This Java Cheat Sheet is designed to serve as a quick reference guide for Java developers, summarizing key concepts, syntax, and best practices. Whether you're a beginner learning Java or an experienced developer needing a quick reminder, this cheat sheet aims to assist you in your Java programming journey.
A.2 Java Basics
A.2.1 Hello World Program
// Example: Hello World Program
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
A.2.2 Data Types
Primitive Data Types:
- byte, short, int, long (integer types)
- float, double (floating-point types)
- char (character type)
- boolean (boolean type)
Reference Data Types:
- Objects (e.g., String, ArrayList)
- Arrays
A.2.3 Variables
// Example: Variables
int age = 25;
double salary = 50000.50;
char grade = 'A';
boolean isStudent = true;
String name = "John";
A.2.4 Constants
// Example: Constants
final double PI = 3.14;
A.2.5 Operators
Arithmetic Operators: +, -, *, /, %
Comparison Operators: ==, !=, <, >, <=, >=
Logical Operators: &&, ||, !
A.3 Control Flow
A.3.1 Conditional Statements
if Statement
// Example: if Statement
if (condition) {
// code
} else if (anotherCondition) {
// code
} else {
// code
}
switch Statement
// Example: switch Statement
switch (variable) {
case value1:
// code
break;
case value2:
// code
break;
default:
// code
}
A.3.2 Looping Statements
for Loop
// Example: for Loop
for (int i = 0; i < 5; i++) {
// code
}
while Loop
// Example: while Loop
while (condition) {
// code
}
do-while Loop
// Example: do-while Loop
do {
// code
} while (condition);
A.4 Methods
A.4.1 Method Declaration
// Example: Method Declaration
public returnType methodName(parameterType parameterName) {
// code
return result;
}
A.4.2 Calling a Method
// Example: Calling a Method
int result = methodName(argument);
A.4.3 Overloading
// Example: Overloading
void printMessage() {
// code
}
void printMessage(String message) {
// code
}
A.5 Object-Oriented Programming (OOP)
A.5.1 Classes and Objects
// Example: Classes and Objects
public class MyClass {
// fields (variables)
private int age;
// constructor
public MyClass(int initialAge) {
this.age = initialAge;
}
// methods
public void printAge() {
System.out.println("Age: " + age);
}
}
// Creating an object
MyClass myObject = new MyClass(30);
// Accessing fields and methods
myObject.printAge();
A.5.2 Inheritance
// Example: Inheritance
public class ChildClass extends ParentClass {
// code
}
A.5.3 Polymorphism
// Example: Polymorphism
public interface Shape {
void draw();
}
public class Circle implements Shape {
public void draw() {
// code to draw a circle
}
}
public class Square implements Shape {
public void draw() {
// code to draw a square
}
}
// Using polymorphism
Shape circle = new Circle();
Shape square = new Square();
A.6 Exception Handling
A.6.1 Try-Catch Block
// Example: Try-Catch Block
try {
// code that may throw an exception
} catch (ExceptionType1 e1) {
// handle exception type 1
} catch (ExceptionType2 e2) {
// handle exception type 2
} finally {
// code to be executed regardless of exceptions
}
A.6.2 Throwing Exceptions
// Example: Throwing Exceptions
if (condition) {
throw new CustomException("This is a custom exception.");
}
A.7 Collections
A.7.1 List
// Example: List
List<String> list = new ArrayList<>();
list.add("Item 1");
list.add("Item 2");
list.add("Item 3");
for (String item : list) {
System.out.println(item);
}
A.7.2 Set
// Example: Set
Set<Integer> set = new HashSet<>();
set.add(10);
set.add(20);
set.add(30);
for (int number : set) {
System.out.println(number);
}
A.7.3 Map
// Example: Map
Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
map.put("Two", 2);
map.put("Three", 3);
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + ": " + entry.getValue());
}
A.8 Input/Output (I/O)
A.8.1 Reading from Console
// Example: Reading from Console
Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.println("Hello, " + name + "!");
A.8.2 File I/O
// Example: File I/O
// Reading from a file
try (BufferedReader reader = new BufferedReader(new FileReader("filename.txt"))) {
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
} catch (IOException e) {
e.printStackTrace();
}
// Writing to a file
try (BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"))) {
writer.write("Hello, World!");
} catch (IOException e) {
e.printStackTrace();
}
A.9 Java 8 Features
A.9.1 Lambda Expressions
// Example: Lambda Expressions
// Syntax: (parameters) -> expression
MathOperation add = (a, b) -> a + b;
System.out.println("Result: " + add.operation(5, 3));
A.9.2 Stream API
// Example: Stream API
List<String> programmingLanguages = Arrays.asList("Java", "Python", "JavaScript", "C++", "Ruby");
// Using stream to filter and print languages starting with 'J'
programmingLanguages.stream()
.filter(language -> language.startsWith("J"))
.forEach(System.out::println);
A.9.3 Optional Class
// Example: Optional Class
Optional<String> language = Optional.of("Java");
System.out.println("Language: " + language.orElse("Unknown"));
A.10 Best Practices
A.10.1 Code Organization
- Use meaningful names for packages, classes, methods, and variables.
- Follow the Java Naming Conventions.
A.10.2 Code Formatting
- Use consistent indentation (preferably 4 spaces).
- Place opening braces on the same line as control statements.
A.10.3 Comments
- Use JavaDoc comments for documenting classes and methods.
- Write comments for complex or non-intuitive code.
A.10.4 Exception Handling
- Catch specific exceptions rather than using a generic
Exceptioncatch. - Log exceptions using a logging framework.
A.10.5 Testing
- Write unit tests for each method in your codebase.
- Integrate automated builds and tests into a continuous integration system.
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