DEV Community: nantha kumar

JDK

nantha kumar — Wed, 21 Jan 2026 13:12:04 +0000

Module 2

nantha kumar — Mon, 19 Jan 2026 02:52:37 +0000

Control flow statement :

Control flow statement decide which statement is execute, how many times it runs, which order it runs.

Types of control flow statement :

Decision making statement
Looping statement
Jumping statement

Decision making statement :

Decision making statement take decision based on the condition.

If statement :

if is a java keyword.
Execute code only if condition is true.

Else statement :

else is a java keyword.
Execute code only when if condition is false.

Else-if statement :

else if is a java keyword.
If we want check multiple condition , we go for else-if statement.

Switch case :

Switch-case is used when you want to execute different code based on one value.
Switch case allowed these primitive data type such as byte, short, int, char in expression.
If no case matches, the default block will execute.

Switch case syntax :

switch (expression) {
    case value1:
        // code
        break;
    case value2:
        // code
        break;
    default:
        // code
}

Example code 1 :

int day = 3;

switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday");
        break;
    default:
        System.out.println("Invalid day");
}

Example code 2 :

char grade = 'A';

switch (grade) {
    case 'A':
    case 'B':
    case 'C':
        System.out.println("Passed");
        break;
    case 'D':
        System.out.println("Failed");
}

After java 8 features :

Switch case allowed String data type in expression after java 8th version.

After java 12+ version :

If we use arrow symbol in switch case, don't need to mention break keyword.

Example code 1 :

char grade = 'A';

switch (grade) {
    case 'A', 'B', 'C' -> System.out.println("Passed");
    case 'D' -> System.out.println("Failed");
}

Example code 2 :

int day =1;
        int result = switch (day) {
        case 1 -> 10;
        case 2 -> 20;
        default -> 0;
    };

Example code 3 :

int day = 1;
int result = switch (day) {
    case 1 -> 10;
    case 2 -> 20;
    default -> 0;
};

Abstraction

nantha kumar — Mon, 19 Jan 2026 01:22:07 +0000

What is abstraction :

Abstraction means hiding implementation details and showing only essential features to the user.

How to achieve abstraction :

Using abstract keyword

Where you used abstract keyword :

We can use abstract keyword in class level and method level.

Can an abstract class have:

Constructors? ✅
Private methods? ✅
Static methods? ✅
Final methods? ✅

Answer : Yes

Notes :

abstract class does not have method definition.
We can't create object directly.

Can we create an object of an abstract class?

Answer: ❌ No But reference can be created

BankService service = new UPIService();

Can an abstract class have instance variable ?

Answer : Yes

Interface

nantha kumar — Sat, 17 Jan 2026 17:14:37 +0000

What is interface :

An interface is a set of rules.

Simple Definition :

An interface tells a class what to do, not how to do it.

Why used interface :

It is used to achieve 100% abstraction and multiple inheritance.
To define common behavior across different classes

Notes :

An interface contains only abstract method.
All methods are public and abstract by default in interface.
All variables are public static final in interface.
We can't create object for interface.
We can't create constructor in interface.

After java 8th version :

Default Methods :

We can create method with body in interface using default keyword.

Static Methods :

We can create static method using static keyword in interface.
We can call the static method using interface name.
We can't override the static method.

Multiple Inheritance Problem Solved :

interface A {
    default void show() {
        System.out.println("A");
    }
}

interface B {
    default void show() {
        System.out.println("B");
    }
}

class Test implements A, B {
    @Override
    public void show() {
        A.super.show();   // calls A's default method
        // B.super.show(); // you can call this also if needed
    }

    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

Private Methods (Java 9+) :

private method used to reuse code inside interface.
Can't accessed by implementing classes.

Inheritance

nantha kumar — Sat, 17 Jan 2026 16:36:36 +0000

What is Inheritance :

Child object behaving as a parent object.

How to Achive Inheritance :

To achive inheritance, we should use extends keyword.

Types of inheritance :

Single Inheritance
Multilevel Inheritance
Hierarchical Inheritance
Hybrid Inheritance
Multiple Inheritance

Single Inheritance :

One child class inherits from one parent class.

Multilevel Inheritance :

A class inherits from another class, and then another class inherits from that class. Example : Grandparent → Parent → Child

Hierarchical Inheritance :

Multiple child classes inherit from a single parent class.

Hybrid Inheritance :

Combination of two or more types of inheritance.

Multiple Inheritance :

if we use multiple inheritance, ambiquity problem will occure.
So Java does not support multiple inheritance. but it supports it via interfaces.

Encapsulation

nantha kumar — Sat, 17 Jan 2026 16:24:39 +0000

What is encapsulation :

Encapsulation is data protection.

Why used encapsulation :

Encapsulation provides data security, better control.

Benefits of Encapsulation :

Data Hiding → Sensitive data is not directly accessible.
Control over Data → You can validate values before setting them.
Improves Security → External code can’t directly change internal variables.

How to protect the data :

Using access modifier.

What is access modifier :

An access modifier in Java defines who can access a class, variable, method, or constructor.

Real time example :

Access Modifier	Within Class	Same Package	Subclass (other package)	Outside Package
private	✅ Yes	❌ No	❌ No	❌ No
default (no keyword)	✅ Yes	✅ Yes	❌ No	❌ No
protected	✅ Yes	✅ Yes	✅ Yes	❌ No
public	✅ Yes	✅ Yes	✅ Yes	✅ Yes

Polymorphism

nantha kumar — Sat, 17 Jan 2026 16:08:55 +0000

What is polymorphism ?

It means the same method name can behave differently based on the object that is calling polymorphism.

Types of Polymorphism ?

Compile-Time Polymorphism (Method Overloading)
Runtime Polymorphism (Method Overriding)

Method overloading :

Method overloading is same method name and different type of arguments or different number of arguments in same class.

Method overriding :

Method overriding is same method signature(method name & arguments) in different class.