Java Cheat Sheet

Java is a programming language and platform that has been widely used since its development by James Gosling in 1991. It follows the Object-oriented Programming concept and can run programs written on any OS platform. Java is a high-level, object-oriented, secure, robust, platform-independent, multithreaded, and portable programming language all those words are collectively called Java Buzzwords.

It is commonly used for programming web-based, Windows, enterprise, and mobile applications. This Java Cheat Sheet article has been written by experts in Java and is based on the experience of students who have recently undergone Java interviews.

This Core Java Cheat Sheet has been designed by Java experts, based on the experience of students who have recently undergone Java interviews. Whether you are a beginner or an experienced Java developer, this Java Cheat Sheet for competitive programming is a valuable resource for quickly accessing essential syntax, concepts, and best practices related to Java Programming .

Java Cheat Sheet for Competitive Programming: Basics to Advanced Concepts

• Java Programming Terminologies
• Java Basics
• Java Program to Print “Hello World”
• Dataypes in Java
• Java Comments
• Java Variables
• Access Modifiers in Java
• Operators in Java
• Identifiers in Java
• Control Flow in Java
• Methods in Java
• Java Input-output (I/O operations)
• Java Polymorphism
• Java Inheritance
• Java Maths Class
• Typecasting In Java
• Arrays in Java
• Strings in Java
• Java Regex
• Java Exception Handling
• Java Commands
• Java Generics
• Java Multithreading
• java Collections

1. Java Programming Terminologies

• JVM: executes the bytecode generated by the compiler.
• Bytecode: The Javac compiler of JDK compiles the Java source code into bytecode so that it can be executed by JVM.
• JDK: It is a complete Java development kit that includes everything including compiler, Java Runtime Environment (JRE), java debuggers, java docs, etc.
• JRE: allows the Java program to run, however, we cannot compile it.
• Garbage Collector: To delete or recollect that memory JVM has a program called Garbage Collector.
• Finalize method: this function is triggered by the garbage collector just before an object is deleted or destroyed.

2. Java Basics

Now, we will explore some of the fundamental concepts often utilized in the Java programming language.

Object – An object refers to an entity that possesses both behavior and state, such as a bike, chair, pen, marker, table, and car. These objects can be either tangible or intangible, including the financial system as an example of an intangible object.

There are three characteristics of an object:

• State: The data (value) of an object is represented by its state.
• Behaviour: The functionality of an object, such as deposit, withdrawal, and so on, is represented by the term behaviour.
• Identity: A unique ID is often used to represent an object’s identification. The value of the ID is hidden from the outside user. The JVM uses it internally to uniquely identify each object.

Class – A class is a collection of objects with similar attributes. It’s a blueprint or template from which objects are made. It’s a logical thing. It can’t be physical. In Java, a class definition can have the following elements:

• Modifiers: A class can be private or public, or it can also have a default access level
• class keyword: To construct a class, we use the class keyword.
• class name: The name of the class should usually start with a capital letter.
• Superclass (optional): If the class has any superclass, we use the extends keyword and we mention the name of the superclass after the class name.
• Interface (optional): If the class implements an interface, we use the implements keyword followed by the name of the interface after the class name.

Constructors: In Java, a constructor is a block of code similar to a method. Whenever a new class instance is created, the constructor is called. The memory allocation for the object only happens when the constructor is invoked.

There are two types of constructors in Java. They are as follows:-

Default Constructor – A default constructor is a type of constructor that does not require any parameters. When we do not declare a constructor for a class, the compiler automatically generates a default constructor for the class with no arguments.

Parameterised Constructor – A parameterized constructor is a type of constructor that requires parameters. It is used to assign custom values to a class’s fields during initialization.

Keyword – In Java, Reserved words are also known as keywords. These are particular terms that hold specific meanings. Java has 61 Reserved Keywords that are predefined and cannot be used as variable, object, or class names. Here’s a list of the keywords used in Java:-

3. Java Program to Print “Hello World”

Hello World!

Editing, Compiling, and Executing

4. Data Types in Java

Data Types in Java are the different values and sizes that can be stored in the variable according to the requirements.

Java Datatype are further two types:-

1. Primitive Data Type in Java

In Java, primitive data types serve as the foundation for manipulating data. They are the most basic types of data that the Java programming language uses. Java has several primitive data types, including:

Type

Size

Example Literals

Range of values

characters representation of ASCII values

1.23e100f , -1.23e-100f , .3f ,3.14F

upto 7 decimal digits

1.23456e300d , -123456e-300d , 1e1d

upto 16 decimal digits

2. Non-primitive Data Type

Non-primitive datatypes are created from primitive datatypes. Examples of non-primitive datatypes include arrays, stacks, and queues.

