The role and design principles of interfaces and abstract classes in Java

The role of interfaces and abstract classes: Interface: Defines necessary behaviors and forces implementation classes to comply with specific specifications. Abstract class: Defines common behavior, forces subclasses to inherit it, provides partial implementation, and allows subclasses to customize specific behaviors. Design Principles: Interface: Keep it small and focused, defining the necessary behavior. Abstract class: declares only abstract methods and provides concrete methods to achieve common behavior. Example: In the shape class hierarchy, the Shape interface defines the behavior of obtaining the area, the AbstractShape abstract class provides the width and height fields, and the Rectangle and Circle classes inherit the abstract class and provide the area calculation method for a specific shape.

The role and design principles of interfaces and abstract classes in Java

Interface

• Define methods but do not provide implementations.
• Declare a series of public constants.
• Force the class that implements it to provide implementation of all methods.
• Syntax: public interface InterfaceName { ... }

Abstract class

• Definition method , can be an abstract method or a concrete method.
• Abstract methods are not implemented and must be implemented by subclasses.
• Fields and constructors are allowed.
• Syntax: public abstract class AbstractClassName { ... }

Function

Interface

• Achieve decoupling, allowing classes to focus on their own responsibilities.
• Promote modularity and extensibility so that components can be easily interchanged and extended.
• Define behavior contracts to force implementation classes to comply with specific specifications.

Abstract class

• Defines a common behavior and forces subclasses to inherit it.
• Provide partial implementation to reduce repeated code in subclasses.
• Provide hook methods to allow subclasses to customize specific behaviors.

Design principles

Interface

• Define only necessary behavior.
• Method names should be clear and concise, indicating expected behavior.
• Interfaces should be kept small and focused.

Abstract class

• Only declares abstract methods and defines public behavior.
• Provide concrete methods to implement common behavior.
• Consider using abstract classes instead of interfaces when shared implementations are required.

Practical case

Consider a simple shape class hierarchy:

**`java
interface Shape {

double getArea();

}

abstract class AbstractShape implements Shape {

protected double width;
protected double height;

public AbstractShape(double width, double height) {
    this.width = width;
    this.height = height;
}

public double getWidth() {
    return width;
}

public double getHeight() {
    return height;
}

}

class Rectangle extends AbstractShape {

public Rectangle(double width, double height) {
    super(width, height);
}

@Override
public double getArea() {
    return width * height;
}

}

class Circle extends AbstractShape {

public Circle(double radius) {
    super(radius, radius);
}

@Override
public double getArea() {
    return Math.PI * width * width;
}

}

public class Main {

public static void main(String[] args) {
    Shape rectangle = new Rectangle(4, 5);
    Shape circle = new Circle(3);

    System.out.println("Rectangle area: " + rectangle.getArea());
    System.out.println("Circle area: " + circle.getArea());
}

}

在这个例子中：

* `Shape` 接口定义了一个通用行为：获取形状面积。
* `AbstractShape` 抽象类提供了通用行为的通用实现，例如宽度和高度字段。
* `Rectangle` 和 `Circle` 类通过继承 `AbstractShape` 类来扩展形状类层次结构，并提供特定形状的具体方法来计算面积。

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