In public classes, use accessor methods, not public-fields

Occasionally, you may be tempted to write degenerate classes that server no purpose other than to group instance fields:

//Degenerate classes like this should not be public!
class Point {
  public long x;
  public long y;
}

Because the data fields of such classes are accessed directly, these classes do not offer the benefits of encapsulation. You can’t change the representation without change the API.

If a class is accessible outside its package, provide accessor methods.

If a class is package-private or is a private nested class, there is nothing inherently wrong with exposing its data fields.

While it’s never a good idea for a static public class to expose fields directly, it is less harmful if the fields are immutable.

// Public class with exposed immutable fields - questionable
public final class Time {
  private static final int HOURS_PER_DAY = 24;
  private static final int MINUTES_PER_HOUR = 60;

  public final int hour;
  public final int minutes;
}

In summary, public classes should never expose mutable fields. It is less harmful, though still questionable, for public classes to expose immutable fields.

It is, however, sometimes desirable for package-private or private nested classes to expose fields, whether mutable or immutable.