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.

System.out.println([text])

Taking Input

It will take string input from the user

import java.util.Scanner; //import scanner class

// create an object of Scanner class
Scanner input = new Scanner(System.in);

// take input from the user
String varName = input.nextLine();

Primitive Type Variables

The eight primitives defined in Java are int, byte, short, long, float, double, boolean, and char those aren’t considered objects and represent raw values.

byte

byte is a primitive data type it only takes up 8 bits of memory.

age = 18;

long

long is another primitive data type related to integers. long takes up 64 bits of memory.

viewsCount = 3_123_456L;

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.

price = 100INR;

char

Char is a 16-bit integer representing a Unicode-encoded character.

letter = 'A';

boolean

The simplest primitive data type is boolean. It can contain only two values: true or false. It stores its value in a single bit.

isEligible = true;

int

int holds a wide range of non-fractional number values.

var1 = 256;

short

If we want to save memory and byte is too small, we can use short.

short var2 = 786;

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.

final float INTEREST_RATE = 0.04;

Arithmetic Expressions

These are the collection of literals and arithmetic operators.

Addition

It can be used to add two numbers

int x = 10 + 3;

Subtraction

It can be used to subtract two numbers

int x = 10 - 3;

Multiplication

It can be used to multiply add two numbers

int x = 10 * 3;

Division

It can be used to divide two numbers

int x = 10 / 3;
float x = (float)10 / (float)3;

Modulo Remainder

It returns the remainder of the two numbers after division

int x = 10 % 3;

Augmented Operators

Addition assignment

var += 10 // var = var + 10

Subtraction assignment

var -= 10 // var = var - 10

Multiplication assignment

var *= 10 // var = var * 10

Division assignment

var /= 10 // var = var / 10

Modulus assignment

var %= 10 // var = var % 10

Escape Sequences

It is a sequence of characters starting with a backslash, and it doesn’t represent itself when used inside string literal.

Tab

It gives a tab space

\t

Backslash

It adds a backslash

\\

Single quote

It adds a single quotation mark

\'

Question mark

It adds a question mark

\?

Carriage return

Inserts a carriage return in the text at this point.

\r

Double quote

It adds a double quotation mark

\"

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

// int x = 45;
double var_name = x;

Narrowing Type Casting

It means converting a higher data type into a lower

double x = 165.48
int var_name = (int)x;

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 of an if-else statement.

variable = (condition) ? expressionTrue : expressionFalse;

Switch Statements

It allows a variable to be tested for equality against a list of values (cases).

switch(expression) {
case a:
// code block
break;
case b:
// code block
break;
default:
// code block
}

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

while (condition) {
// code block
}

for Loop

for loop is used to run a block of code several times

for (initialization; termination; increment) {
statement(s)
}

for-each Loop

for(dataType item : array) {
...
}

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

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

Break statement

break keyword inside the loop is used to terminate the loop

break;

Continue statement

continue keyword skips the rest of the current iteration of the loop and returns to the starting point of the loop

continue;

Arrays

Arrays are used to store multiple values in a single variable

Declaring an array

Declaration of an array

String[] var_name;

Defining an array

Defining an array

String[] var_name = {''Harry", "Rohan", "Aakash"};

Accessing an array

Accessing the elements of an array

String[] var_name = {''Harry", "Rohan", "Aakash"};
System.out.println(var_name[index]);

Changing an element

Changing any element in an array

String[] var_name = {''Harry", "Rohan", "Aakash"};
var_name[2] = "Shubham";

Array length

It gives the length of the array

System.out.println(var_name.length);

Loop through an array

It allows us to iterate through each array element

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[2][3] matrix = new int[2][3]; 
matrix[0][0] = 10; 
// Shortcut 
int[2][3] 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);

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: "+(a+b)) ;
}
void sum (float x, float y)
{
System.out.println("Sum is: "+(a+b));
}
Public static void main (String[] args)
{
Calculate calc = new Calculate();
calc.sum (5,4); //sum(int x, int y) is method 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. And 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

