Java Interfaces and Abstract Classes: Mastering the Difference Makes a Master Programmer

php editor Zimo brings you the differences between Java interfaces and abstract classes to become a programming master. Interfaces and abstract classes in Java are two commonly used object-oriented programming concepts, each with its own characteristics and applicable scenarios. By having an in-depth understanding of its differences and applications, you can better improve your programming skills and use them flexibly in project development. Interfaces emphasize specifications, while abstract classes pay more attention to structure. Mastering the differences will make you more comfortable on the road to programming!

• The interface is a pure abstract type without any method implementation.
• The interface only contains method declarations and constant definitions.
• Classes inherit their method signatures by implementing interfaces and must implement all declared methods.
• Interfaces can implement multiple inheritance (a class can implement multiple interfaces).
• Interface cannot instantiate objects.

Abstract class

• Abstract classes contain abstract methods and concrete methods.
• Abstract methods are not implemented and must be implemented by subclasses.
• Abstract classes can only be inherited once, so multiple inheritance cannot be achieved.
• Abstract classes can instantiate objects, but only their subclasses.

The difference between interface and abstract class

feature	interface	Abstract class
Method implementation	No	There can be specific methods
Method declaration	can only be an abstract method	Can be abstract and concrete methods
Class implementation	Must fully implement the interface	Abstract methods can be optionally overridden or implemented
inherit	Support multiple inheritance	Only supports single inheritance
Instantiation	Cannot instantiate object	Can instantiate subclasses

Choose interface or abstract class

Choosing to use an interface or an abstract class depends on the specific scenario:

• Using interface:
  • When a set of method signatures need to be defined without implementation.
  • When multiple inheritance needs to be implemented.
  • When you need to ensure that the class implements all the functions of the interface.
• Use abstract class:
  • When it is necessary to provide a default implementation of a method, but allow subclasses to override it.
  • When it is necessary to instantiate an object with a partial implementation.
  • When you need to restrict subclasses to inherit from only one class.

Example

Consider the following example:

interface:

interface Shape {
double getArea();
double getPerimeter();
}

Abstract class:

abstract class PolyGon {
int numSides;

abstract double getArea();

double getPerimeter() {
// 默认实现，适用于所有多边形
}
}

Concrete class:

Implementation interface:

class Circle implements Shape {
@Override
public double getArea() { ... }

@Override
public double getPerimeter() { ... }
}

Inherit abstract class:

class Square extends Polygon {
@Override
public double getArea() { ... }

@Override
public double getPerimeter() { ... } // 可覆盖默认实现
}

Understanding the difference between interfaces and abstract classes is crucial for designing robust and scalable code in Java. By choosing the right abstract type wisely, you can improve the reusability, scalability, and maintainability of your code.

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