Chapeter 4 Classes and Interfaces

Item 14: In public classes, use accessor methods, not public fields

1. Degenerate classes:
   • serve no purpose other than to group instance fields
   • it does not offer the benefits of encapsulation

   • should always be replaced by classes with private fields and public accessor methods (getters)

               ====================Degenerate classes===================
               // Degenerate classes like this should not be public!
               class Point {
                   public double x;
                   public double y;
               }
     
               =============accessor methods and mutators================
               // Encapsulation of data by accessor methods and mutators
               // to preserve the flexibility to change the class’s internal representation.
               class Point {
                   private double x;
                   private double y;
                   public Point(double x, double y) {
                       this.x = x;
                       this.y = y;
                   }
                   public double getX() { return x; }
                   public double getY() { return y; }
                   public void setX(double x) { this.x = x; }
                   public void setY(double y) { this.y = y; }
               }
     

2. violate the advice that public classes should not expose fields directly

   • if a class is package-private or is a private nested class, there is nothing inherently wrong with exposing its data fields
   • the Point and Dimension classes in the java.awt package
     • it resulted in a serious performance problem that is still with us today.
   • it is less harmful if the fields are immutable

In summary, public classes should never expose mutable fields.

(1) For public classes to expose immutable fields. It is less harmful, though still questionable,

(2) Sometimes desirable for package-private or private nested classes to expose fields, whether mutable or immutable.