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);
    }
}

Escape Sequences

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

Tab

It gives a tab space.

public class HelloWorld {
    public static void main(String args[]) {
        System.out.print("\t");
    }
}

Backslash

It adds a backslash.

public class HelloWorld {
    public static void main(String args[]) {
        System.out.print("\\");
    }
}

Single quote

It adds a single quotation mark.

public class HelloWorld {
    public static void main(String args[]) {
        System.out.print("\'");
    }
}

Question mark

It adds a question mark.

public class HelloWorld {
    public static void main(String args[]) {
        System.out.print("?");
    }
}

Carriage return

Inserts a carriage return in the text at this point.

public class HelloWorld {
    public static void main(String args[]) {
        System.out.print("\r");
    }
}

Double quote

It adds a double quotation mark.

public class HelloWorld {
    public static void main(String args[]) {
        System.out.print("\"");
    }
}

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

Basics

Boilerplate

Showing Output

Taking Input

Primitive Type Variables

byte

long

float

char

int

short

Comments

Single line comment

Multi-line comment

Constants

Arithmetic Expressions

Addition

Subtraction

Multiplication

Division

Modulo Remainder

Augmented Operators

Addition assignment

Subtraction assignment

Multiplication assignment

Division assignment

Modulus assignment

Escape Sequences

Tab

Backslash

Single quote

Question mark

Carriage return

Double quote

Type Casting

Widening Type Casting

Narrowing Type Casting

Decision Control Statements

if Statement

if-else Statement

if else-if Statement

Ternary Operator

Syntax

Example

Switch Statements

Iterative Statements

while Loop

for Loop

for-each Loop

do-while Loop

Break statement

Continue statement

Arrays

Declaring an array

Defining an array

Accessing an array

Changing an element

Array length

Loop through an array

Multi-dimensional Arrays

Methods

Declaration

Calling a method

Example

Method Overloading

Recursion

Strings

Creating String Variable

String Length

String Methods toUpperCase()

toLowerCase()

indexOf()

concat()

Math Class

Methods max() method

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