5. Java Comments

There are three types of comments in Java

1. Single Line comment

To comment on a single line of code, you can use a double forward slash “//” followed by your message. This syntax is commonly used for coding.

2. Multi-line comment

If you need to comment on multiple lines of code, you can utilize the syntax of a double forward slash “/*”. Simply enter your message between the two symbols, and complete the comment with “*/”. This is a widely used syntax in coding.

3. JavaDoc Type Comment

When working on a project or software package, it is often helpful to use this method as it can assist in creating a documentation page for reference. This page can provide information on available methods, their parameters, and more, making it a useful tool for learning about the project.

Syntax:

/** Comment starts 
*This is
*sample comment */

6. Java Variables

Variables are the containers that save the data values. Each variable is assigned according to the data type it is assigned.

Syntax:

data_type var_name;

Types of Variables

There are 3 types of Data types in Java as mentioned below:

1. Local Variables
A variable defined within a block or method or constructor is called a local variable.

2. Instance Variables
Instance variables are non-static variables and are declared in a class outside of any method, constructor, or block.

3. Static Variables
Static variables are also known as class variables. The static variables are declared using the static keyword within a class outside of any method, constructor, or block.

Below is the Example of the above topic:

Before Calling function count:0 10 After Calling function count:1 Instance variable value:11

7. Access Modifiers in Java

Access modifiers help to restrict the scope of a class, constructor, variable, method, or data member. It provides security, accessibility, etc to the user depending upon the access modifier used with the element

Types of Access Modifiers in Java

There are four types of access modifiers available in Java:

• Default – No keyword required
• Private
• Protected
• Public

8. Operators in Java

Comparison Operators are the Operators which return a boolean value as the result means the answer will be either true or false.

Less Than Equal to

Greater Than Equal to

Precedence and Associativity of Operator in Java

Here is the table representing the precedence order of Java operators from high to low as we move from top to bottom.

9. Identifiers in Java

The name that we give to the class, variable, and methods are formally called identifiers in Java. and for defining Identifier there are some rules of it we need to take care of that while defining Identifiers.

Rules of defining Java identifiers:-

• Valid Java Identifiers have strict guidelines to avoid compile-time errors. Similar rules apply to C and C++
• Only letters (both upper and lowercase), numbers, dollar signs, and underscores are allowed as identifiers in Java. Special characters like “@” are not permitted.
• Numbers should not be used to begin identifiers ([0-9]). �geeks,” for example, is not a valid Java identifier.
• Case matters when it comes to Java Identifiers. For example ?it’ and ?IT’ would be considered as different identifiers in Java.
• The length of the identifier is not limited, however, it is recommended that it be kept to a maximum of 4� letters.
• Using reserved words as identifiers is not allowed in Java. This includes the term “while,” as it is one of the 53 reserved terms.
• The length of the identifier is not limited, however, it is recommended that it be kept to a maximum of 4� letters.

10. Control Flow in Java

1. If, else-if, else

2. Nested if – else
A nested if is an if statement that is the target of another if or else. Nested if statements mean an if statement inside an if statement.

i is smaller than 15 i is smaller than 12 too

2. Switch Statement

Friday

Loops in Java

Loops are used for performing the same task multiple times. There are certain loops available in Java as mentioned below:

1. For Loop
2. While Loop
3. do-while

                          Java

                                 Java

1 2 3 4 5

11. Methods in Java

Java Methods are collections of statements that perform some specific task and return the result.

Syntax of Method

  ( list_of_parameters)

 //body
>

Below is the implementation of the above method:

0 1 2 1 2 3 2 3 4

12. Java Input-output (I/O operations)

We can only print using System.out but can use different print varieties in it:

1. print

The cursor remains on the same line

System.out.print(" GFG " + x);

2. println

Cursor Moves to a new line

System.out.println(" GFG " + x);

3. printf

The cursor remains on the same line

System.out.printf(" GFG %d",x);

Formatted Print

System.out.printf("%7.5f", Math.PI);

Explanation of the above statement:

7 is the Field width and .5 is the precision fo the floating number printed . So, answer will be 3.14159

Taking Input in Java

1. Parsing Command-line arguments

We can take input using the Command line simp

javac GFG.java
java GFG This is just to check 2

Output:

This
is 
just 
to 
Check
2

2. Buffer Reader Class

InputStreamReader() is a function that converts the input stream of bytes into a stream of characters so that it can be read as BufferedReader expects a stream of characters.

