DESCO; Assistant Engineer; Exam: 20.05.2023 (DESCO)প্রশ্ন ও সমাধান

Access specifiers (access modifiers) control the visibility/accessibility of class members (fields, methods) in OOP languages like Java and C++.

classDiagram
class Base {
  +publicMember : public, accessible everywhere
  #protectedMember : protected, class + subclass
  -privateMember : private, class only
}
class Derived
Base <|-- Derived

Specifier	Same class	Subclass (derived)	Same package/file	Other packages/outside
Public	Yes	Yes	Yes	Yes
Protected	Yes	Yes	Yes	No
Default (package-private, Java only)	Yes	Only if same package	Yes	No
Private	Yes	No	No	No

• Public: part of the class's open interface — used for methods/data meant to be freely used by any code.
• Private: hides implementation details (encapsulation) — accessible only inside the class itself.
• Protected: exposes members to subclasses (supports inheritance) while still hiding them from unrelated outside code.
• Default: Java-specific — no keyword written, visible only within the same package.

Exam takeaway: visibility from most to least open is Public > Protected > Default (package-private) > Private — always choose the tightest specifier that still lets the class fulfil its role, for strong encapsulation.

SQL joins combine rows from two or more tables based on a related column.

erDiagram
EMPLOYEE }o--|| DEPARTMENT : belongs_to

Join type	Returns	SQL example
INNER JOIN	Only matching rows in both tables	SELECT * FROM Emp e INNER JOIN Dept d ON e.DeptID=d.DeptID;
LEFT (OUTER) JOIN	All rows from left, plus matched rows from right (NULL if no match)	SELECT * FROM Emp e LEFT JOIN Dept d ON e.DeptID=d.DeptID;
RIGHT (OUTER) JOIN	All rows from right, plus matched rows from left (NULL if no match)	SELECT * FROM Emp e RIGHT JOIN Dept d ON e.DeptID=d.DeptID;
FULL OUTER JOIN	All rows from both tables; NULL where no match on either side	SELECT * FROM Emp e FULL OUTER JOIN Dept d ON e.DeptID=d.DeptID;
CROSS JOIN	Cartesian product — every row of A with every row of B (no condition)	SELECT * FROM Emp CROSS JOIN Dept;
SELF JOIN	A table joined with itself (e.g. employee-manager)	SELECT e1.Name, e2.Name AS Manager FROM Emp e1 JOIN Emp e2 ON e1.ManagerID=e2.EmpID;

Exam takeaway: INNER returns only matches; LEFT/RIGHT return all of one side plus matches; FULL OUTER returns all of both sides; CROSS returns every combination; SELF joins a table to itself via a foreign key referencing its own primary key.