2026-01-04

Topics Studied

• OOPS Fundamentals in Java
  • Overview of OOPS
  • Objects and Classes
  • Pillars of OOPS (Just definitions)
    • Data Abstraction
    • Encapsulation
    • Inheritance
    • Polymorphism
  • Is-a & Has-a Relationships
• Java Basic Overview
  • JDK, JRE, JVM
  • Execution Phase
  • First Java Program
  • Variables
    • Primitive Data Types

My Understanding

• Data Abstraction is achieved through Interface and Abstract Classes
• Data Encapsulation (DATA-HIDING) achieved by:
  • Declare variable of a Class as private
  • Provide a public getters and setters to view and modify the value of the variables.
• Multiple Inheritance is not possible in Java
  • It is achieved by Interfaces in java by a special functionality override.
• Polymorphism is of two types:
  • Method Overloading/Static Polymorphism
    • Compile Time
  • Method Overriding/Dynamic Polymorphism
    • Run-Time
• Is-a Relationship is basically Inheritance
  • Ex: Dog is-a Animal
• Has-a relationship -> whenever an object is used inside other class
• Association is relationship between two different objects.
  • Aggregation
    • Both objects, can survive individually means ending of one object will not end other object.
    • Ex: School and Student
  • Composition
    • Ending of one object will end other object
    • Ex- School and Room
• Java Agents are loaded before the main() method executes.
• Negative numbers are 2s Compliment of Positive
  • Ex: -3 is 2s Compliment of +3
  • Step-by-Step: 2’s Complement of 3 (to get -3)
    1. Binary representation of +3 (8-bit): 0000 0011
    2. Step 1 - Find 1’s Complement (invert all bits):
       • 0000 0011 → 1111 1100
    3. Step 2 - Add 1 to the 1’s Complement:
       • 1111 1100 + 0000 0001 = 1111 1101
    4. Result: 1111 1101 is the 2’s complement representation of -3
    5. Verification: In Java, byte b = -3; internally stores 1111 1101
  • Binary Verification: 3 + (-3) = 0 ``` 0000 0011 (+3)
    • 1111 1101 (-3) ———– 1 0000 0000 (9 bits) ```
    • The leftmost bit (carry bit) overflows and is discarded in 8-bit arithmetic
    • Final result: 0000 0000 = 0 ✅
    • This proves the 2’s complement representation is correct!
• Never use float or double for industry dealing money/currency
  • Use BigDecimal

Confusions

• Difference of two float values can be different than expected.
  • 0.3f - 0.1f gives 0.20000002
  • Use BigDecimal for better and precision calculation

Interview Notes