Below is the implementation of the above method:

Output:

ABC
11
Entered String : ABC
Entered Integer : 11

3. Scanner Class

Syntax:

Scanner scn = new Scanner(System.in);

Below is the implementation of the above method:

Output:

ABC
65
Name :ABC
Marks :65

13. Java Polymorphism

Polymorphism: It is the ability to differentiate between entities with the same name efficiently.

Compile-time polymorphism: Static polymorphism, also called compile-time polymorphism, is achieved through function overloading in Java. Static polymorphism, also called compile-time polymorphism, is achieved through function overloading in Java.

Method Overloading: If there are multiple functions that have the same name but different parameters, it is known as overloading. Alterations in the number or type of arguments can result in the overloading of functions.

Example:

3 7.370000000000001

14. Java Inheritance

Inheritance: It is the mechanism in Java by which one class is allowed to inherit the features (fields and methods) of another class.

Salary : 70000 Benefits : 15000

15. Java Maths Class

In Java, there exists a Math Library that can simplify intricate calculations

Library:

import java.lang.Math;

Use:

Math.function_name

Returns or Calculates

minimum of a and b

maximum of a and b

toRadians( int θ )

Convert angle in degrees(θ) to radians

toDegrees( int θ )

Convert angle int radians(θ) to degrees

exponential value e a

natural log -> log _e a

pow( int a, int b )

round to the nearest integer

returns the random value in [0,1)

the square root of a

value of e (constant value)

Value of π(constant value)

Note: All the functions mentioned above can be used with either data-types not bounded any single data-types.

16. Typecasting In Java

1. Type Casting in Java

Type Casting is a method in which we convert from one data type to another.

2. Type Conversion in Java

Type Conversion in Java is a technique where we can convert from double, int, and float to double.

Below is the implementation of the above methods:

                    display the casted value  Java

17. Arrays in Java

Arrays are the type of data structure that can store data of similar data types. Arrays are allocated in contiguous memory allocation with a fixed size.

Syntax:

// Both Methods are correct
int arr[]=new int[20];
int[] arr=new int[20];

Below is the implementation of the above method:

arr[0] :10 arr[1] :20 arr[2] :30 arr[3] :40 arr[4] :50

Multidimensional Arrays in Java

Arrays are not bounded to be single-dimensional but can store elements in multidimensional.

Syntax:

int[][] arr= new int[3][3];
int arr[][]=new int[3][3];

Below is the implementation of the above method:

1 2 3 4 5 6 7 8 9

18. Strings in Java

Strings are the type of objects that can store the character of values. A string acts the same as an array of characters in Java.

Syntax:

String abc=" ";

CharSequence Interface in Java

1. StringBuffer

StringBuffer s = new StringBuffer("GeeksforGeeks");

2. StringBuilder

StringBuilder str = new StringBuilder();
str.append("GFG");

3. String Tokenizer

public StringJoiner(CharSequence delimiter)

19. Java Regex

Regular Expressions, or Regex for short, is a Java API that allows users to define String patterns for searching, manipulating, and modifying strings. It is commonly used for setting limits on various areas of strings such as email validation and passwords. The java.util.regex package contains regular expressions and consists of three classes and one interface. The following table lists the three classes included in the java.util.regex package:

Pattern class: This class does not have any public constructors. Instead, it consists of a group of regular expressions that can be utilized to define different types of patterns. To do this, simply execute the compile() method with a regular expression as the first input. After execution, the method will return a pattern.

The table below provides a list of the methods in this class along with their descriptions.

Matcher Class: T o evaluate previously described patterns through match operations on an input string in Java, the ‘object’ is utilized. No public constructors are defined here. One can execute a matcher() on any pattern object to achieve this. The table below displays the methods present in this class along with their descriptions:To evaluate previously described patterns through match operations on an input string in Java, the ‘object’ is utilized. No public constructors are defined here. One can execute a matcher() on any pattern object to achieve this.

The table below displays the methods present in this class along with their descriptions:

20. Java Exception Handling

Exception: – An Exception is an unexpected error that occurs during the run-time of a program.

Error vs Exception: What is the difference?

