java_cheat_sheet

Core Java Concepts Cheat Sheet by 😸CattoDoesCode

Table of Contents

• Comments
• Syntax
• Variables
• String
• If...Else
• Switch
• Loops
• Array
• Method
• References

Syntax

Main.java

public class Main {

    public static void main(String[] args) {
	    // write your code here
    }
}

Printing a line to the console

System.out.println("Hello World");

Comments

Single-line Comment

// This is a sample comment.

Multi-line Comment

/* This is a sample
multi-line comment. */

Variables

Primitive Data Types

Data Type	Size	Description	Range
byte	8-bit	signed two's complement integer	-128 to 127
short	16-bit	signed two's complement integer	-32,768 to 32,767
int	32-bit	signed two's complement integer	-231 to 231-1 (i.e., -2147483648 to 2147483647)
long	64-bit	two's complement	-263 to 263-1 (i.e., -9223372036854775808 to `9223372036854775807
float	32-bit	IEEE 754 floating point	Negative range: -3.4028235E+38 to -1.4E-45 Positive range: 1.4E-45 to 3.4028235E+38
double	64-bit	IEEE 754 floating point	Negative range: -1.7976931348623157E+308 to -4.9E-324 Positive range: 4.9E-324 to 1.7976931348623157E+308
boolean	1-bit	has only two possible values	true or false
char	16-bit	single 16-bit Unicode character	'\u0000' (or 0) to '\uffff' (or 65,535 inclusive)

String in Java is not a primitive data type but an object that are backed by char array.

Declaring a Variable syntax:

<DataType> <variableName>;

sample code:

int numVariable; // variable name is in camelCase
char my_variable; // variable name is in snake_case

Default Values

Data Type	Default Value
byte	0
short	0
int	0
long	0L
float	0.0f
double	0.0d
char	'\u0000'
boolean	false
String(or any object)	null

Assigning a Variable a value:

<variableName> = <value or expression>;

sample code:

numVariable = 100; 
my_variable = 'x'; // single quotation marks ('') for char and double quotation marks for string("")

String

Creating a variable of type String

String myString = "Java";

String Methods

Method	Description
lenth()	returns the length of characters inside the string
toUpperCase	converts a string into upper case
toLowerCase	converts a string into lower case
indexOf("locate")	returns the index (the position) of the first occurrence of a specified text in a string (including whitespace)

for more string methods: https://www.w3schools.com/java/java_ref_string.asp

sample code:

public class Main {

    public static void main(String[] args) {
        String vowels = "AEIOU";
        System.out.println("length of vowels: " + vowels.length());
    }
}

output:

length of vowels: 5

Conditionals

If, Else if, Else statement

syntax:

if (condition1) {
    // block of code to be executed if condition1 is true
} 
else if (condition2) {
    // block of code to be executed if the condition1 is false and condition2 is true
} else {
    // block of code to be executed if the condition1 is false and condition2 is false
}

sample code:

public class Main {
    public static void main(String[] args) {

        int x = 100;
        int y = 50;

        if (x < y) {
            System.out.println("x is less than y");
        }
        else if (x == y) {
            System.out.println("x is equal to y");
        }
        else {
            System.out.println("x is greater than y");
        }
    }
}

output:

x is greater than y

ternary operator

syntax:

variable = (condition) ? expressionTrue : expressionFalse  

sample code:

public class Main {
    public static void main(String[] args) {

        int x = 100;
        int y = 50;

        String result = (x < y) ? "x is less than y" : "x is greater than y";
        System.out.println(result);

    }
}

output:

x is greater than y

Switch

syntax:

switch(expression) {
    case x:
        // code block
        break;
    case y:
        // code block
        break;
    default:
        // code block
}

The break keyword terminates a switch statement or a loop statement.

sample code:

public class Main {
    public static void main(String[] args) {

        int choice = 3;
        switch(choice) {
            case 1:
                System.out.println("one");
                break;
            case 2:
                System.out.println("two");
                break;
            case 3:
                System.out.println("three");
                break;
        }

    }
}

enhanced switch statement

public class Main {
    public static void main(String[] args) {

        int choice = 3;
        switch (choice) {
            case 1 -> System.out.println("one");
            case 2 -> System.out.println("two");
            case 3 -> System.out.println("three");
        }

    }
}

output:

three

Loops

for loop

syntax:

for ((initialization); (condition); (increment)) {
    // code block to be executed
}

sample code:

public class Main {
    public static void main(String[] args) {

        for (int i = 0; i <= 10; i+=1) {
            System.out.println(i);
        }

    }
}

output:

0
1
2
3
4
5
6
7
8
9
10

while loop

syntax:

while(condition) {
    // code block to be executed    
}

sample code:

public class Main {
    public static void main(String[] args) {

        int i = 0;
        while(i <= 10) {
            System.out.println(i);
            i++;
        }

    }
}

output:

0
1
2
3
4
5
6
7
8
9
10

do while loop

syntax:

do {
        // code block to be executed
}
while (condition);

sample code:

public class Main {
    public static void main(String[] args) {

        int i = 0;
        do {
            System.out.println(i);
            i++;
        }
        while(i <= 10);

    }
}

output:

0
1
2
3
4
5
6
7
8
9
10

Array

Creating an Array

syntax:

<dataType>[] <variable> = <value>;

sample code:

String[] ones = ["one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"]

Access Array elements

public class Main {
    public static void main(String[] args) {

        int[] tens = {10, 20, 30, 40, 50, 60, 70, 80, 100};
        System.out.println(tens[2]);

    }
}

output:

30

Change Array elements

public class Main {
    public static void main(String[] args) {

        int[] tens = {10, 20, 30, 40, 50, 60, 70, 80, 100};
        tens[8] = 90;
        System.out.println(tens[8]);

    }
}

output:

90

Loop through an Array

public class Main {
    public static void main(String[] args) {

        String[] days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};

        for (String d : days) {
            System.out.println(d);
        }
        
    }
}

output:

Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday

Method

Creating a Method syntax

static <void> <methodName>() {
    // code to be executed    
}

keywords:

static means that the method belongs to the Main class and not an object of the Main class. void means that this method does not have a return value.

or

static <dataType> <methodName>() {
    // code to be executed  
    return <value or expression>  
}

If you want the method to return a value, you can use a primitive data type (such as int, char, etc.) instead of void, and use the return keyword inside the method:

void method

sample code:

public class Main {
    static void hello_world(){
        System.out.println("Hello, world!");
    }

    public static void main(String[] args) {
        hello_world();
    }
}

output:

Hello, world!

value-returning method

sample code:

public class Main {
    static String greet_user(String user){
        return "Hello, " + user;
    }

    public static void main(String[] args) {
        System.out.println(greet_user("Bob"));
    }
}

output:

Hello, Bob

References

• Java Documentation | docs.oracle.com
• mathcenter.oxford.emory.edu
• w3schools.com
• javatpoint.com
• geeksforgeeks.org