代码：

public class Main{
    private double yibian;
    private double erbian;
    private double sanbian;
    public Main(double yibian,double erbian,double sanbian){        //初始化变量不用void
        this.yibian=yibian;
        this.erbian=erbian;
        this.sanbian=sanbian;
    }
    public double SJXZC(){
        return yibian+erbian+sanbian;
    }
    public double SJXMJ(){
        double p=SJXZC()/2;
        double area=Math.sqrt(p*(p-yibian)*(p-erbian)*(p-sanbian));
        return area;
    }
    public static void main(String[] args){
        Main SJX = new Main(3,4,5);
        double Mj = SJX.SJXMJ();
        System.out.println(Mj);
        double Zc=SJX.SJXZC();
        System.out.println(Zc);

    }}

收获：

看注释，看注释。

若有了void的报错

java: 无法将类 Main中的构造器 Main应用到给定类型;
  需要: 没有参数
  找到: int,int,int
  原因: 实际参数列表和形式参数列表长度不同