Complete Java Interview Prep Sheet

Complete Java Interview Prep Sheet (Final Version)

1. Core OOP Concepts

4 Fundamental Principles:

• Abstraction: Hide implementation details, show only essential features (abstract classes/interfaces). (what to implement).
• Encapsulation: Bundle data + methods, hide internals using private fields + public getters/setters (data hiding).
• Inheritance: Child class acquires parent class properties (is-a relationship, single inheritance only).
• Polymorphism: Same interface, multiple forms (compile-time: method overloading; runtime: method overriding).

Key Points:

• Java not pure OOP (uses primitives, no multiple class inheritance).
• Object class is root of all classes: equals(), hashCode(), toString(), wait()/notify().
• Multiple inheritance via interfaces only (no diamond problem).

2. Java Basics

Class/Object/Method:

• Class = blueprint/template
• Object = runtime instance (new ClassName())
• Method = block of code performing specific task

Variables:

• Local: method scope only
• Instance: per object (non-static)
• Static: class-level, shared by all objects

Data Types:

Primitives (8): byte, short, int, long, float, double, char, boolean
Reference: String, arrays, classes, interfaces, wrappers

Arrays: Fixed size, same type elements, objects themselves (int[] arr = new int[5];)

Packages: Organize classes (java.lang auto-imported, java.io for streams)

3. Wrapper Classes

Purpose: Convert primitives to objects for Collections, APIs

int → Integer    | char → Character
long → Long      | boolean → Boolean
float → Float    | double → Double

Key Methods: Integer.parseInt("123"), Integer.valueOf(5) Autoboxing: Integer i = 5; (Java 5+ auto conversion)

4. JVM/JDK/JRE

JDK = Development Kit (javac + JVM + tools)
JRE = Runtime Environment (JVM + libraries)
JVM = Executes bytecode (platform independence)

Flow: .java → javac → .class (bytecode) → java → Run anywhere

5. Exception Handling

Syntax:

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try { risky code }
catch (SpecificException e) { handle }
catch (Exception e) { general handle }
finally { cleanup always runs }

Custom: class MyException extends Exception {}

6. Collections Framework

Performance:

ArrayList: O(1) get, O(n) insert middle
LinkedList: O(1) insert/delete, O(n) get
HashMap: O(1) average get/put
TreeMap: O(log n) sorted operations

Critical: hashCode() + equals() contract MUST be consistent

7. Multithreading

Create Threads:

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// Way 1
class MyThread extends Thread { public void run() {} }

// Way 2 (preferred)
class MyRunnable implements Runnable { public void run() {} }
Thread t = new Thread(new MyRunnable());

Synchronization:

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synchronized void method() {}  // method-level
synchronized(this) { }         // block-level

wait()/notify(): Inter-thread communication (must be in synchronized block)

Modern (java.util.concurrent):

• ExecutorService + thread pools
• Callable + Future (return values)
• ConcurrentHashMap, CopyOnWriteArrayList

8. Java 8+ Features

Lambdas: (params) -> expression

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list.forEach(x -> System.out.println(x));
Comparator.comparing(Person::getName)

Streams:

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list.stream()
    .filter(x -> x > 10)
    .map(x -> x * 2)
    .collect(Collectors.toList())

Optional: Avoid NullPointerException

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Optional.ofNullable(value).orElse("default")

9. Memory & Garbage Collection

Memory Areas:

Heap: Objects, instance variables
Stack: Method frames, local variables

GC: Automatic memory reclamation of unreachable objects

• Use try-with-resources for cleanup
• Tune with JVM flags (-Xmx, -Xms)

10. Spring Framework (Interview Essentials)

Core Concepts:

• IoC Container: Manages object lifecycle
• DI: @Autowired injects dependencies

Annotations:

@Component, @Service, @Repository, @Controller
@RestController, @RequestMapping
@RequestParam, @PathVariable

Spring Boot:

• @SpringBootApplication
• application.properties
• Auto-configuration

11. Hibernate/JPA

Core Annotations:

@Entity, @Table
@Id, @GeneratedValue
@OneToMany, @ManyToOne, @JoinColumn

Key Concepts:

• Lazy vs Eager loading
• Persistence Context
• Dirty Checking (auto updates)
• Caching (1st/2nd level)

Quick Review Checklist

✅ OOP 4 pillars + Object class
✅ Collections + hashCode contract
✅ Exceptions hierarchy
✅ Multithreading basics + sync
✅ Java 8 lambdas/streams/Optional
✅ JVM basics
✅ Spring DI + annotations
✅ Hibernate relationships

Print this sheet. 2 pages max. You’re ready for interviews.