//
Java Cheatsheet
Basics
Basic syntax and functions from the Java programming language.
Boilerplate
class HelloWorld { public static void main(String args[])
{ System.out.println("Hello World"); }
}
Showing Output
It will print something to the output console.
class HelloWorld { public static void main(String args[])
{ System.out.println("Hello World");
}
}
Taking Input
It will take string input from the user.
import java.util.Scanner; class HelloWorld { public static void
main(String args[]) { Scanner sc = new Scanner(System.in);
String name = sc.nextLine();
System.out.println(name);
}
}
It will take integer input from the user.
//
import java.util.Scanner; class HelloWorld { public static void
main(String args[]) { Scanner sc = new Scanner(System.in); int x =
sc.nextInt();
System.out.println(x);
}
}
It will take float input from the user.
import java.util.Scanner; class HelloWorld { public static void
main(String args[]) { Scanner sc = new Scanner(System.in); float x
= sc.nextFloat(); System.out.println(x);
}
}
It will take double input from the user.
import java.util.Scanner; class HelloWorld { public static void
main(String args[]) { Scanner sc = new Scanner(System.in);
double x = sc.nextDouble(); System.out.println(x);
}
}
Primitive Type Variables
The eight primitives defined in Java are int , byte , short , long , float , double , boolean ,
and char . These aren't considered objects and represent raw values.
byte
byte is a primitive data type that only takes up 8 bits of memory.
//
class HelloWorld { public static void main(String args[]) { byte age
= 18;
System.out.println(age);
}
}
long
long is another primitive data type related to integers. long takes up 64 bits of memory.
class HelloWorld { public static void main(String args[]) { long var
= 900L;
System.out.println(var);
}
}
float
We represent basic fractional numbers in Java using the float type. This is a single-precision
decimal number, which means if we get past six decimal points, this number becomes less
precise and more of an estimate.
class HelloWorld { public static void main(String args[]) { float
price = 100.05f;
System.out.println(price);
}
}
char
char is a 16-bit integer representing a Unicode-encoded character.
class HelloWorld { public static void main(String args[]) { char
letter = 'A';
System.out.println(letter);
//
}
}
int
int holds a wide range of non-fractional number values.
class HelloWorld { public static void main(String args[]) { int var1
= 256;
System.out.println(var1);
}
}
short
If we want to save memory and byte is too small, we can use short .
class HelloWorld { public static void main(String args[]) { short
var2 = 5666; System.out.println(var2);
}
}
Comments
A comment is the code that is not executed by the compiler, and the programmer uses it to keep
track of the code.
Single line comment
// It's a single line comment
//
Multi-line comment
/* It's a multi-
line comment
*/
Constants
Constants are like a variable, except that their value never changes during program execution.
public class Declaration { final double PI =
3.14;
public static void main(String[] args) {
System.out.println("Value of PI: " + PI); }
}
Arithmetic Expressions
These are the collection of literals and arithmetic operators.
Addition
It can be used to add two numbers.
public class HelloWorld { public static void main(String args[]) {
int x = 10 + 3;
System.out.println(x);
}
}
//
Subtraction
It can be used to subtract two numbers.
public class HelloWorld { public static void main(String args[]) {
int x = 10 - 3;
System.out.println(x);
}
}
Multiplication
It can be used to multiply two numbers.
public class HelloWorld { public static void main(String args[]) {
int x = 10 * 3;
System.out.println(x);
}
}
Division
It can be used to divide two numbers.
public class HelloWorld { public static void main(String args[]) {
int x = 10 / 3;
System.out.println(x);
}
}
Modulo Remainder
It returns the remainder of the two numbers after division.
