Java Interview Questions

In Java, data types specify the type and size of data that can be stored in a variable. Java supports two categories of data types: primitive data types and reference data types.

Primitive Data Types

Primitive data types are predefined by the language and named by a reserved keyword. There are 8 primitive data types in Java:

1. byte

   • Size: 8-bit
   • Range: -128 to 127
   • Example: byte b = 100;

2. short

   • Size: 16-bit
   • Range: -32,768 to 32,767
   • Example: short s = 10000;

3. int

   • Size: 32-bit
   • Range: -2^31 to 2^31 - 1
   • Example: int i = 100000;

4. long

   • Size: 64-bit
   • Range: -2^63 to 2^63 - 1
   • Example: long l = 100000L;

5. float

   • Size: 32-bit floating point
   • Range: approximately ±3.40282347E+38F (6-7 significant decimal digits)
   • Example: float f = 10.5f;

6. double

   • Size: 64-bit floating point
   • Range: approximately ±1.79769313486231570E+308 (15 significant decimal digits)
   • Example: double d = 20.5;

7. char

   • Size: 16-bit Unicode character
   • Range: '\u0000' (or 0) to '\uffff' (or 65,535)
   • Example: char c = 'A';

8. boolean

   • Size: not precisely defined, typically a single bit
   • Values: true or false
   • Example: boolean b = true;

Reference Data Types

Reference data types are objects that are instances of classes. They store references (addresses) to the actual data, rather than the data itself. Reference types include:

1. Classes

   • Used to define objects that encapsulate data and methods.
   • Example:

     class Car {
         String model;
         int year;
     }
     
     Car myCar = new Car();
     myCar.model = "Toyota";
     myCar.year = 2020;

2. Interfaces

   • Used to define abstract methods that classes can implement.
   • Example:

     interface Vehicle {
         void start();
     }
     
     class Car implements Vehicle {
         public void start() {
             System.out.println("Car started.");
         }
     }
     
     Vehicle myCar = new Car();
     myCar.start();  // Output: Car started.

3. Arrays

   • Containers that hold fixed-size sequential collections of elements of the same type.
   • Example:

     int[] numbers = {1, 2, 3, 4, 5};
     System.out.println(numbers[0]);  // Output: 1

4. Enums

   • Special classes that represent a group of constants (unchangeable variables).
   • Example:

     enum Day {
         SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
     }
     
     Day today = Day.MONDAY;
     System.out.println(today);  // Output: MONDAY

Summary

• Primitive Data Types: byte, short, int, long, float, double, char, boolean.
• Reference Data Types: Class, Interface, Array, Enum.

Primitive data types store actual values, while reference data types store references to the objects. Understanding these data types is crucial for efficient memory management and effective Java programming.