String var_name = "Harry";
System.out.println("The length of the string is: " + var_name.length());

String Methods toUpperCase()

Convert the string into uppercase

String var_name = "Harry";
System.out.println(var_name.toUpperCase());

toLowerCase()

Convert the string into lowercase

String var_name = ""Harry"";
System.out.println(var_name.toLowerCase());

indexOf()

Returns the index of specified character from the string

String var_name = "Harry";
System.out.println(var_name.indexOf("a"));

concat()

Used to concatenate two strings

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

Math.max(25, 45);

min() method

It is used to find the smaller number among the two

Math.min(8, 7);

sqrt() method

It returns the square root of the supplied value

Math.sqrt(144);

random() method

It is used to generate random numbers

Math.random(); //It will produce random number b/w 0.0 and 1.0

int random_num = (int)(Math.random() * 101); //Random num b/w 0 and 100

Object-Oriented Programming

It is a programming approach that primarily focuses on using objects and classes. The objects can be any real-world entities.

object

An object is an instance of a Class.

className object = new className();

class

A class can be defined as a template/blueprint that describes the behavior/state that the object of its type support.

class ClassName {
// Fields
// Methods
// Constructors
// Blocks
}

Encapsulation

Encapsulation is a mechanism of wrapping the data and code acting on the data together as a single unit. In encapsulation, the variables of a class will be hidden from other classes and can be accessed only through the methods of their current class.

public class Person { 
private String name; // using private access modifier 

// Getter 
public String getName() { 
return name; 
} 

// Setter 
public void setName(String newName) { 
this.name = newName; 
} 
}

Inheritance

Inheritance can be defined as the process where one class acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.

class Subclass-name extends Superclass-name 
{ 
//methods and fields 
}

Polymorphism

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

// A class with multiple methods with the same name 
public class Adder { 
// method 1 
public void add(int a, int b) { 
System.out.println(a + b); 
} 

// method 2 
public void add(int a, int b, int c) { 
System.out.println(a + b + c); 
} 

// method 3 
public void add(String a, String b) { 
System.out.println(a + " + " + b); 
} 
} 

// My main class 
class MyMainClass { 
public static void main(String[] args) { 
Adder adder = new Adder(); // create a Adder object 
adder.add(5, 4); // invoke method 1 
adder.add(5, 4, 3); // invoke method 2 
adder.add("5", "4"); // invoke method 3 
} 
}

File Operations

File handling refers to reading or writing data from files. Java provides some functions that allow us to manipulate data in the files.

canRead method

Checks whether the file is readable or not

file.canRead()

createNewFile method

It creates an empty file

file.createNewFile()

canWrite method

Checks whether the file is writable or not

file.canWrite()

exists method

Checks whether the file exists

file.exists()

delete method

It deletes a file

file.delete()

getName method

It returns the name of the file

file.getName()

getAbsolutePath method

It returns the absolute pathname of the file

file.getAbsolutePath()

length Method

It returns the size of the file in bytes

file.length()

list Method

It returns an array of the files in the directory

file.list()

mkdir method

It is used to create a new directory

file.mkdir()

close method

It is used to close the file

file.close()

To write something in the file

import java.io.FileWriter; // Import the FileWriter class
import java.io.IOException; // Import the IOException class to handle errors

public class WriteToFile {
public static void main(String[] args) {
try {
FileWriter myWriter = new FileWriter("filename.txt");
myWriter.write("Laal Phool Neela Phool, Harry Bhaiya Beautiful");
myWriter.close();
System.out.println("Successfully wrote to the file.");
} catch (IOException e) {
System.out.println("An error occurred.");
e.printStackTrace();
}
}
}

Exception Handling

An exception is an unusual condition that results in an interruption in the flow of the program.

try-catch block

try statement allow you to define a block of code to be tested for errors. catch block is used to handle the exception.

try {
// Statements
}
catch(Exception e) {
// Statements
}

finally block

finally code is executed whether an exception is handled or not.

try {
//Statements
}
catch (ExceptionType1 e1) { 
// catch block
}
finally {
// finally block always executes
}