WebReference.com - Part 4 of chapter 5 from Beginning Java 2 SDK 1.4 Edition, Wrox Press Ltd (6/8)

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Beginning Java 2 SDK 1.4 Edition

Specifying Access Attributes

As you probably gathered from the diagrams that we just looked at, to specify an access attribute for a class member, you just add the keyword to the beginning of the declaration. Here is the Point class you saw earlier, but now with access attributes defined for its members:

Try It Out--Accessing the Point Class

Make the following changes to your Point class. If you save it in a new directory, do make sure Line.java is copied there as well. It will be useful later if they are in a directory with the name Geometry.

public class Point {
  // Create a point from its coordinates
  public Point(double xVal, double yVal) {
    x = xVal;
    y = yVal;
  }
  // Create a Point from an existing Point object
  public Point(final Point aPoint) {
    x = aPoint.x;
    y = aPoint.y;
  }
 
  // Move a point
  public void move(double xDelta, double yDelta) {
    // Parameter values are increments to the current coordinates
    x += xDelta;
    y += yDelta;
  }
  // Calculate the distance to another point
  public double distance(final Point aPoint) {
    return Math.sqrt((x--aPoint.x)*(x--aPoint.x)+(y--aPoint.y)*(y--aPoint.y));
  }
  // Convert a point to a string 
  public String toString() {
    return Double.toString(x) + ", " + y;    // As "x, y"
  }
  // Coordinates of the point
  private double x;
  private double y;
}

The members have been re-sequenced within the class with the private members appearing last. You should maintain a consistent ordering of class members according to their access attributes, as it makes the code easier to follow. The ordering adopted most frequently is for the most accessible members to appear first, and the least accessible last, but a consistent order is more important than the particular order you choose.

How It Works

Now the instance variables x and y cannot be accessed or modified from outside the class as they are private. The only way these can be set or modified is through methods within the class, either with constructors, or the move() method. If it is necessary to obtain the values of x and y from outside the class, as it might well be in this case, a simple function would do the trick. For example:

public double getX() {
  return x;  
}

Couldn't be easier really, could it? This makes x freely available, but prevents modification of its value from outside the class. In general, such methods are referred to as accessor methods, and usually have the form getXXX(). Methods that allow a private data member to be changed are called mutator methods, and are typically of the form setXXX() where a new value is passed as an argument. For example:

public void setX(double inputX) {
  x = inputX;  
}

It may seem odd to use a method to alter the value of a private data member when you could just make it public. The main advantage of using a method in this way is that you can apply validity checks on the new value that is to be set.

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Created: July 29, 2002
Revised: July 29, 2002

URL: https://webreference.com/programming/java/beginning/chap5/4/6.html