//
public class HelloWorld { public static void main(String args[]) {
int x = 10 % 3;
System.out.println(x);
}
}
Augmented Operators
Addition assignment
public class HelloWorld { public static void main(String args[]) {
int var = 1; var += 10;
System.out.println(var);
}
}
Subtraction assignment
public class HelloWorld { public static void main(String args[]) {
int var = 1; var -= 10;
System.out.println(var);
}
}
Multiplication assignment
public class HelloWorld { public static void main(String args[]) {
int var = 1; var *= 10;
System.out.println(var);
}
}
//
Division assignment
public class HelloWorld { public static void main(String args[]) {
int var = 1; var /= 10;
System.out.println(var);
}
}
Modulus assignment
public class HelloWorld { public static void main(String args[]) {
int var = 1; var %= 10;
System.out.println(var);
}
}
Type Casting
Type Casting is a process of converting one data type into another.
Widening Type Casting
It means converting a lower data type into a higher.
class HelloWorld { public static void main(String args[]) { int x =
45; double var_name = x;
System.out.println(var_name);
}
}
Narrowing Type Casting
It means converting a higher data type into a lower.
//
class HelloWorld { public static void main(String args[]) { double x
= 40005; int var_name = (int) x;
System.out.println(var_name);
}
}
Decision Control Statements
Conditional statements are used to perform operations based on some condition.
if Statement
if (condition) {
// block of code to be executed if the condition is true
}
if-else Statement
if (condition) {
// If condition is True then this block will get executed
} else {
// If condition is False then this block will get executed
}
if else-if Statement
if (condition1) { // Codes
} else if(condition2) { // Codes
} else if (condition3) { // Codes }
else {
// Codes
}
//
Ternary Operator
It is shorthand for an if-else statement.
Syntax
variable = (condition) ? expressionTrue : expressionFalse;
Example
public class TernaryOperatorExample { public static void
main(String args[]) { int x, y; x = 20;
y = (x == 1) ? 61 : 90;
System.out.println("Value of y is: " + y); y = (x == 20) ? 61 : 90;
System.out.println("Value of y is: " + y); }
}
Switch Statements
It allows a variable to be tested for equality against a list of values (cases).
class SwitchExample { public static void main(String args[])
{ int day = 4; switch (day) { case 1:
System.out.println("Monday"); break;
case 2:
System.out.println("Tuesday"); break;
case 3:
System.out.println("Wednesday"); break;
case 4:
System.out.println("Thursday"); break;
case 5:
System.out.println("Friday"); break;
case 6:
System.out.println("Saturday"); break;
case 7:
System.out.println("Sunday"); break;
}
//
}
}
Iterative Statements
Iterative statements facilitate programmers to execute any block of code lines repeatedly and can
be controlled as per conditions added by the coder.
while Loop
It iterates the block of code as long as a specified condition is True.
public class WhileExample { public static void main(String[] args)
{ int i = 1; while(i <= 10) { System.out.println(i); i++;
}
}
}
for Loop
for loop is used to run a block of code several times.
class HelloWorld { public static void main(String args[]) { for(int i
= 1; i < 100; i++) { System.out.println(i);
}
}
}
for-each Loop
public class HelloWorld { public static void main(String args[]) {
int[] arr = {2, 4, 5, 7, 8, 0, 3, 5}; for (int i : arr) {
System.out.println(i);
//
}
}
}
do-while Loop
It is an exit-controlled loop. It is very similar to the while loop with one difference, i.e., the body of
the do-while loop is executed at least once even if the condition is False.
public class HelloWorld { public static void main(String args[]) {
int i = 1; do {
System.out.println(i); i++;
} while(i <= 100);
}
}
Break statement
break keyword inside the loop is used to terminate the loop.
class HelloWorld { public static void main(String args[]) { for(int i
= 1; i < 100; i++) { System.out.println(i); if(i == 50) break;
}
}
}
Continue statement
continue keyword skips the rest of the current iteration of the loop and returns to the starting
point of the loop.
class HelloWorld { public static void main(String args[]) { for(int i
= 1; i < 100; i++) {
System.out.println(i);
if(i == 50) continue;
//
}
}
}
Arrays
Arrays are used to store multiple values in a single variable.
