

• JRE (Java Runtime Environment) is a software package that provides Java class libraries, Java Virtual Machine (JVM), and other components that are required to run Java applications.

• JDK (Java Development Kit) is a software development kit required to develop applications in Java. When you download JDK, JRE is also downloaded with it.

What is JDK?

JDK (Java Development Kit) is a software development kit required to develop applications in Java. When you download JDK, JRE is also downloaded with it.

In addition to JRE, JDK also contains a number of development tools (compilers, JavaDoc, Java Debugger, etc).

Java Development Kit

If you want to develop Java applications, download JDK.

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Relationship between JVM, JRE, and JDK.

Relationship between JVM, JRE, and JDK

Java Operators

Operators in Java can be classified into 5 types:

1. Arithmetic Operators

2. Assignment Operators

3. Relational Operators

4. Logical Operators

5. Unary Operators

6. Bitwise Operators

Arithmetic Operator

Operator	Operation
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulo Operation (Remainder after division)

Logical Operators

Operator	Description
~	Bitwise Complement
<<	Left Shift
>>	Right Shift
>>>	Unsigned Right Shift
&	Bitwise AND
^	Bitwise exclusive OR

Java Unary Operators

Operator	Meaning
+	Unary plus: not necessary to use since numbers are positive without using it
-	Unary minus: inverts the sign of an expression
++	Increment operator: increments value by 1
--	Decrement operator: decrements value by 1
!	Logical complement operator: inverts the value of a boolean

Java Assignment Operators

Operator	Example	Equivalent to
=	a = b;	a = b;
+=	a += b;	a = a + b;
-=	a -= b;	a = a - b;
*=	a *= b;	a = a * b;
/=	a /= b;	a = a / b;
%=	a %= b;	a = a % b;

Java Relational Operators

Operator	Example	Equivalent to
=	a = b;	a = b;
+=	a += b;	a = a + b;
-=	a -= b;	a = a - b;
*=	a *= b;	a = a * b;
/=	a /= b;	a = a / b;
%=	a %= b;	a = a % b;

Java Logical Operators

Operator	Example	Meaning
&& (Logical AND)	expression1 && expression2	true only if both expression1 and expression2 are true
|| (Logical OR)	expression1 || expression2	true if either expression1 or expression2 is true
! (Logical NOT)	!expression	true if expression is false and vice versa

Java Bitwise Operators

Operator	Description
~	Bitwise Complement
<<	Left Shift
>>	Right Shift
>>>	Unsigned Right Shift
&	Bitwise AND
^	Bitwise exclusive OR

Java Basic Input and Output

Java Output

System.out.println(); or
System.out.print(); or
System.out.printf();

Java Input

import java.util.Scanner;
// create an object of Scanner
Scanner input = new Scanner(System.in);
// take input from the user
int number = input.nextInt();

Java Flow Control

Java if...else Statement

Working of if Statement

Working of Java if statement

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How the if...else statement works?

Working of Java if-else statements

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How the if...else...if ladder works?

Working of if...else...if ladder

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class Main {
  public static void main(String[] args) {

    int number = 0;

    // checks if number is greater than 0
    if (number > 0) {
      System.out.println("The number is positive.");
    }

    // checks if number is less than 0
    else if (number < 0) {
      System.out.println("The number is negative.");
    }
    
    // if both condition is false
    else {
      System.out.println("The number is 0.");
    }
  }
}

Output

The number is 0.

Java switch Statement

switch (expression) {

  case value1:
    // code
    break;
  
  case value2:
    // code
    break;
  
  ...
  ...
  
  default:
    // default statements
  }

Java for Loop

Flowchart of Java for loop

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for-each Loop Sytnax

// print array elements 
class Main {
  public static void main(String[] args) {
    // create an array
    int[] numbers = {3, 9, 5, -5};
    // for each loop 
    for (int number: numbers) {
      System.out.println(number);
    }
  }
}

/* Output
3
9
5
-5
*/

Java while loop

while (testExpression) {
    // body of loop
}

Flowchart of Java while loop

Flowchart of Java do while loop

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do {
    // body of loop
} while(textExpression);

Break Statement

How break statement works?

Working of Java break Statement

if (number < 0.0) {
    break;
}

Java break and Nested Loop

Working of break Statement with Nested Loops

Labeled break Statement

Working of the labeled break statement in Java

Java continue Statement

Working of Java continue statement

Working of Java continue Statement

Java continue with Nested Loop.

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Java continue with Nested Loop

Labeled continue Statement

Java Arrays

String[] array = new String[100];
// declare arrray 
dataType[] arrayName;

// declare an array
int[] age = new int[5];

// initialize array
age[0] = 12;
age[1] = 4;
age[2] = 5;
..

Java Arrays initialization

int[][] a = new int[3][4];

Here, we have created a multidimensional array named a. It is a 2-dimensional array, that can hold a maximum of 12 elements,

2-dimensional Array