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☕ Java Notes

Java

1. Basics of Java

  • Java: High-level, object-oriented, platform-independent programming language.
  • Key Features:
    • Platform Independent – Write Once, Run Anywhere (via JVM).
    • Object-Oriented – Everything is represented as objects.
    • Robust – Strong memory management, exception handling.
    • Secure – No explicit pointer, bytecode verification.
    • Multithreaded – Support for concurrent execution.
    • Portable – Compiled bytecode runs on any platform with JVM.

2. Java Architecture

  • Source Code (.java) → Compiler (javac) → Bytecode (.class) → JVM → Machine Code
  • JVM Components:
    • Class Loader – Loads class files.
    • Memory Areas – Method area, Heap, Stack, PC register.
    • Execution Engine – Interpreter, JIT compiler.
    • Garbage Collector – Frees unused objects.

3. Data Types

  • Primitive:
    • byte (1 byte), short (2), int (4), long (8)
    • float (4), double (8)
    • char (2), boolean (1 bit)
  • Non-Primitive:
    • Strings, Arrays, Classes, Interfaces.

4. OOP Concepts

Concept Description Example
Encapsulation Wrapping data and methods in one unit (class). private fields + getters/setters
Inheritance One class acquires properties/methods of another. extends keyword
Polymorphism Same method name, different behavior. Overloading / Overriding
Abstraction Hiding implementation details. Abstract classes / Interfaces

5. Important Keywords

  • final – Variable (constant), method (cannot override), class (cannot extend).
  • static – Belongs to class, not object.
  • this – Refers to current object.
  • super – Refers to parent class.
  • synchronized – Thread-safe block/method.
  • volatile – Tells JVM value may change unexpectedly (thread communication).
  • transient – Excluded from serialization.

6. Java Collections Framework

  • List – Ordered, duplicates allowed → ArrayList, LinkedList.
  • Set – No duplicates → HashSet, LinkedHashSet, TreeSet.
  • Map – Key-value pairs → HashMap, LinkedHashMap, TreeMap.
  • Queue – FIFO → PriorityQueue, ArrayDeque.

7. Exception Handling

  • Checked – Compile-time (IOException, SQLException).
  • Unchecked – Runtime (NullPointerException, ArithmeticException).
  • Keywords: try, catch, finally, throw, throws.

Example:

try {
    int a = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("Always runs");
}

8. Java 8 Features

  • Lambda Expressions – (param) -> expression
  • Functional Interfaces – Single abstract method (@FunctionalInterface)
  • Streams API – Functional-style operations on collections.
  • Optional – Avoid null checks.
  • Default & Static Methods in Interfaces.

9. Multithreading

  • Thread creation:
    • Extending Thread class.
    • Implementing Runnable interface.
  • Thread lifecycle:
    • New → Runnable → Running → Waiting/Sleeping → Terminated.
  • Synchronization to avoid race conditions.

10. Memory Management

  • Heap – Stores objects.
  • Stack – Stores method calls and local variables.
  • Garbage Collection – Automatic removal of unused objects.