Non-Primitive Datatypes (Reference Types)
Introduction
In Java, data types are divided into two main categories:
- Primitive Data Types – predefined and store simple values like int, float, boolean, etc.
- Non-Primitive Data Types (Reference Types) – used to store objects and complex data structures.
Unlike primitive types, non-primitive data types refer to memory locations rather than storing values directly.
They are created by the user or Java API and include:
- Strings
- Arrays
- Classes
- Interfaces
- Enums
1. Strings
A String in Java is a sequence of characters enclosed in double quotes. It is an object of the String class and is immutable (once created, it cannot be changed).
Example:
public class ShikshaSanchar {
public static void main(String[] args) {
String name = "ShikshaSanchar";
System.out.println("Welcome to " + name);
System.out.println("Length: " + name.length());
System.out.println("Uppercase: " + name.toUpperCase());
}
}Output:
Welcome to ShikshaSanchar
Length: 14
Uppercase: SHIKSHASANCHAR
Key Points
- Strings are stored in the String Pool for memory efficiency.
- Common methods:
- length() → returns number of characters.
- toUpperCase() / toLowerCase() → changes case.
- concat() → joins two strings.
- equals() → compares two strings.
2. Arrays
An Array is a collection of elements of the same data type stored in contiguous memory. Arrays in Java are objects, meaning they belong to the reference data type category.
Example:
public class ShikshaSanchar {
public static void main(String[] args) {
int[] marks = {85, 90, 78, 92};
System.out.println("Marks List:");
for (int m : marks) {
System.out.println(m);
}
}
}Output:
Marks List:
85
90
78
92
Key Points
- Array size is fixed once declared.
- Supports both 1D and multi-dimensional arrays.
- Common operations: traversing, updating, sorting.
Syntax:
int[] arr = new int[5]; // Declaration + memory allocation
arr[0] = 10; // Initialization3. Classes
A Class is a blueprint for creating objects. It defines attributes (variables) and behaviors (methods) that an object can have.
Example:
class Student {
String name;
int age;
void display() {
System.out.println("Name: " + name + ", Age: " + age);
}
}
public class ShikshaSanchar {
public static void main(String[] args) {
Student s1 = new Student(); // Object creation
s1.name = "Simran";
s1.age = 21;
s1.display();
}
}Output:
Name: Simran, Age: 21
Key Points
- A class defines properties and methods.
- Objects are created using the new keyword.
- Example: Student s1 = new Student();
- Promotes object-oriented programming concept like encapsulation and reusability.
4. Interfaces
An Interface in Java is a blueprint of a class that contains abstract methods (no body). It is used to achieve abstraction and multiple inheritance.
Example:
interface Animal {
void sound(); // Abstract method
}
class Dog implements Animal {
public void sound() {
System.out.println("Dog barks");
}
}
public class ShikshaSanchar {
public static void main(String[] args) {
Dog d = new Dog();
d.sound();
}
}Output:
Dog barks
Key Points
- All methods in interfaces are abstract by default.
- A class implements an interface using the implements keyword.
- One class can implement multiple interfaces.
- Helps in achieving loose coupling and polymorphism.
5. Enums
An Enum (Enumeration) is a special Java type used to define a collection of constants. Enums make code more readable and type-safe.
Example
enum Day {
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}
public class ShikshaSanchar {
public static void main(String[] args) {
Day today = Day.FRIDAY;
System.out.println("Today is " + today);
}
}Output:
Today is FRIDAY
Key Points
- Enums are implicitly static and final.
- Can have fields, methods, and constructors.
- Commonly used in switch cases, configuration, and representing fixed sets of values.
Summary:
- Non-primitive data types store references to objects, not actual values.
- They support object-oriented features like encapsulation, inheritance, and polymorphism.
- Used widely in real-world Java applications — from data structures to frameworks.
| Concept | Description | Example |
|---|---|---|
| String | Sequence of characters, immutable object. | String name = "Java"; |
| Array | Collection of similar data types. | int[] arr = {1,2,3}; |
| Class | Blueprint for objects. | class Student { } |
| Interface | Contains abstract methods, enables multiple inheritance. | interface Animal { } |
| Enum | Defines fixed constants. | enum Days { MON, TUE } |