Java Cheat Sheet
Complete reference guide for Java with interactive examples and live playground links
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Basic Syntax
Hello World
The classic first program: outputting text to the console.
Java
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}Variables and Data Types
Variable declaration and basic data types in Java
Java
// Primitive types
int age = 30;
double height = 5.9;
boolean isActive = true;
char grade = 'A';
// Reference types
String name = "Java";
Integer count = 42;
Double price = 19.99;
Boolean isValid = true;
// Type conversion
int num = 42;
double decimal = (double) num; // Explicit casting
int backToInt = (int) decimal; // Explicit castingString Operations
Common string operations and formatting techniques
Java
String text = "Hello World";
// String methods
System.out.println(text.toUpperCase()); // HELLO WORLD
System.out.println(text.toLowerCase()); // hello world
System.out.println(text.replace("World", "Java")); // Hello Java
System.out.println(text.split(" ")); // [Hello, World]
System.out.println(text.length()); // 11
// String formatting
String name = "Alice";
int age = 25;
System.out.printf("My name is %s and I'm %d years old%n", name, age);
System.out.println(String.format("My name is %s and I'm %d years old", name, age));Arrays and Collections
Working with arrays and collections
Java
// Arrays
int[] numbers = {1, 2, 3, 4, 5};
String[] fruits = new String[3];
fruits[0] = "apple";
fruits[1] = "banana";
fruits[2] = "cherry";
// ArrayList
ArrayList<String> list = new ArrayList<>();
list.add("apple");
list.add("banana");
list.add("cherry");
list.remove("banana");
list.set(0, "orange");
// HashMap
HashMap<String, Integer> map = new HashMap<>();
map.put("apple", 1);
map.put("banana", 2);
map.put("cherry", 3);
map.remove("banana");
int value = map.get("apple");Input/Output
Basic input and output operations
Java
// Console output
System.out.println("Hello World");
System.out.printf("Name: %s, Age: %d%n", name, age);
System.out.print("No new line");
// Console input
Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.print("Enter your age: ");
int age = scanner.nextInt();
scanner.close();Comments
Different ways to add comments and documentation
Java
// Single line comment
/*
* Multi-line comment
* This is a block comment
*/
/**
* Javadoc comment
* Used for documentation
* @param name the name parameter
* @return the greeting string
*/
public String greet(String name) {
return "Hello, " + name;
}Control Flow
Conditional Statements
Conditional logic and decision making
Java
// If-else
int age = 18;
if (age >= 18) {
System.out.println("Adult");
} else if (age >= 13) {
System.out.println("Teenager");
} else {
System.out.println("Child");
}
// Ternary operator
String status = (age >= 18) ? "Adult" : "Minor";
// Switch statement
switch (age) {
case 18:
System.out.println("Just became adult");
break;
case 13:
System.out.println("Just became teenager");
break;
default:
System.out.println("Other age");
}Loops
Different types of loops and iterations
Java
// For loop
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
// Enhanced for loop
String[] fruits = {"apple", "banana", "cherry"};
for (String fruit : fruits) {
System.out.println(fruit);
}
// While loop
int count = 0;
while (count < 5) {
System.out.println(count);
count++;
}
// Do-while loop
int num = 0;
do {
System.out.println(num);
num++;
} while (num < 5);Exception Handling
Error handling with try-catch blocks and custom exceptions
Java
try {
// Risky code
int result = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Cannot divide by zero!");
} catch (Exception e) {
System.out.println("An error occurred: " + e.getMessage());
} finally {
System.out.println("This always executes");
}
// Custom exception
class ValidationException extends Exception {
public ValidationException(String message) {
super(message);
}
}
// Throwing exceptions
public void validate(int age) throws ValidationException {
if (age < 0) {
throw new ValidationException("Age cannot be negative");
}
}Object-Oriented Programming
Classes and Objects
Class definition, constructors, and methods
Java
public class Person {
// Fields
private String name;
private int age;
// Constructor
public Person(String name, int age) {
this.name = name;
this.age = age;
}
// Getters and Setters
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
// Methods
public void introduce() {
System.out.printf("Hi, I'm %s and I'm %d years old%n", name, age);
}
}
// Usage
Person person = new Person("Alice", 25);
person.introduce();Inheritance and Interfaces
Inheritance, abstract classes, and interfaces
Java
// Abstract class
abstract class Animal {
protected String name;
public Animal(String name) {
this.name = name;
}
public abstract void makeSound();
}
// Interface
interface Movable {
void move();
void stop();
}
// Class implementing interface and extending abstract class
class Dog extends Animal implements Movable {
public Dog(String name) {
super(name);
}
@Override
public void makeSound() {
System.out.println("Woof!");
}
@Override
public void move() {
System.out.println("Dog is running");
}
@Override
public void stop() {
System.out.println("Dog stopped");
}
}Collections Framework
Lists and Sets
Working with Lists and Sets
Java
// ArrayList
List<String> list = new ArrayList<>();
list.add("apple");
list.add("banana");
list.add("cherry");
list.remove("banana");
String first = list.get(0);
// LinkedList
List<Integer> linkedList = new LinkedList<>();
linkedList.add(1);
linkedList.add(2);
linkedList.add(3);
// HashSet
Set<String> set = new HashSet<>();
set.add("apple");
set.add("banana");
set.add("cherry");
set.add("apple"); // Duplicate, won't be added
// TreeSet
Set<Integer> treeSet = new TreeSet<>();
treeSet.add(3);
treeSet.add(1);
treeSet.add(2);
// Will be stored in order: 1, 2, 3Maps
Map implementations and operations
Java
// HashMap
Map<String, Integer> map = new HashMap<>();
map.put("apple", 1);
map.put("banana", 2);
map.put("cherry", 3);
// Accessing values
int value = map.get("apple");
boolean exists = map.containsKey("banana");
map.remove("cherry");
// TreeMap
Map<String, Integer> treeMap = new TreeMap<>();
treeMap.put("cherry", 3);
treeMap.put("apple", 1);
treeMap.put("banana", 2);
// Keys will be stored in order: apple, banana, cherry
// Iterating
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.printf("%s: %d%n", entry.getKey(), entry.getValue());
}Streams and Lambdas
Stream Operations
Working with Java Streams API
Java
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
// Filter and map
List<Integer> evenSquares = numbers.stream()
.filter(n -> n % 2 == 0)
.map(n -> n * n)
.collect(Collectors.toList());
// Reduce
int sum = numbers.stream()
.reduce(0, Integer::sum);
// Grouping
Map<String, List<Person>> byAge = people.stream()
.collect(Collectors.groupingBy(p -> p.getAge() >= 18 ? "Adult" : "Minor"));
// Parallel stream
int sum = numbers.parallelStream()
.mapToInt(Integer::intValue)
.sum();Lambda Expressions
Lambda expressions and functional programming
Java
// Functional interfaces
@FunctionalInterface
interface Calculator {
int calculate(int a, int b);
}
// Lambda expressions
Calculator add = (a, b) -> a + b;
Calculator subtract = (a, b) -> a - b;
Calculator multiply = (a, b) -> a * b;
// Method references
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
names.forEach(System.out::println);
// Comparator
List<Person> people = new ArrayList<>();
people.sort((p1, p2) -> p1.getName().compareTo(p2.getName()));
// or
people.sort(Comparator.comparing(Person::getName));Java - Interactive Developer Reference
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