DEV Community: velvizhi Muthu

Module 3: Encapsulation

velvizhi Muthu — Mon, 16 Feb 2026 08:30:37 +0000

01.What is Encapsulation?

Encapsulation means wrapping (hiding) variables (data) and providing access only through methods (getters/setters).

Real-Time Example: Bank Account :

Account balance should not be directly accessible.

Only deposit/withdraw methods should modify it.

Getters provide safe access to balance.

02.Access modifier types?
1.public
2.protected
3.default
4.private

03.what is public?

public → accessible everywhere.
public → Surname (everyone knows).

ex:
public → everyone can know
public int dad_amount = 1000;

04.What is the protected?

protected → accessible within the same package and by subclasses (even in other packages).

protected → Property (children can inherit, brother in the same family can access).

ex:
protected → accessible to all children (even in other packages)
protected String property = "House";

05.What is the default?

default (no modifier) → accessible only within the same package.
default → Family name (known inside same package/family).
ex:
default → only family members in the same package can access
String familyName = "Kumar Family";

06.What is the private?
private → accessible only within the same class
private → Secret (only parent knows).

ex:
private → only parent knows
private String secret = "Parent's Secret";

07.What is the Import statement?
Import is a keyword.
We can use it to access the other package. That time we call the "import" keyword.
Import keyword, no need to call the same package.

08.What is return?
return is a keyword in java
return mainwork is retunted the variable value.
if i use the return keyword,then no need to call the object.

09.What is the purpose of poja class?
POJO classes are used to create readable and reusable code,
variable used to create the get method and get the value.

10.What is get balance?
Retrieves current account balance.

11.What is set balance?
Updates value of private balance.

Module 3: Abstraction

velvizhi Muthu — Mon, 16 Feb 2026 08:30:27 +0000

What is abstraction? Abstraction is showing only the necessary data, hiding the unwanted data.

keyword: abstract

How to achieve the abstraction?
keyword: abstract
Use the keyword and achieve the abstraction.
We can't create the object in abstract class.
We can use the abstract keyword in method and class also.
Where is use to the abstract method?
We can create the multiple definition by using the abstract method.

EX: Login page.

Module 3: Interface

velvizhi Muthu — Mon, 16 Feb 2026 08:30:17 +0000

What is an Interface?

A set of rules for classes to follow, without providing the actual implementation of those rules.

Interface is a keyword.

Without a method definition that time we need to mention the "abstract" keyword.

We called the interface class, that time, no need to mention the "abstract" keyword in rules method.

We can mention the Non-abstract method but no need to mention the open brace and close brace.

We can't create the Object in the main method.

Key word: Implements
Implements is a Java keyword.

No need to called the abstract keyword in class and main method. Already automatically "abstract "keyword available in class and main method because hide the "abstract" keyword.

02.Why we use the interface in java?
We can hide the implementation details and just expose the method signatures.

Module 3:Dynamic binding

velvizhi Muthu — Mon, 16 Feb 2026 08:30:07 +0000

What is Dynamic binding?
1. In general, an object can be created using the same class name, but it is possible to create an object using two different class names. It's called a Dynamic binding.
2. cannot convert from Parent to Child.
3. Child activity can't access the parent, but the parent activity can access the Child class.
4. Then, override method we can access the child and parent class, it's called as dynamic binding.
5. Only the same class name can be used on the "new" keyword.

Example:
Parent pa = new Parent
instead of
Parent pa = new Child or Child ch =new Parent.

Module 4: Return and void keyword

velvizhi Muthu — Mon, 16 Feb 2026 08:29:56 +0000

What is the "return" keyword?
The return keyword in Java is used inside a method to:

Exit from the method immediately.

Used just to exit the method (no value returned).

Optionally send back a value to the caller (if the method has a return type other than void).

The type of the value returned must match the method’s declared return type.

If the method is declared as void, you can use plain return or omit it.

return immediately ends method execution; any code after it won’t run.

You can use return inside loops, conditionals, or directly in methods.

What are "Void" keywords?
The keyword void is a return type used for methods that do not return any value.

Note: Primitive data type is a Java keyword because the first letter is lowercase.

Non primitive data type is a Java.lang
The non primitive data type -first letter is an uppercase.

