Chapter 1: Variables & Data Types

Keywords in Java

Keywords are words which are reserved and used by the Java Compiler. They cannot be used as an Identifier.

Access Modifiers

public private protected

Data Types

int double float char boolean byte short long

Control Flow

if else switch case for while do break continue

Class & Object

class interface extends implements new this super

Note: Go to docs.oracle.com for a comprehensive list of all Java keywords!

Variables in Java

A variable is a container that holds data that can be changed during the execution of a program.

Variable Naming Rules:

✅ Valid Names

age

studentName

_count

$price

number1

❌ Invalid Names

2number (starts with digit)

class (keyword)

my-name (contains hyphen)

my name (contains space)

@value (invalid character)

Data Types in Java

Data types specify the different sizes and values that can be stored in the variable.

Java is statically typed - variables must be declared before use.

Primitive Data Types

There are 8 primitive data types supported by Java:

Data Type	Size	Range	Default Value
`byte`	1 byte	-128 to 127	0
`short`	2 bytes	-32,768 to 32,767	0
`int`	4 bytes	-2,147,483,648 to 2,147,483,647	0
`long`	8 bytes	-2^63 to 2^63-1	0L
`float`	4 bytes	3.4e-038 to 3.4e+038	0.0f
`double`	8 bytes	1.7e-308 to 1.7e+308	0.0d
`char`	2 bytes	0 to 65,535	'\u0000'
`boolean`	1 bit	true or false	false

How to choose data types for our Variables

Primitive Data Types

Integral (int)

byte, short, int, long

Floats (Decimal)

float(f), double(d)

Char, Boolean

char, boolean(true/false)

In order to choose the data type we first need to find the type of data we want to store. After that we need to analyze the Min & Max Value we might use.

Literals

A constant value which can be assigned to the variable is called a literal.

Types of Literals:

101 → Integer Literal
10.1f → Float Literal
10.1 → Double Literal (default type for decimals)
'A' → Character Literal
true → Boolean Literal
"Harry" → String Literal

Reading Data from the Keyboard

In order to read data from the keyboard, Java has a Scanner class.

Scanner class has a lot of methods to read data from the keyboard.

Scanner Setup:

InputExample.java

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // Read different types of input
        System.out.print("Enter your name: ");
        String name = scanner.nextLine();
        
        System.out.print("Enter your age: ");
        int age = scanner.nextInt();
        
        System.out.print("Enter your salary: ");
        double salary = scanner.nextDouble();
        
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Salary: " + salary);
        
        scanner.close();
    }
}

Scanner Methods:

Method	Description	Example
`nextInt()`	Reads an integer	`int num = scanner.nextInt();`
`nextDouble()`	Reads a double	`double price = scanner.nextDouble();`
`nextFloat()`	Reads a float	`float rate = scanner.nextFloat();`
`next()`	Reads a single word	`String word = scanner.next();`
`nextLine()`	Reads entire line	`String line = scanner.nextLine();`
`nextBoolean()`	Reads a boolean	`boolean flag = scanner.nextBoolean();`

Practice Set

Exercise 1.1: Student Percentage Calculator

Problem: Write a program to calculate percentage of a given student in CBSE board exam. His marks from 5 subjects must be taken as input from the keyboard. (Marks are out of 100).

Chapter 1 - Practice Set

1. Sum of Three Numbers

Write a program to sum three numbers in Java.

2. CGPA Calculator

Write a program to calculate CGPA using marks of three subjects (out of 100).

3. Greeting Program

Write a Java program which asks the user to enter his/her name and greets them with "Hello <name>, have a good day" text.

4. Unit Conversion

Write a Java program to convert Kilometers to miles.

5. Number Type Detection

Write a Java program to detect whether a number entered by the user is integer or not.

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