Basics
Basic Boilerplate Code - Hello World Program
public class test{
public static void main(String[] a){
System.out.println("Hello World");
}
}
Comments
Comments are used to make code more readable or to explain java code. Comments are ignored while executing the code.
Single Line Comment
Single Line comments starts with double forward slashes(//). Syntax for Single Line Comment:
//This is comment and will be ignored by java while executing.
System.out.println("HelloWorld");Multi-Line Comments
Multi-line comments starts with /* and ends with */.
Any code between these two will be ignored.
/*
This block will be ignored
The code below will print hello world.
Example of multi-line comment
*/
System.out.println("HelloWorld");User Input in Java
To take user input, first we will import Scanner class. Learn more about Scanner class here
public class test{
public static void main(String[] a){
//Creating an object of scanner class.
Scanner input = new Scanner(System.in);
//Taking Integer Input from user
int testVar = input.nextInt();
}
}Data Types
Primitive Data Type
| Data Type | Example | Size |
|---|---|---|
| Byte | byte a = 100; | 1 bytes |
| Short | short a = 500; | 2 bytes |
| Integer | int a = 5; | 4 bytes |
| Long | long a =1241351866L; | 8 bytes |
| Float | float a = 15.87; | 4 bytes |
| Double | double a =2156.4674; | 8 bytes |
| Boolean | bool isValid = true; | 1 bit |
| Character | char letter = “J”; | 2 bytes |
Java Operator
Operator are used to perform mathematical operations on variables and values.
Types of Operators
- Arithmetic Operators
- Logical Operators
- Relational Operators
- Bitwise Operators
Arithmetic Operators
| Operator | Name | About | Example |
|---|---|---|---|
| + | Addition | Adds the variables | a + b |
| - | Subtraction | Subtracts the variables | a - b |
| * | Multiplication | Multiply the variables | a * b |
| / | Division | Divides the variables | a / b |
| % | Modulus | prints the remainder of division | a % b |
| ++ | Increment | Increment the values by 1 | a++ |
| — | Decrement | Decrement the values by 1 | a— |
Logical Operators
| Operator | Name | About | Example |
|---|---|---|---|
| && | Logical AND | If any one of the condition is false, then result becomes false. | A && B (If A is true and B is false then result is FALSE) |
| || | Logical OR | If any one of the condition is true then result becomes true | A || B (If A is true and B is false then result is TRUE) |
| ! | Logical NOT | It is used to reverse the logical state of operand. If condition is FALSE then NOT operator will make it TRUE | !A (Here If A is TRUE the !A(NOT A) becomes FALSE) |
Relational Operators
| Operator | Name | About | Example |
|---|---|---|---|
| == | Equals to | Check two variables if they are equal or not | a == b |
| < | Less than | Checks if left variable is less than or not of right variable | a < b |
| > | Greater than | Checks if left variable is greater than or not of right variable | a > b |
| ⇐ | Less than or equal to | Check if left variable is less than or equal to right variable | a ⇐ b |
| >= | Greater than or equal to | Checks if left variable is greater than or equal to right variable | a>=b |
Bitwise Operator
In progress
Conditional Statements
If Statements
if statements are used to specify block of code to be executed if condition is true
if(condition){
//block of code
}
//EXAMPLE:
int a = 5;
int b = 2;
if(a > b){
System.out.println("A is greater");
}Else Statement
else block is executed if the if block is false
if(condition){
//code
}
else{
//executed this block if above "if" block is false
}Else if Statements
else if is used to specify multiple if condition.
if(condition){
//code 1
}
else if(condition2){
// execute this block if above "if" is false.
}
else if(condition3){
// execute this block if above "else if" block is false.
}
else{
// execute this if all above block is false.
}Switch Statement
switch(condition){
case A:
// code
break;
case B:
//code
break;
case C:
//code
break;
default:
//code
}Loops
While Loop
while(condition){
//code
}Example Code:
//print 1 to 10
int i=1;
while(i<=10){
System.out.println(i);
i++;
}For Loop
for(initialization; condition; increment/decrement){
//code
}
//print 1 to 10
for(int i=1; i<=10; i++){
System.out.println(i);
}Do While Loop
do{
//loop body or code
}
while(condition);
//Example Code:
int i=1;
do{
System.out.println(i);
i++;
}
while(i<=10);Break/Continue Statements
break statement is used to jump out of the loop.
for(int i=1; i<=10;i++){
//break loop if value of i becomes 5
if(i == 5){
break;
}
System.out.print(i + " ");
}
//OUTPUT:
// 1 2 3 4continue statement is used skip the one iteration in the loop.
for(int i=1; i<=10; i++){
if(i==5){
continue;
}
System.out.print(i+" ");
}
// OUTPUT:
// 1 2 3 4 6 7 8 9 10
// Here 5 is skippedArrays
Arrays are used to store multiple values of same data type in single variable. Array Index starts from 0.
Declaring Array
int[] id; //declaring array
id = new int[5]; //allotting memory to array
//Another Way to declare array
int[] id = new int[5];Defining Array
int[] id = {1,2,3,4,5};Access an Array Element
String[] arr = {'ABC','XYZ', 'PQR'};
System.out.println(arr[1]);
//Changing Array Element
arr[2] = "RST";
//"PQR" will be changed to "RST"Array Length
System.out.println(arr.length);2D Array
//Creating 2x2 Matrix Array
int[2][2] matrix = new int[2][2];
//the two brackets( [2][2] ) represents no. of rows and columns
// Here array of 2 row and 2 col are declared.
//defining array
int[2][2] matrix = {
// COL 1 COL 2
{12, 54}, // ROW 1
{67, 23} // ROW 2
};Methods (Functions)
Declaring Method
//SYNTAX
returnType methodName(parameters){
//code
}
//EXAMPLE
int printNum(int a){
return a;
}
//CALLING METHOD
//SYNTAX : methodName(arguments);
// EXAMPLE:
printNum(5);
// ↑ ↑
// Method Arguments
// NameMethod Overloading
Two or more methods can have same name but different parameters, such methods are called “overloaded methods”.
//Method 1 (0 Arguments)
static void greet(){
System.out.println("Good Morning");
}
//Method 2 (1 Arguments)
static void greet(String name){
System.out.println("Good Morning " + name);
}
//Calling Both Methods
greet(); //OUTPUT: Good Morning
greet("Carl Johnson") //OUTPUT: Good Morning Carl JohnsonRecursion
Recursion is the technique of making a function call itself.
//Factorial of Number in Recursion
static int factorial(int n){
if(n == 0){
return 1;
}
return n * factorial(n-1);
// ↑
// This is recursive function call.
}