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Java Quick Revision

Last Updated : 26 May, 2026

title: Quick Revision Notes

Notes295 words8 headingsTables included

title: Quick Revision Notes description: Java ke important concepts ki quick revision

Core Java

JVM bytecode execute karta hai | JRE = JVM + libraries | JDK = JRE + tools
8 primitive types: byte, short, int, long, float, double, char, boolean
String immutable hai → String Pool mein store hoti hai
== reference compare | .equals() value compare
Autoboxing: int → Integer | Unboxing: Integer → int

OOP Pillars

Encapsulation: private fields + public getters/setters
Inheritance: extends keyword, IS-A relationship
Polymorphism: Overloading (compile-time) | Overriding (runtime)
Abstraction: Abstract class (partial) | Interface (full)

Access Modifiers

Modifier	Same Class	Same Package	Subclass	Everywhere
private	✅	❌	❌	❌
default	✅	✅	❌	❌
protected	✅	✅	✅	❌
public	✅	✅	✅	✅

Exception Handling

Checked: compile time (IOException, SQLException)
Unchecked: runtime (NullPointerException, ArrayIndexOutOfBoundsException)
throw → manually exception throw | throws → method declaration
finally → hamesha execute hota hai

Collections

List: ordered, duplicates allowed → ArrayList, LinkedList
Set: unique elements → HashSet (no order), LinkedHashSet (insertion), TreeSet (sorted)
Map: key-value → HashMap (fast), LinkedHashMap (insertion), TreeMap (sorted)
Queue: FIFO → PriorityQueue, ArrayDeque

Multithreading

Thread create: extends Thread | implements Runnable | Lambda | ExecutorService
States: NEW → RUNNABLE → RUNNING → WAITING → TERMINATED
synchronized → race condition rokta hai
wait() → lock release | sleep() → lock nahi release karta
Deadlock: do threads ek doosre ka lock wait karein

Java 8

Lambda: (params) -> expression
Stream: filter, map, reduce, collect
Optional: null safe wrapper
Default methods: interface mein concrete methods

Collections Time Complexity

Operation	ArrayList	LinkedList	HashMap	TreeMap
Get	O(1)	O(n)	O(1)	O(log n)
Add	O(1)*	O(1)	O(1)	O(log n)
Remove	O(n)	O(1)	O(1)	O(log n)
Search	O(n)	O(n)	O(1)	O(log n)

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Java Quick Revision.

Interview Use

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