When a program encounters an error, it means there is a significant issue that a well-designed program should not try to fix. On the other hand, an exception indicates a particular situation that a well-designed program should try to handle.

Types of Exceptions:-

Checked Exception :- This mainly includes IO Exceptions and Compile time Exceptions

Unchecked Exceptions: – This covers both Runtime Exceptions and Null Pointer Exceptions.

Handle Exceptions

try block : The try block comprises a set of statements that may throw an exception.

Catch Block: When there is an uncertain condition in the try block, the catch block comes in handy to handle it. It is mandatory to have a catch block right after the try block to handle any exceptions that may be thrown by the try block.

Finally Block: When programming in Java, the finally block is a section of code that will always run, regardless of whether or not an exception has been caught. If there is a catch block following a try block, it will be executed before the finally block. However, if there is no catch block, the finally block will be executed immediately after the try block.

Example:-

try
// block of code to monitor for errors
// the code you think can raise an exception
> catch (ExceptionType1 exOb)
// exception handler for ExceptionType1
> catch (ExceptionType2 exOb)
// exception handler for ExceptionType2
>
// optional
finally < // block of code to be executed after try block ends
>

final vs finally vs finalize

Below is a table that outlines the distinctions between the terms final, finally, and finalize:

final

• A final keyword can be used with classes, methods, and variables.
• A final class cannot be inherited.
• A final method cannot be overridden.
• A final variable cannot be reassigned.

finally

• A finally block is always executed, regardless of whether an exception is thrown or not.
• The finally block is executed after the try block and catch block, but before the program control returns to the calling method.
• The finally block can be used to close resources, such as files and database connections.

finalize

• The finalize() method is called by the garbage collector when an object is no longer needed.
• The finalize() method can be used to perform cleanup operations, such as releasing resources or writing data to a file.
• The finalize() method is not guaranteed to be called, so it should not be used to perform critical operations.

throw keyword:- When using Java, the throw keyword is utilized to throw an exception from a block of code or a method. It is possible to throw either a checked or unchecked exception. The throw keyword is frequently used for throwing custom exceptions.

throws keyword:- If a try/catch block is not present, the throws keyword can be used to manage exceptions. This keyword specifies the specific exceptions that a method should throw if an exception happens.

21. Java Commands

Here are the most commonly used Java commands:

• java -version : Displays the version of the Java Runtime Environment (JRE) that is installed on your computer.
• java -help : Displays a list of all of the Java commands.
• java -cp : Specifies the classpath for the Java program that you are running.
• java -jar : Runs a Java Archive (JAR) file.
• java -D : Sets a system property.
• java -X : Specifies a non-standard option for the Java Virtual Machine (JVM).

Here are some other useful Java commands:

• javac : Compiles a Java source file into bytecode.
• javap : Disassembles a Java class file.
• jdb : A Java debugger.
• jconsole : A Java monitoring and management tool.
• jvisualvm : A Java profiling and diagnostic tool.

22. Java Generics

Generics means parameterized types. The idea is to allow type (Integer, String, … etc., and user-defined types) to be a parameter to methods, classes, and interfaces. Using Generics, it is possible to create classes that work with different data types. An entity such as class, interface, or method that operates on a parameterized type is a generic entity.

Types of Java Generics

Generic Method: Generic Java method takes a parameter and returns some value after performing a task. It is exactly like a normal function, however, a generic method has type parameters that are cited by actual type. This allows the generic method to be used in a more general way. The compiler takes care of the type of safety which enables programmers to code easily since they do not have to perform long, individual type castings.

// To create an instance of generic class 
BaseType obj = new BaseType ()

Generic Functions: We can also write generic functions that can be called with different types of arguments based on the type of arguments passed to the generic method. The compiler handles each method.

Output

java.lang.Integer = 11 java.lang.String = GeeksForGeeks java.lang.Double = 1.0

23. Java Multithreading

Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such program is called a thread. So, threads are light-weight processes within a process.

Threads can be created by using two mechanisms :

• Extending the Thread class
• Implementing the Runnable Interface

Example: – Thread creation by extending the Thread class

To create a new thread in Java, we can extend the java.lang.Thread class and override its run() method. This is where the thread’s execution starts. Then, we can create an instance of our class and call the start() method to begin the execution of the thread. The start() method will then call the run() method of the Thread object.

