java求圆的面积pta

import java.util.*;

class Circle{
	private int radius;
	
	public Circle() {
		radius=2;
		System.out.println("This is a constructor with no para.");
	}
	
	public Circle(int radius) {
		this.radius=radius;
		System.out.println("This is a constructor with para.");
	}

	
	
	public int getRadius() {
		return radius;
	}

	public void setRadius(int radius) {
		if(radius<=0) {
			radius=2;
		}
		else
		this.radius = radius;
	}

	public float getArea() {
		double area=radius*radius*Math.PI;
		return (float)((Math.round(area * 100)) / 100.0);
	}

	@Override
	public String toString() {
		return "Circle [radius=" + radius + "]";
	}
	
}

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc=new Scanner(System.in);
		Circle c1=new Circle();
		System.out.println(c1.toString());
		System.out.println(c1.getArea());
		
		Circle c2=new Circle();
		System.out.println(c2.toString());
		System.out.println(c2.getArea());
		
	
		int radius=sc.nextInt();
		c2.setRadius(radius);
		System.out.println(c2.toString());
		System.out.println(c2.getArea());
	
		Circle c3=new Circle(sc.nextInt());
		System.out.println(c3.toString());
		System.out.println(c3.getArea());
		sc.close();
	}

}

1. return (float)((Math.round(area * 100)) / 100.0),将Math方法运算后产生的整数转化为小数
2. Circle c1=new Circle()，创建对象
3. c2.setRadius(radius)，指明要使用的方法名与参数
4. Circle c3=new Circle(sc.nextInt())，从键盘传入数据