🧠 Programming Session Day 7: Mastering Loops, Conditions & Database Logic in Java

programming session:

---

🌐 Why Do We Connect Many Databases in a Server?

In real-world applications, one server connects multiple databases because:

✅ Different modules need different data.

✅ Easy to scale (add/remove DBs).

✅ Security: Finance DB ≠ Student DB.

✅ Performance: Less load per DB.

✅ Backup and Recovery made easier.

🎓 Example: A college server may have:

• students_db for student data
• staff_db for faculty info
• fees_db for payment records

• attendance_db for daily logs

  • 📌 Example: Amazon may have separate DBs for Products, Orders, and Customers.

---

🔁 Reversing a Number using while

Let's take this number:

int num = 1576;

while (num > 0) {
    System.out.println(num % 10);  // prints last digit
    num = num / 10;                // removes last digit
}

🟢 Output:

6
7
5
1

👉 % 10 → gets the last digit

👉 / 10 → removes it

---

🔢 Counting Digits in a Number

int num = 1576;
int count = 0;

while (num > 0) {
    count++;
    num = num / 10;
}
System.out.println("Total Digits: " + count);

🟢 Output: Total Digits: 4

---

🔁 Different Variations of While Loop

✅ Case 1: while (num >= 0)

int num = 1576;
while (num >= 0) {
    System.out.println(num % 10);
    num = num / 10;
}

⚠️ Infinite Loop: num becomes 0 → %10 = 0, /10 = 0 → stuck.

---

✅ Case 2: while (num > 1)

int num = 1576;
while (num > 1) {
    System.out.println(num % 10);
    num = num / 10;
}

🟢 Output:

6
7
5

📌 Skips the final 1.

---

✅ Case 3: % 100 Instead of % 10

int num = 1576;
while (num > 0) {
    System.out.println(num % 100);
    num = num / 10;
}

🟢 Output:

76
7
0
0

---

✅ Case 4: num = num / 100;

int num = 1576;
while (num > 0) {
    System.out.println(num % 10);
    num = num / 100;
}

🟢 Output:

6
5

---

✅ Case 5: % 100 and / 100

int num = 1576;
while (num > 0) {
    System.out.println(num % 100);
    num = num / 100;
}

🟢 Output:

76
15

✨ Useful for date, time, or currency formats.

---

🔀 What is switch-case?

Switch case is a clean way to write multiple fixed value conditions.

✅ Syntax:

String day = "Monday";

switch (day) {
    case "Monday":
        System.out.println("Weekday");
        break;

    case "Sunday":
        System.out.println("Weekend");
        break;

    default:
        System.out.println("Invalid");
}

---

❓ Why and Where to Use switch?

• When one variable has many fixed values
• Example: Menu options, Days of week, Payment modes

Use switch when:

• You check one value against many known cases
• The values are fixed like "Low", "Medium", "High"

Use if when:

• You check ranges, like x > 10
• You use logical operators: &&, ||, !

---

🔥 Switch Case vs If Statement

Feature	switch	if-else
Use case	Fixed values	Complex logic, ranges
Data types	int, char, String	All types
Readability	Very clean	Can be messy if long
Flexibility	Limited	Highly flexible

---

🧐 Why Keywords like switch, case are Lowercase?

➡️ To separate language keywords from user-defined names.

➡️ Capital letters are reserved for classes like String, Scanner.

✅ switch, case, break, if, for = keywords → lowercase

✅ Main, String, ArrayList = classes → Capital case

It’s a syntax rule across C, Java, C#, JS…

---

📆 Example: Switch Case with Days

String day = "Tuesday";

switch (day) {
    case "Monday":
        System.out.println("Weekday");
        break;

    case "Tuesday":
        System.out.println("It's Tuesday!");
        break;

    default:
        System.out.println("Invalid day");
}

🟢 Output:

It's Tuesday!

---

❌ What If We Remove default?

No crash.

But if no case matches, nothing prints.

---

❌ What If We Remove break?

String day = "Monday";

switch (day) {
    case "Monday":
        System.out.println("Monday");
    case "Tuesday":
        System.out.println("Tuesday");
    case "Sunday":
        System.out.println("Sunday");
}

🟢 Output:

Monday
Tuesday
Sunday

⛔ This is called fall-through. Once matched, it keeps printing below cases.

---

🔁 Can I Move default or break?

✅ default can be at the top, middle, or bottom.

🚫 But break should be placed after each case to stop flow.

---

❓ Is Only int Allowed in Switch?

No!

Java supports:

• int, byte, short, char
• String (from Java 7)
• enum types

⛔ float, double, boolean are NOT allowed.

---

❓ Why Use default?

To handle unexpected input or invalid values.

Like:

default:
    System.out.println("Oops! Not a valid day");

---

💥 Learn from Errors — Missing break

int option = 2;

switch (option) {
    case 1:
        System.out.println("Option 1");
    case 2:
        System.out.println("Option 2");
    case 3:
        System.out.println("Option 3");
}

🟢 Output:

Option 2
Option 3

✅ Without break, everything falls through!

---

🐍 Why No Switch Case in Python?[TBD]

Python loves simplicity and readability.

Instead of switch, it uses if-elif-else.

But from Python 3.10, you can now use match-case like switch:

day = "Tuesday"

match day:
    case "Monday":
        print("Weekday")
    case "Sunday":
        print("Weekend")
    case _:
        print("Invalid")

---

✅ Final Thoughts

In this blog,i am learned:

• Why servers connect to multiple DBs
• Reversing numbers using while
• Switch vs If — where, why, and when
• Keyword rules in Java
• How to learn from break/default mistakes
• Python’s switch alternative

---