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Catalog / Java Programming Cheatsheet

Java Programming Cheatsheet

A comprehensive cheat sheet covering essential Java syntax, data structures, and commonly used libraries, designed to boost your coding efficiency.

Java Basics

Basic Syntax

//

Single-line comment.

/* ... */

Multi-line comment.

public class Main { ... }

Basic class definition.

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

Main method - entry point of execution.

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

Prints text to the console.

int x = 5;

Integer variable declaration and initialization.

Data Types

int

Integer (4 bytes).

double

Double-precision floating-point number (8 bytes).

boolean

Boolean (true/false).

String

Sequence of characters.

char

Single character.

float

Single-precision floating-point number (4 bytes).

Operators

+, -, *, /, %

Arithmetic operators (addition, subtraction, multiplication, division, modulus).

==, !=, >, <, >=, <=

Comparison operators (equal to, not equal to, greater than, less than, greater than or equal to, less than or equal to).

&&, ||, !

Logical operators (and, or, not).

=

Assignment operator.

+=, -=, *=, /=, %=

Compound assignment operators.

++, --

Increment and decrement operators.

Control Flow

Conditional Statements

if statement:

if (condition) {
 // code to execute if condition is true
}

if-else statement:

if (condition) {
 // code to execute if condition is true
} else {
 // code to execute if condition is false
}

if-else if-else statement:

if (condition1) {
 // code to execute if condition1 is true
} else if (condition2) {
 // code to execute if condition2 is true
} else {
 // code to execute if all conditions are false
}

switch statement:

switch (expression) {
 case value1:
  // code to execute if expression == value1
  break;
 case value2:
  // code to execute if expression == value2
  break;
 default:
  // code to execute if expression doesn't match any case
}

Looping Statements

for loop:

for (initialization; condition; increment/decrement) {
 // code to execute repeatedly
}

while loop:

while (condition) {
 // code to execute repeatedly while condition is true
}

do-while loop:

do {
 // code to execute at least once
} while (condition);

enhanced for loop (for-each loop):

for (DataType item : collection) {
 // code to execute for each item in the collection
}

Branching Statements

break

Terminates the loop or switch statement.

continue

Skips the current iteration and proceeds to the next iteration of the loop.

return

Exits from the current method.

Object-Oriented Programming

Classes and Objects

Class Definition:

public class Dog {
 // Fields (attributes)
 String breed;
 int age;

 // Constructor
 public Dog(String breed, int age) {
  this.breed = breed;
  this.age = age;
 }

 // Method (behavior)
 public void bark() {
  System.out.println("Woof!");
 }
}

Object Creation:

Dog myDog = new Dog("Golden Retriever", 3);
myDog.bark(); // Output: Woof!

Inheritance

Basic Inheritance:

class Animal {
 String name;
 public void eat() {
  System.out.println("Animal is eating");
 }
}

class Dog extends Animal {
 public void bark() {
  System.out.println("Dog is barking");
 }
}

Using Inheritance:

Dog myDog = new Dog();
myDog.name = "Buddy";
myDog.eat();  // Output: Animal is eating
myDog.bark(); // Output: Dog is barking

Polymorphism

Method Overriding:

class Animal {
 public void makeSound() {
  System.out.println("Generic animal sound");
 }
}

class Dog extends Animal {
 @Override
 public void makeSound() {
  System.out.println("Woof!");
 }
}

Using Polymorphism:

Animal myAnimal = new Dog();
myAnimal.makeSound(); // Output: Woof!

Encapsulation

Example of Encapsulation:

class BankAccount {
 private double balance;

 public double getBalance() {
  return balance;
 }

 public void deposit(double amount) {
  balance += amount;
 }

 public void withdraw(double amount) {
  if (balance >= amount) {
  balance -= amount;
  } else {
  System.out.println("Insufficient funds");
  }
 }
}

Using Encapsulation:

BankAccount account = new BankAccount();
account.deposit(1000);
account.withdraw(500);
System.out.println("Balance: " + account.getBalance()); // Output: Balance: 500.0

Collections Framework

Lists

ArrayList:

import java.util.ArrayList;
import java.util.List;

List<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
System.out.println(list.get(0)); // Output: Apple

LinkedList:

import java.util.LinkedList;
import java.util.List;

List<String> linkedList = new LinkedList<>();
linkedList.add("Car");
linkedList.add("Bike");
System.out.println(linkedList.get(1)); // Output: Bike

Sets

HashSet:

import java.util.HashSet;
import java.util.Set;

Set<String> set = new HashSet<>();
set.add("Red");
set.add("Blue");
System.out.println(set.contains("Red")); // Output: true

TreeSet:

import java.util.TreeSet;
import java.util.Set;

Set<String> treeSet = new TreeSet<>();
treeSet.add("Cat");
treeSet.add("Dog");
System.out.println(treeSet); // Output: [Cat, Dog] (sorted order)

Maps

HashMap:

import java.util.HashMap;
import java.util.Map;

Map<String, Integer> map = new HashMap<>();
map.put("Alice", 25);
map.put("Bob", 30);
System.out.println(map.get("Alice")); // Output: 25

TreeMap:

import java.util.TreeMap;
import java.util.Map;

Map<String, Integer> treeMap = new TreeMap<>();
treeMap.put("Charlie", 35);
treeMap.put("David", 40);
System.out.println(treeMap); // Output: {Charlie=35, David=40} (sorted order by key)
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