Quick Recall_Java

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Java Quick Review (Exam Fast Revision)

Unit 1: Java Fundamentals

What is Java?

• High level, object-oriented programming language
• Platform independent → “Write Once, Run Anywhere”
• Uses JVM (Java Virtual Machine)

OOP Concepts

1. Class → blueprint
2. Object → real instance
3. Encapsulation → data hiding
4. Inheritance → reuse code
5. Polymorphism → many forms
6. Abstraction → hide details

Basic Program

class Hello {
    public static void main(String args[]) {
        System.out.println("Hello");
    }
}

JDK

• JDK = Java Development Kit
• Contains compiler (javac) + JVM + libraries

Data Types

Type	Example
int	10
float	2.5f
double	5.67
char	'A'
boolean	true
String	"Hello"

Variables & Constants

int a = 5;
final int x = 10;

final = constant value cannot change.

Operators

Operator	Example
Arithmetic	+ - * / %
Relational	> < == !=
Logical	&& || !
Assignment	= += -=

Type Casting

double x = 5.5;
int y = (int)x;

Control Statements

if

if(a>5){
   System.out.println("Yes");
}

switch

switch(n){
 case 1:
   break;
 default:
}

Loops

for

for(int i=0;i<5;i++)

while

while(condition)

do while

do{
}while(condition);

Array

int a[] = {1,2,3};

String

String s = "Java";
s.length();
s.toUpperCase();

---

Unit 2: Classes, Objects & Methods

Class and Object

class Student{
   int id;
}

Student s = new Student();

Method

void show(){
   System.out.println("Hello");
}

Constructor

Student(){
   System.out.println("Constructor");
}

• Same name as class
• No return type
• Automatically called

this keyword

this.id = id;

Used to refer current object.

static keyword

static int count;

Shared by all objects.

---

Unit 3: Inheritance & Interfaces

Inheritance

class A{}
class B extends A{}

Types:

• Single
• Multilevel
• Hierarchical

super keyword

super();
super.x;

Calls parent class members.

Method Overriding

class A{
   void show(){}
}

class B extends A{
   void show(){}
}

Polymorphism

One method → many forms.

A obj = new B();

final keyword

Usage	Meaning
final variable	cannot change
final method	cannot override
final class	cannot inherit

Abstract Class

abstract class A{
   abstract void run();
}

Cannot create object.

Interface

interface A{
   void show();
}

Used for full abstraction.

Access Specifiers

Specifier	Access
public	everywhere
private	inside class
protected	package + subclass
default	same package

---

Unit 4: Exception Handling & Multithreading

Exception Handling

try{
}
catch(Exception e){
}
finally{
}

Keywords:

• try
• catch
• throw
• throws
• finally

Thread

Extending Thread

class A extends Thread{
   public void run(){
   }
}

Runnable Interface

class A implements Runnable{
   public void run(){
   }
}

Start thread:

t.start();

---

Unit 5: File Handling & I/O

Scanner

Scanner sc = new Scanner(System.in);
int x = sc.nextInt();

File Writing

FileWriter fw = new FileWriter("a.txt");
fw.write("Hello");
fw.close();

File Reading

FileReader fr = new FileReader("a.txt");

Streams

Stream	Meaning
Byte Stream	binary data
Character Stream	text data

Random Access File

RandomAccessFile f =
new RandomAccessFile("a.txt","rw");

Can access any position directly.

---

Unit 6: Swing & JDBC

Swing

Used for GUI.

Components

Component	Use
JButton	button
JTextField	text box
JCheckBox	checkbox
JList	list

Event Handling

button.addActionListener(this);

Layout Managers

Layout	Work
FlowLayout	left to right
BorderLayout	north south east west
GridLayout	grid form

JDBC

Java Database Connectivity

Steps

1. Import package
2. Load driver
3. Create connection
4. Execute query
5. Close connection

JDBC Example

Connection con =
DriverManager.getConnection(
"jdbc:mysql://localhost/test",
"root",""
);

Statement st = con.createStatement();

st.executeUpdate(
"insert into student values(1,'Ram')"
);

---

Most Important Viva Questions

Difference between JDK, JRE, JVM

Term	Meaning
JVM	runs bytecode
JRE	JVM + libraries
JDK	JRE + compiler

Difference between Method and Constructor

Method	Constructor
has return type	no return type
called manually	called automatically

Difference between Abstract Class and Interface

Abstract Class	Interface
partial abstraction	full abstraction
can have normal methods	mainly abstract methods

---

Super Important Programs For Exam

1. Class and Object
2. Constructor
3. Inheritance
4. Method Overriding
5. Interface
6. Exception Handling
7. Multithreading
8. File Handling
9. Swing GUI
10. JDBC CRUD

---

Last Minute Tips

• Remember syntax carefully.
• Write semicolon ;
• main() syntax is important

public static void main(String args[])

• extends → inheritance
• implements → interface
• try-catch-finally always together
• Thread starts with start(), not run()

Good luck for your exam 👍

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