How to write an abstract class in Java?

In Java, an abstract class is a class that cannot be instantiated, and it can contain both abstract and non-abstract methods. The steps to write an abstract class in Java are as follows:

1. Declare an abstract class using the keyword “abstract”.
2. In an abstract class, you can include both abstract methods, which are declared using the keyword “abstract,” and non-abstract methods, which are implemented directly.
3. If a class inherits an abstract class and does not implement the abstract methods, then that class must also be declared as an abstract class.

Here is a simple example demonstrating how to write an abstract class.

// 抽象类
abstract class Shape {
    // 抽象方法
    public abstract double getArea();

    // 非抽象方法
    public void printShape() {
        System.out.println("This is a shape.");
    }
}

// 继承抽象类
class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    // 实现抽象方法
    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

public class Main {
    public static void main(String[] args) {
        Circle circle = new Circle(5);
        System.out.println("Area of circle: " + circle.getArea());
        circle.printShape();
    }
}

In the above example, Shape is an abstract class that contains an abstract method getArea() and a non-abstract method printShape(). The Circle class inherits from the Shape class and implements the getArea() method. The Circle class is instantiated in the Main class, where its methods are called.