Module 4: Scanner

velvizhi Muthu — Mon, 16 Feb 2026 08:29:45 +0000

What is a Scanner in Java? Scanner is a class in "Java. util" package that is used to read input. (System.in) → user input Different data types: nextInt()

Common Methods in Scanner

nextInt() → reads integer

nextDouble() → reads decimal numbers

nextLine() → reads a full line of text

next() → reads single word

hasNextInt(), hasNextLine() → check before reading

Module 4-Array

velvizhi Muthu — Mon, 16 Feb 2026 08:29:36 +0000

What is an array? An array is a collection of similar data types.

Array Syntax : []
Declaration:
datatype[] arrayName; or datatype arrayName[];
Example:
int[] marks = {50,80,70};
boolean[] result ={true,false,true};
float[] s = {6.0f,5.0f,4.8f};

An array is index-based.
marks[0] = 50;
marks[1] = 80;
marks[2] = 70;

Program explanation:

int[] marks = {50,80,70};
length = 3
index starts from 0. --> length -1 (2)

Object:
Objects in Java ->Arrays are objects stored in heap memory.
arrayName = new datatype[size];
Example:
int[] marks = new int[3];

Why do we use arrays in Java?
We use arrays in Java because they make it easy to store and manage multiple values of the same type under a single variable name, instead of declaring many separate variables.
example:
int tamil = 50;
int english = 80;
int maths = 70;
Key Points about Arrays:

Fixed size – Once created, the size of an array cannot be changed.

Same data type – All elements must be of the same type.

Index-based – Elements are accessed using an index, starting from 0 in most programming languages.

Homogeneous data stores only elements of the same data types.
First element-> index 0.
Second element-> index length - 1.
Print all elements in the array
Reverse the array.
Sum the elements in the array.
Store all values in a single array

Module 4 -String

velvizhi Muthu — Mon, 16 Feb 2026 08:29:26 +0000

01. What is a String in Java?

String is a non-Primitive data type.
String is a class.
The package name is java.lang
A string is a sequence of characters.
But in Java, a string is an object that represents a sequence of characters.
The Java.lang. -The String class is used to create a string object.

02. How to create a string object?
There are two ways to create a String object:

By string literal
By a new keyword

03. How to create a String Literal in Java?
A Java String literal is created by using double quotes.

For Example:
String s = "welcome"; String Literal.

04. Why is String immutable in Java?
String objects are immutable.
Immutable means unchangeable.
Once a string object is created, it cannot be changed.

05. String Literals (Without the new keyword):

Each time we create a string literal, the JVM checks the "string constant pool" first.
If the string already exists in the pool, a reference to the pooled instance is returned.
If the string doesn't exist in the pool, a new string instance is created and placed in the pool.

Note: String objects are stored in a special memory area known as the "string constant pool".

06. Advantage of string
immutable
Reduce memory usage by reusing existing strings.

**07. Disadvantage of string:
**Memory Consumption

Uses extra memory due to immutability
Performance Overhead

Slower for modifications & concatenation
Security Risks

Encoding Issues

08. How to create a string (With a new keyword)?
Creates a new object in heap memory.
String s1= new String("Hello")

Module 4 -Java string method

velvizhi Muthu — Mon, 16 Feb 2026 08:29:15 +0000

JAVA String method:

The java.lang.The String class provides many useful methods to perform operations on a sequence of char values.

String-First letter is uppercase, so string is a class.

Method 1:
char charAt(int index)
It returns a char value for the particular index.
Character at given index
Return type: Char

Output comment:
char in =s.charAt(0);
System.out.println(in);

Method 2:
int length()
The String class length() method returns the length of the specified String.
Number of characters in string
Return type: int

Output Comment:
int l=s.length ();
System.out.println(l);

Method 3:
trim()
Removes leading and trailing spaces
Return type: String

Output:
String v= s.trim();
System.out.println(v);

Method 4:
startsWith(prefix)
Checks if the string starts

Return type: boolean

Output:
boolean bo= s.startsWith("Sa");
System.out.println(bo);//True

Method 5:
endsWith(suffix)
Checks if the string ends with a specified sequence.

Return type: boolean

Output:
boolean lo= s.endsWith("Sa");
System.out.println(lo);//false

Method 6:
toCharArray()
Converts to a char array

Return type: char[]

Output:

Method 7:
isEmpty()
Checks if empty

Return type: boolean

Output:

isBlank()
The method is a predefined method in the String class that checks whether a string is empty or contains only whitespace characters.

