Data Types & Variables

Java First Program

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

Java Comments

Comments can be used to explain  Java code, and to make it more  readable. It can also be used to  prevent execution when testing  alternative code.

Java single-line  Comments

Single-line comments start with two  forward slashes (//).

This example uses a single-line  comment before a line of code:

Java multi-line  Comments

Multi-line comments start with /*  and ends with */.

Example

/* The code below will print the  words Hello World

to the screen, and it is amazing */  System.out.println(“Hello World”);

Java Variables

Variables are containers for  storing data values.

In Java, there are different  types of variables, for  example:

Variables Declaration and Initialization

• Declaration: This involves  specifying the data type  and the variable name.
• Initialization: This involves  assigning a value to the  variable.

// Declarationint a; //  Initialization a = 10; //  Declaration and  Initializationint b = 20;

Types of Variables in Java

• Variables Local Variables
• Instance Variables
• Static (Class) 

Local Variables

• Declared inside a method, constructor, or block.
• Created when the method, constructor, or block is entered and destroyed once exited.
• Scope is limited to the method, constructor, or block.

Instance Variables

• Declared inside a class but outside any method, constructor, or block.
• Created when an object is created and destroyed when the object is destroyed.
• Instance variables are unique to each instance of a class.

Static (Class) Variables

• Declared with the static keyword inside a class, outside any method, constructor, or block.
• Created when the program starts and destroyed when the program stops.
• Shared among all instances of a class.

Java Data Types

In Java, data types specify the size  and type of values that can be  stored in variables. Java supports  two categories of data types:

1. Primitive Data Types

Introduction

• Java supports eight built-in data types known as primitive data types.
• Primitive data types are predefined by the language and named by a keyword.

 Categories of Primitive Data Types

• Integer Types
• Floating-Point Types
• Character Type
• Boolean Type

Integer Types

byte

• Size: 8-bit
• Range: -128 to 127

short

• Size: 16-bit
• Range: -32,768 to 32,767

int

• Size: 32-bit
• Range: -2^31 to 2^31 – 1

long

• Size: 64-bit
• Range: -2^63 to 2^63 – 1

Floating-Point Types

• float
  • Size: 32-bit
  • Single-precision
• double
  • Size: 64-bit
  • Double-precision

boolean

Size: Not precisely defined (depends on JVM)

• Values: true or false

Character Type

• char
  • Size: 16-bit
  • Represents a single 16-bit Unicode character
  • Range: ‘\u0000’ (0) to ‘\uffff’ (65,535)

Default Values

• byte: 0
• short: 0
• int: 0
• long: 0L
• float: 0.0f
• double: 0.0d
• char: ‘\u0000’
• boolean: false

2. Non-Primitive Data Types

Introduction

• Non-primitive data types, also known as reference types, refer to objects.
• Unlike primitive types, non-primitive types are created by the programmer.

 Categories of Non Primitive Data Types

• Strings
• Arrays
• Classes
• Interfaces
• Collections

Strings

• A sequence of characters.
• Defined using the String class.
• Immutable: once created, their values cannot be changed.

public class StringExample 
{ 
    public static void main(String[] args)
     { 
       String str = "Hello, World!"; 
       System.out.println(str); 
     }
}

Arrays

• A collection of elements of the same type.
• Fixed in size once created.
• Can be one-dimensional or multi-dimensional.

public class ArrayExample
 {
      public static void main(String[] args)
    {
       int[] arr = {1, 2, 3, 4, 5}; for (int num : arr) 
       { 
          System.out.println(num);
        }
     }
 }

Classes

• Blueprint for creating objects.
• Contains fields (variables) and methods to define behaviors.

class Dog 
{
    String breed;
    int age;
    void bark()
    {
        System.out.println("Woof!");
    }
}


public class ClassExample 
{
    public static void main(String[] args)
    {
        Dog myDog = new Dog();
        myDog.breed = "Golden Retriever";
        myDog.age = 5;
        myDog.bark();
        System.out.println("Breed: " + myDog.breed + ", Age: " + myDog.age);
    }
}

Interview Questions

Q-1:- What is type casting? Explain with an example.

A-1:- Type casting is converting a variable from one data type to another. There are two

          types of casting in Java:                    

Q-2:-  What is the difference between float and double?

A-2:- float: Single precision (32 bits), approximately 6-7 decimal digits.

          double: Double precision (64 bits), approximately 15-16 decimal digits.

Q-3:-   What are the major differences in data types between Java and JavaScript?

A-3:-     Java has strong and static typing. Data types must be declared explicitly, and type                

              checking is done at compile-time. Examples include int, double, char, boolean, 

              String.

              JavaScript has dynamic typing. Variables can hold any type of data, and types are 

              checked at runtime. Examples include number, string, boolean, object, 

              undefined, null.

JAVA MCQ

Q-1:- What is the default value of a local variable in Java?

      A) 0

   B) null

   C) false

   D) No default value

Q-2:- Which of the following is not a primitive data type in Java?

A) int

B) float

C) String

D) boolean

Q-3:- Which of the following is not a keyword in Java?

A) static

B) Boolean

C) void

D) private