Constructor (a special method)

1. Definitions

The same method name with the class name.

No return value, but can not add void.

package 构造方法;
public class Circle extends Shape{
    private double radius;
    private double pi=3.1415926;
    public Circle(){
    }
    public Circle(double radius){
        this.radius = radius;
    }
    
    public double getRadius() {
        return radius;
    }
    
    public void setRadius(double radius) {
        this.radius = radius;
    }
    
    public  double getArea(){
         return radius*radius*pi;
     }
    
    public  double getLength() {
        return 2*pi*radius;
    } 
    
    
}

2, the default constructor

     There must be a class constructor, if writing, the system automatically adds constructor with no arguments.

3, a constructor call another constructor (this)

 1 public class ConstructionMethodDemo {
 2     public static float radius;
 3     private double pi = Math.PI;
 4 
 5     public ConstructionMethodDemo(float radius) {
 6         this.radius = radius;
 7     }
 8 
 9     public ConstructionMethodDemo() {
10         this(radius);
11         System.out.println("圆的面积为:" + radius * radius * pi);
12     }
13 
14     public static void main(String[] args) {
15         ConstructionMethodDemo ccm = new ConstructionMethodDemo(2);
16         ConstructionMethodDemo ccm1 = new ConstructionMethodDemo();
17     }
18 }

Benefits 4, constructor

    Complete object initialization