Method 8:
toUpperCase()
The Java String toUpperCase () method converts it into the String's lowercase letters
Converts to upper case
Return type: String

Output:
String vv =s.toUpperCase();
System. out.println(vv);//SACHIN HELLO

Method 9:
toLowerCase()
The Java String toLowerCase () method converts it into the String's lowercase letters
Converts to lower case

**Return type: **String

Output:
String sv =s.toLowerCase();
System.out.println(sv);//sachin hello

Method 10:
subString method:
Returns substring from index to end
Return type: String (i)

Output:
String sub =s.substring(0)
System.out.println(sub);

Method 11:
replace() method:
Replace character or substring

Output:
String re= s.replace('S', 'H');
System.out.println(re);
Return type: String('i','j');

Method 12:
replaceAll();
Replace all words.

Output:
String str =s.replaceAll("Sachin", "Hello");
System.out.println(str);
Return type: String("String","String");

Method 13:
split();
Splits a string based on a regex.

Output:

Return type:

Method 14:
join();
The method combines multiple strings using a delimiter.

Output:
String result = si.join(" ", "I","Love", "Java");
System.out.println(result);
Return type: String();

Method 15:
contains()
Checks if a sequence of characters is present in the string.

SQL

velvizhi Muthu — Mon, 16 Feb 2026 08:29:03 +0000

01.SQL full form?
SQL-Structured Query Language(SQL)

What is SQL?
SQL stands for Structured Query Language and is a standardised programming language used for storing, manipulating, and retrieving data in relational databases.
What is a database?
Collection of data.
How many databases?
-Relational database (Table format)
-Hierarchical database.
-NoSQL database. (Without table format)
Below are the NoSQL Database types:
Key value
Document based
column based
Graph based
What is the DBMS?
A database management system
The DBMS is a software.
Interface between the database and the end user.
Software to store, retrieve, define and manage data in a database.
Easy CRUD operations. (CRUD- Create, read, update, delete)
Takes care of authentication, concurrency, logging, backup, optimisation, etc.
What is RDBMS? (Relational database management software)
RDBMS is a software. Below are the RDBMS software
-MySQL-Open source
-SQL server-microsoft
-Oracle -IBM
-PostgreSQL-Open source.

07.

Module 4-Block and static block

velvizhi Muthu — Mon, 16 Feb 2026 08:28:53 +0000

01. What is the Non-Static block?
The non-static block will be executed every time an object of the class is created, before the constructor is called as non static block.

02. How to use the non-static block?
-Non-static blocks are not marked by any keyword.
-simply appear as { /* code */ } inside the class.
-Each time an instance (object) of the class is created, the non-static block executes, allowing for instance-level logic.

Example:

public class Example {
{ // Non-static block
System.out.println("Non-static block executed.");
}

public Example() {
    System.out.println("Constructor executed.");
}

}
Output
// Non-static block executed.
// Constructor executed.

Workflow:

*1. Static block: *
-The class is loaded, and static blocks execute (only once per class).
-Static is a class.

-Static blocks are written using static { ... } inside the class body.

2. Non-static block:
An object is created. Non-static blocks execute in sequence.

*3. Constructor: *
-The constructor executes after non-static blocks.

-The constructor name must match the class name.

-It has no return type.

-It is called automatically during object creation.

-It initialises the object's state

4. Instance block:

-It runs automatically whenever an instance (object) of the class is created.

-It executes before the constructor code.

-It is used for code or logic common to all constructors to avoid duplication.

-It cannot accept parameters (unlike constructors).

5. Main method:
Executed after class loading and static block execution.

Module 4 : Main method explaination

velvizhi Muthu — Mon, 16 Feb 2026 08:28:40 +0000

What is the main method?

The Java Virtual Machine (JVM) starts program execution. -Without a properly defined main method, the JVM will not run the program

Main method declaration:
public static void main(String[] args)

Explain:

public: So the JVM can access it from anywhere.

static: So it can be called without creating an object of the class.

void: It does not return any value.

main: The specific name the JVM looks for as the entry point.

String[] args: An array parameter to accept command-line arguments.