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("\nCircle's length is "+ length);
         area = circle.getArea();
         System.out.println(". Area is "+ area +".\n");

         length = trangle.getLength();
         System.out.print("Trangle's length is "+ length);
         area = trangle.getArea();
         System.out.println(". Area is "+ area +".\n");

         area = lader.getArea();
         System.out.println("Lader's area is "+ area +".\n");

         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 +".\n");
     }
 }

trangle类的成员函数加不加public都可以运行成功，什么原因呢？