Quick Revision Notes — Java Programming

Concise Java quick revision notes covering all major topics - OOP, Collections, Multithreading, JDBC, Spring Boot in table format for fast review before exams or interviews.

Core Java Fundamentals

Concept	Key Points
JVM	Executes bytecode, platform-specific
JRE	JVM + core libraries
JDK	JRE + dev tools (javac, jdb)
main()	`public static void main(String[] args)`
Data types	byte(1), short(2), int(4), long(8), float(4), double(8), char(2), boolean
Wrapper	Integer, Double, Character, Boolean (autoboxing/unboxing)
String Pool	Literals stored in pool for reuse; `new String()` creates in heap
final	Variable=constant, Method=no override, Class=no inherit
static	Belongs to class, not instance; shared across all objects

Exception Handling

├── Error (don't catch	OutOfMemoryError, StackOverflowError)
├── Checked (must handle	IOException, SQLException)
└── RuntimeException (unchecked	NullPointer, ArrayIndexOutOfBounds)

Keyword	Purpose
try	Code that may throw
catch	Handle exception
finally	Always executes (cleanup)
throw	Throw an exception
throws	Declare method may throw

Collections Framework

Interface	Implementations	Ordered	Duplicates	Null
List	ArrayList, LinkedList, Vector	✅	✅	✅
Set	HashSet, TreeSet, LinkedHashSet	Varies	❌	1 null (HashSet)
Queue	PriorityQueue, ArrayDeque	FIFO/Priority	✅	❌
Map	HashMap, TreeMap, LinkedHashMap	Varies	Keys: ❌	1 null key (HashMap)

HashMap Internals

Array of buckets (Node[])
hash(key) → bucket index
Collision → Linked List (≤8) → Red-Black Tree (>8)
Load factor: 0.75, resize at 75% full

Multithreading

Topic	Key Point
Create thread	extends Thread OR implements Runnable
States	NEW → RUNNABLE → RUNNING → BLOCKED/WAITING → TERMINATED
synchronized	One thread at a time (monitor lock)
volatile	Ensures visibility across threads
wait()/notify()	Inter-thread communication (inside synchronized)
sleep() vs wait()	sleep doesn't release lock; wait does
ExecutorService	Thread pool management
AtomicInteger	Lock-free thread-safe operations
Deadlock	Two threads waiting for each other's locks

JDBC Quick Reference

java example

// 1. Load driver (auto since JDBC 4.0)
// 2. Get connection
Connection conn = DriverManager.getConnection(url, user, pass);
// 3. Create statement
PreparedStatement ps = conn.prepareStatement("SELECT * FROM users WHERE id=?");
ps.setInt(1, id);
// 4. Execute
ResultSet rs = ps.executeQuery();
// 5. Process results
while (rs.next()) { String name = rs.getString("name"); }
// 6. Close resources (use try-with-resources)

Spring Boot Essentials

Annotation	Purpose
@SpringBootApplication	Main class (Config + AutoConfig + ComponentScan)
@RestController	REST API controller
@GetMapping / @PostMapping	HTTP method mapping
@PathVariable	URL path parameter
@RequestParam	Query parameter
@RequestBody	JSON body → Object
@Autowired	Dependency injection
@Service	Business layer bean
@Repository	Data layer bean
@Entity	JPA database entity
@Transactional	Database transaction
@Valid	Input validation
@ExceptionHandler	Handle exceptions

Spring Boot Layers

Example

Controller → Service → Repository → Database ↑ ↑ ↑ @RestController @Service @Repository DTOs Business Logic JPA/Hibernate

Design Patterns (Key Ones)

Pattern	Purpose	Java Example
Singleton	One instance only	`Runtime.getRuntime()`
Factory	Create objects without specifying class	`Calendar.getInstance()`
Builder	Step-by-step object construction	`StringBuilder`
Observer	Notify dependents of changes	Event listeners
Strategy	Swappable algorithms	`Comparator`
Iterator	Sequential access	`Iterator` interface

