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Classes Vs Abstract Classes Vs Interfaces

By javaplanet.io / 9 May 2025 / 2 minutes of reading

Aspect	Class	Abstract Class	Interface
Definition	A blueprint for creating objects that can have state and behavior.	A class that cannot be instantiated and can contain both abstract and concrete methods.	A contract that defines a set of abstract methods without implementation.
Instantiation	Can be instantiated using the `new` keyword.	Cannot be instantiated.	Cannot be instantiated.
Keyword	`class`	`abstract class`	`interface`
Method Implementation	Can contain fully implemented methods.	Can contain both abstract and concrete methods.	Prior to Java 8, only abstract methods. Java 8+ supports default and static methods.
Constructors	Can have constructors.	Can have constructors, but they cannot instantiate objects.	Cannot have constructors.
Inheritance	Supports single inheritance.	Supports single inheritance.	Can be implemented by multiple classes (Multiple Inheritance).
Access Modifiers	Can use any access modifier.	Can use any access modifier.	Methods are `public` and `abstract` by default (prior to Java 8).
Fields/Variables	Can have instance variables.	Can have instance variables and `final` static constants.	Can only have `public static final` constants.
Method Overriding	Methods can be overridden in subclasses.	Abstract methods must be implemented in subclasses.	All abstract methods must be implemented in implementing classes.
Usage Purpose	To define common properties and behavior.	To provide a base class with common methods and optional method implementation.	To define a set of rules/behaviors that a class must follow.
Multiple Inheritance	Not supported.	Not supported.	Supported using multiple interfaces.
Java 8+ Features	N/A	Can have default and static methods but not private methods.	Can have default, static, and private methods.
Use Case	When a fully defined class with implementation is needed.	When some methods need to be implemented and some need to be defined by subclasses.	When a contract or a set of behaviors needs to be defined without implementation.