Declaring an array
Declaration of an array.
public class HelloWorld { public static void main(String args[]) {
String[] var_name;
}
}
Defining an array
Defining an array.
public class HelloWorld { public static void main(String args[]) {
String[] var_name = {"harry", "rohan", "aakash"}; }
}
Accessing an array
Accessing the elements of an array.
public class HelloWorld { public static void main(String args[]) {
String[] var_name = {"Harry", "Rohan", "Aakash"};
System.out.println(var_name[0]);
}
}
//
Changing an element
Changing any element in an array.
public class HelloWorld { public static void main(String args[]) {
String[] var_name = {"Harry", "Rohan", "Aakash"}; var_name[2] =
"Shubham";
}
}
Array length
It gives the length of the array.
public class HelloWorld { public static void main(String args[]) {
String[] var_name = {"Harry", "Rohan", "Aakash"};
System.out.println(var_name.length);
}
}
Loop through an array
It allows us to iterate through each array element.
public class HelloWorld { public static void main(String args[]) {
String[] var_name = {"Harry", "Rohan", "Aakash"}; for (int i
= 0; i < var_name.length; i++)
{ System.out.println(var_name[i]);
}
}
}
Multi-dimensional Arrays
Arrays can be 1-D, 2-D, or multi-dimensional.
//
// Creating a 2x3 array (two rows, three columns) int[][] matrix = new
int[2][3]; matrix[0][0] = 10; // Shortcut int[][] matrix = {
{ 1, 2, 3 },
{ 4, 5, 6 } };
Methods
Methods are used to divide an extensive program into smaller pieces. It can be called multiple
times to provide reusability to the program.
Declaration
Declaration of a method.
returnType methodName(parameters) { // statements
}
Calling a method
Calling a method.
methodName(arguments);
Example
public static void findEvenOdd(int num) {
// method body if(num % 2 ==
0)
System.out.println(num + " is even"); else
System.out.println(num + " is odd");
}
//
import java.util.Scanner; public class EvenOdd { public static void
main (String args[]) {
// creating Scanner class object
Scanner scan = new Scanner(System.in);
System.out.print("Enter the number: "); // reading value
from the user int num = scan.nextInt(); // method
calling findEvenOdd(num);
}
}
Method Overloading
Method overloading means having multiple methods with the same name, but different
parameters.
class Calculate { void sum(int x, int y) {
System.out.println("Sum is: " + (x + y));
}
void sum(float x, float y) {
System.out.println("Sum is: " + (x + y));
} public static void main(String[] args) { Calculate calc = new Calculate();
calc.sum(5, 4); // sum(int x, int y) is called.
calc.sum(1.2f, 5.6f); // sum(float x, float y) is called. }
}
Recursion
Recursion is when a function calls a copy of itself to work on a minor problem. The function that
calls itself is known as the Recursive function.
void recurse() {
recurse(); }
//
Strings
It is a collection of characters surrounded by double quotes.
Creating String Variable
String var_name = "Hello World";
String Length
Returns the length of the string.
public class str { public static void main(String args[]) { String
var_name = "Harry";
System.out.println("The length of the string is: " + var_name.length()); }
}
String Methods toUpperCase()
Convert the string into uppercase.
public class str { public static void main(String args[]) {
String var_name = "Harry";
System.out.println(var_name.toUpperCase()); }
}
toLowerCase()
Convert the string into lowercase.
public class str { public static void main(String args[]) {
String var_name = "Harry";
System.out.println(var_name.toLowerCase()); }
//
}
indexOf()
Returns the index of a specified character from the string.
public class str { public static void main(String args[]) {
String var_name = "Harry";
System.out.println(var_name.indexOf("a")); }
}
concat()
Used to concatenate two strings.
public class str { public static void main(String args[]) {
String var1 = "Harry";
String var2 = "Bhai";
System.out.println(var1.concat(var2)); }
}
Math Class
Math class allows you to perform mathematical operations.
Methods max() method
It is used to find the greater number among the two.
public class Demo { public static void main(String[] args) {
// using the max() method of Math class
System.out.print("The maximum number is
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