Output

Thread 15 is running Thread 17 is running Thread 14 is running Thread 12 is running Thread 13 is running Thread 18 is running Thread 11 is running Thread 16 is running

24. Java Collections

The collection is a framework in Java that provides an architecture to store and manipulate objects in Java. It contains the interface as mentioned below:

• List Interface
• Queue Interface
• Deque Interface
• Set Interface
• SortedSet Interface
• Map Interface
• Collection Interface
• Iterable Interface

Syntax of the Interfaces:

List al = new ArrayList<> ();

List ll = new LinkedList<> ();

List v = new Vector<> ();

Queue pq = new PriorityQueue<> ();

Queue ad = new ArrayDeque<> ();

Deque ad = new ArrayDeque<> ();

Set hs = new HashSet<> ();

Set lhs = new LinkedHashSet<> ();

Set ts = new TreeSet<> ();

Sorted Set Interface

SortedSet ts = new TreeSet<> ();

Map hm = new HashMap<> ();

Map tm = new TreeMap<> ();

Why Use Java?

Java is simple for programmers to learn. Java is frequently chosen as the first programming language to learn. Furthermore, the sector continues to benefit from Java’s prominence. The majority of websites and apps in the government, healthcare, defence, and educational sectors continue to use Java technology. Java is thus worthwhile to learn and use. If you choose a career in Java, you can follow a number of different career routes. Almost anything that Java can accomplish.

Why is Java so Popular?

Because Java includes a unique tool called the Java Virtual Machine that ensures the code functions consistently everywhere, it is feasible for Java to execute without any issues on many types of computers and machines. This makes Java a popular programming language. A rule in Java also known as WORA states that code only has to be written once and may execute anywhere. Java’s high level of security, which shields the code from hackers, viruses, and other dangers, is another factor in its popularity. Java also enables the creation of many jobs that may run concurrently within a programme, making it quicker and more effective. Java provides a clever method for generating code that primarily concentrates on.

Features of Java

• Java is an Object Oriented Programming language .
• Java is Platform Independent because it comes with its own platform to run applications.
• Simple to learn programming language because doesn’t contain Pointers and operator overloading.
• Java is secure because the Java program runs on Java virtual machine(JVM) so, if there is any problem occurred in the program it will remain inside the JVM only, it will not affect the operating system.
• Java is Architecture neutral because it is independent of the platform so we do not have to design the program for different devices differently.
• Java is portable because, after the compilation of any Java program, it generates a bytecode that can run on any machine which has JVM.
• Java is Robust means Java comes with automatic garbage collection management and strong exception handling.
• Java supports multithreading , It is possible in Java that we can divide a program into two or many subprograms and run these programs/threads at the same time.

Applications of Java Programming language

• Mobile Applications
• Desktop GUI Applications
• Artificial intelligence
• Scientific Applications
• Cloud Applications
• Embedded Systems
• Gaming Applications

Conclusion

This Java Cheat Sheet serves as a quick reference guide for both beginners and experienced developers working with Java. By summarizing essential syntax, key concepts, and common commands, it aims to enhance your productivity and ensure you have the critical information at your fingertips. Whether you’re preparing for an interview, working on a project, or simply refreshing your knowledge, this cheat sheet is designed to make your Java development process more efficient and effective.

Java Cheat Sheet – FAQs

1. Why Use Java?

Java is simple to learn programming language because doesn’t contain concepts like: Pointers and operator overloading and it is secure and portable.

2. What is the difference between C++ and Java?

C++	JAVA
C++ is platform dependent.	Java is platform-independent.
C++ uses a compiler only.	Java uses a compiler and interpreter both.
C++ support pointers and operator overloading.	Java doesn’t support pointers and operator overloading concepts.
C++ does not support the multithreading concept.	Java supports the multithreading concept.

3. What is the most important feature of Java?

The most important features of Java are Platform Independent and Object Oriented. That’s why Java is the most popular among high-level programming languages.

4. What are the main uses of Java?

1. Web Development
2. Software Development
3. Mobile Application Development
4. Distributed Applications
5. Web servers
6. Enterprise Application

5. How is Java useful in real life?

Java is useful in developing real-world web or mobile applications, and also useful to build servers. There are a lot of things you can do in Java, there are multiple libraries, and using that you can do anything. The application built in Java can be distributed over the internet or on any network.

6. What is the scope of Java?

The developer community of Java is so vast and is the strength of the Java language. This technology is growing at a fast pace and job opportunities are also increasing for Java developers having good knowledge of Java technologies. As Java is scalable, you can find Java in mobile applications, software applications, servers, and web applications.