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構造方法名要跟類名一致
構造方法內呼叫構造方法，使用this();

public class circle {
    public double r;
    public String colour;
    public static final double pi=3.14;
    public circle(double r){//構造方法名要跟類名一致
        this.r=r;
        this.colour="紅色";
    }
    public circle(String colour){ //構造方法名要跟類名一致
        this(5);//構造方法內呼叫構造方法，使用this();
        this.colour=colour;
        System.out.println("This circle's colour is:"+colour);
    }
    public double c(){
        double r;
        r=this.r;
        return 2*pi*r;
    }
    public double area(){
        double r;
        r=this.r;
        return pi*r*r;
    }
    public void show(){
        String colour;
        double r,c,s;
        r=this.r;
        c=this.c();
        s=this.area();
        colour=this.colour;
        System.out.println("半徑"+r);
        System.out.println("顏色"+colour);
        System.out.println("周長"+c);
        System.out.println("半徑"+s);
    }
}