A Java Private Method

In this example I create a class called Square. It has a public method called display_area. This has to call a private method called calculate_area before it can display the result. Private methods are only accessible from within the class which contains them:

andrew@UBUNTU:~/Java$ cat Square.java
public class Square
  {
  double width;
  double area;
  public void display_area()
    {
    calculate_area();
    System.out.println("Area = " + area);
    }
  private void calculate_area()
    {
    area = width * width;
    }
  }
andrew@UBUNTU:~/Java$ javac Square.java
andrew@UBUNTU:~/Java$

I create a program to define a square and display its area (I have shown you a similar example already):

andrew@UBUNTU:~/Java$ cat SquareExample1.java
class SquareExample1
  {
  public static void main(String args[])
    {
    Square my_square = new Square();
    my_square.width = 5;
    my_square.display_area();
    }
  }
andrew@UBUNTU:~/Java$ javac SquareExample1.java
andrew@UBUNTU:~/Java$ java SquareExample1
Area = 25.0
andrew@UBUNTU:~/Java$

However, when I try to execute the calculate_area method directly from outside the class, Java returns a compilation error as the method concerned is private:

andrew@UBUNTU:~/Java$ cat SquareExample2.java
class SquareExample2
  {
  public static void main(String args[])
    {
    Square my_square = new Square();
    my_square.width = 5;
    my_square.calculate_area();
    }
  }
andrew@UBUNTU:~/Java$ javac SquareExample2.java
SquareExample2.java:7: calculate_area() has private access in Square
    my_square.calculate_area();
             ^
1 error
andrew@UBUNTU:~/Java$