Item 16: In public classes, use accessor methods, not public fields

// Degenerate classes like this should not be public!
class Point {
    public double x;
    public double y;
}
// Encapsulation of data by accessor methods and mutators
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; }
}

Several classes in the Java platform libraries violate the advice that public classes should not expose fields directly. Prominent examples include the Point and Dimension classes in the java.awt package. Rather than examples to be emulated, these classes should be regarded as cautionary tales. As described in Item 67, the decision to expose the internals of the Dimension class resulted in a serious performance problem that is still with us today.

// 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 minute;

    public Time(int hour, int minute) {
        if (hour < 0 || hour >= HOURS_PER_DAY)
           throw new IllegalArgumentException("Hour: " + hour);
        if (minute < 0 || minute >= MINUTES_PER_HOUR)
           throw new IllegalArgumentException("Min: " + minute);
        this.hour = hour;
        this.minute = minute;
    }
    ... // Remainder omitted
}

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.