Java Cheatsheet
What is JAVA?
Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let programmers write once, run anywhere (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation. Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax of Java is similar to C and C++, but has fewer low-level facilities than either of them. The Java runtime provides dynamic capabilities (such as reflection and runtime code modification) that are typically not available in traditional compiled languages. As of 2019, Java was one of the most popular programming languages in use according to GitHub, particularly for client–server web applications, with a reported 9 million developers.
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.
byte age = 18;long
long is another primitive data type related to integers. long takes up 64 bits of memory.
long 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.
float price = 100f;char
Char is a 16-bit integer representing a Unicode-encoded character.
char 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.
boolean isEligible = true;int
int holds a wide range of non-fractional number values.
int num1 = 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 commentMulti-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 + 10Subtraction assignment
var -= 10 // var = var - 10Multiplication assignment
var *= 10 // var = var * 10Division assignment
var /= 10 // var = var / 10Modulus assignment
var %= 10 // var = var % 10Escape 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
\tBackslash
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.
\rDouble 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.
dataType variable = (condition) ? expression_if_true : expression_if_false;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 = {"Ram","Rahim","Shyam"};Accessing an array
Accessing the elements of an array
String[] var_name = {"Ram","Rahim","Shyam"};
System.out.println(var_name[index]);Changing an element
Changing any element in an array
String[] var_name = {"Ram","Rahim","Shyam"};
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 = {"Ram","Rahim","Shyam"};
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 = "Spaul";
System.out.println("The length of the string is: " + var_name.length());String Methods toUpperCase()
Convert the string into uppercase
String var_name = "Spaul";
System.out.println(var_name.toUpperCase());toLowerCase()
Convert the string into lowercase
String var_name = "Spaul";
System.out.println(var_name.toLowerCase());indexOf()
Returns the index of specified character from the string
String var_name = "Spaul";
System.out.println(var_name.indexOf("a"));concat()
Used to concatenate two strings
String var1 = "Spaul";
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 1.0int 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,
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
}
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