Abstract classes and interfaces are powerful tools in object-oriented programming. They help create flexible, reusable code structures by defining common behaviors and contracts for related classes. These concepts are crucial for understanding inheritance and polymorphism in Java.
In this section, we'll explore how abstract classes provide partial implementations and blueprints for subclasses. We'll also dive into interfaces, which define contracts for classes and enable a form of multiple inheritance in Java.
Abstract Classes
Defining Abstract Classes and Methods
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Top images from around the web for Defining Abstract Classes and Methods
java - abstract classes ,constructor and interface - Stack Overflow View original
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java - abstract classes ,constructor and interface - Stack Overflow View original
Is this image relevant?
java - abstract classes ,constructor and interface - Stack Overflow View original
Is this image relevant?
java - abstract classes ,constructor and interface - Stack Overflow View original
Is this image relevant?
1 of 2
Abstract class serves as a blueprint for other classes, cannot be instantiated directly
Contains abstract methods without implementation, forcing subclasses to provide concrete implementations
May include non-abstract methods with full implementations
Abstract methods declared using
abstract
keyword, end with semicolon instead of method body
Subclasses must implement all abstract methods or be declared abstract themselves
Creating and Using Concrete Classes
Concrete class extends abstract class, provides implementations for all abstract methods
Can be instantiated to create objects
Inherits non-abstract methods and variables from abstract superclass
Uses
extends
keyword to inherit from abstract class
Can override inherited methods to provide specialized behavior
Implementation and Polymorphism
Abstract classes enable polymorphism, allowing objects of different subclasses to be treated uniformly
Promote code reuse by defining common attributes and behaviors in abstract superclass
Support partial implementations, useful for defining framework or template for related classes
Cannot be instantiated using
new
keyword, but can be used as reference type
Abstract class constructors called when concrete subclasses are instantiated
Interfaces
Defining and Implementing Interfaces
Interface declares a set of abstract methods without implementations
Serves as a contract for classes that implement it
Declared using
interface
keyword instead of
class
Methods in interfaces implicitly
public
and
abstract
Classes implement interfaces using
implements
keyword
Can contain constant variables (implicitly
public
,
static
, and
final
)
Multiple Interface Inheritance
Java supports multiple interface inheritance, allowing a class to implement multiple interfaces
Enables a form of multiple inheritance without the complexity of multiple class inheritance
Class must provide implementations for all methods from all implemented interfaces
Resolves diamond problem by requiring explicit implementation of conflicting methods
Interfaces can extend other interfaces using
extends
keyword
Default and Static Methods in Interfaces
Default methods introduced in Java 8, provide default implementation in interfaces
Declared using
default
keyword, have method body unlike traditional interface methods
Allow adding new methods to interfaces without breaking existing implementations
Static methods in interfaces can be called without an instance of the implementing class
Both default and static methods can be overridden in implementing classes if needed