Core Java Fundamentals

Concept	Key Points
JVM	Executes bytecode, platform-specific
JRE	JVM + core libraries
JDK	JRE + dev tools (javac, jdb)
main()	`public static void main(String[] args)`
Data types	byte(1), short(2), int(4), long(8), float(4), double(8), char(2), boolean
Wrapper	Integer, Double, Character, Boolean (autoboxing/unboxing)
String Pool	Literals stored in pool for reuse; `new String()` creates in heap
final	Variable=constant, Method=no override, Class=no inherit
static	Belongs to class, not instance; shared across all objects

Exception Handling

├── Error (don't catch	OutOfMemoryError, StackOverflowError)
├── Checked (must handle	IOException, SQLException)
└── RuntimeException (unchecked	NullPointer, ArrayIndexOutOfBounds)

Keyword	Purpose
try	Code that may throw
catch	Handle exception
finally	Always executes (cleanup)
throw	Throw an exception
throws	Declare method may throw

Collections Framework

Interface	Implementations	Ordered	Duplicates	Null
List	ArrayList, LinkedList, Vector	✅	✅	✅
Set	HashSet, TreeSet, LinkedHashSet	Varies	❌	1 null (HashSet)
Queue	PriorityQueue, ArrayDeque	FIFO/Priority	✅	❌
Map	HashMap, TreeMap, LinkedHashMap	Varies	Keys: ❌	1 null key (HashMap)

HashMap Internals

Array of buckets (Node[])
hash(key) → bucket index
Collision → Linked List (≤8) → Red-Black Tree (>8)
Load factor: 0.75, resize at 75% full

Multithreading

Topic	Key Point
Create thread	extends Thread OR implements Runnable
States	NEW → RUNNABLE → RUNNING → BLOCKED/WAITING → TERMINATED
synchronized	One thread at a time (monitor lock)
volatile	Ensures visibility across threads
wait()/notify()	Inter-thread communication (inside synchronized)
sleep() vs wait()	sleep doesn't release lock; wait does
ExecutorService	Thread pool management
AtomicInteger	Lock-free thread-safe operations
Deadlock	Two threads waiting for each other's locks

JDBC Quick Reference

java example

// 1. Load driver (auto since JDBC 4.0)
// 2. Get connection
Connection conn = DriverManager.getConnection(url, user, pass);
// 3. Create statement
PreparedStatement ps = conn.prepareStatement("SELECT * FROM users WHERE id=?");
ps.setInt(1, id);
// 4. Execute
ResultSet rs = ps.executeQuery();
// 5. Process results
while (rs.next()) { String name = rs.getString("name"); }
// 6. Close resources (use try-with-resources)

Spring Boot Essentials

Annotation	Purpose
@SpringBootApplication	Main class (Config + AutoConfig + ComponentScan)
@RestController	REST API controller
@GetMapping / @PostMapping	HTTP method mapping
@PathVariable	URL path parameter
@RequestParam	Query parameter
@RequestBody	JSON body → Object
@Autowired	Dependency injection
@Service	Business layer bean
@Repository	Data layer bean
@Entity	JPA database entity
@Transactional	Database transaction
@Valid	Input validation
@ExceptionHandler	Handle exceptions

Spring Boot Layers

Example

Controller → Service → Repository → Database ↑ ↑ ↑ @RestController @Service @Repository DTOs Business Logic JPA/Hibernate

Design Patterns (Key Ones)

Pattern	Purpose	Java Example
Singleton	One instance only	`Runtime.getRuntime()`
Factory	Create objects without specifying class	`Calendar.getInstance()`
Builder	Step-by-step object construction	`StringBuilder`
Observer	Notify dependents of changes	Event listeners
Strategy	Swappable algorithms	`Comparator`
Iterator	Sequential access	`Iterator` interface

Quick Revision Notes — Java Programming

Core Java Fundamentals

Exception Handling

Collections Framework

HashMap Internals

Multithreading

JDBC Quick Reference

Spring Boot Essentials

Spring Boot Layers

Design Patterns (Key Ones)

Continue learning this concept

Quick Revision Notes — Java Programming

Core Java Fundamentals

Exception Handling

Collections Framework

HashMap Internals

Multithreading

JDBC Quick Reference

Spring Boot Essentials

Spring Boot Layers

Design Patterns (Key Ones)

Continue learning this concept