类中定义成员方法。加不加public有什么区别？

class Trangle{
    double sideA, sideB, sideC, area, length;
    boolean flag;
    Trangle(double a, double b, double c){//public
        if(a < (b + c) && b < (a + c) && c < (a + b)){
            sideA = a;
            sideB = b;
            sideC = c;
            flag = true;
        }
        else
            flag = false;
    }
    double getLength(){//public
        length = sideA + sideB + sideC;
        return length;
    }
    double getArea(){//public
        if(flag){
            double p = (sideA + sideB + sideC)/2.0;
            area = Math.sqrt(p*(p-sideA)*(p-sideB)*(p-sideC));
            return area;
        }
        else{
            System.out.println("This is not a trangle");
            return 0;
        }
    }
    void setABC(double a, double b, double c){//public
        if(a < (b + c) && b < (a + c) && c < (a + b)){
            sideA = a;
            sideB = b;
            sideC = c;
            flag = true;
        }
        else{
            flag = false;
            sideA = sideB =sideC = 0;        
        }
    }    
}

class Lader{
    double above,botton,height,area;
    Lader(double a,double b, double h){
        above =a;
        botton = b;
        height = h;
    }
    public double getArea(){
        area = (above + botton)*height/2.0;
        return area;
    }
}

class Circle{
    double radius, area;
    Circle(double r){
        if(r > 0)
            radius = r;
        else 
            radius = 0;
    }
    double getRadius(){
        return radius;
    }
    double getArea(){
        area = Math.PI * radius * radius;
        return area;
    }
    double getLength(){
        return 2 * Math.PI * radius;
    }
    void setRadius(double r){
        if(r > 0)
            radius = r;
        else 
            radius = 0;
    }
}

public class AreaAndLength{
    public static void main(String[] args){
        double length, area;
        Circle circle = null;
        Trangle trangle = null;
        Lader lader = null;
        circle = new Circle(2.0);
        trangle = new Trangle(5.0, 3.0, 4.0);
        lader = new Lader(4, 8, 5);
        
        length = circle.getLength();
        System.out.print("
Circle's length is "+ length);    
        area = circle.getArea();
        System.out.println(". Area is "+ area +".
");
        
        length = trangle.getLength();
        System.out.print("Trangle's length is "+ length);    
        area = trangle.getArea();
        System.out.println(". Area is "+ area +".
");
        
        area = lader.getArea();
        System.out.println("Lader's area is "+ area +".
");
        
        trangle.setABC(12, 34, 1);
        area = trangle.getArea();
        System.out.print("This nwe trangle's area is "+ area);
        length = trangle.getLength();
        System.out.println(". Length is "+ length +".
");    
    }
}

trangle类的成员函数加不加public都可以运行成功，什么原因呢？