public fieldsDon't use public fields when you really want to wrap the internal behavior of a class. Take java.io.BufferedReader for example. It has the following field:
private boolean skipLF = false; // If the next character is a line feed, skip it
skipLF is read and written in all read methods. What if an external class running in a separate thread maliciously modified the state of skipLF in the middle of a read? BufferedReader will definitely go haywire.
public fieldsTake this Point class for example:
class Point {
private double x;
private double y;
public Point(double x, double y) {
this.x = x;
this.y = y;
}
public double getX() {
return this.x;
}
public double getY() {
return this.y;
}
public void setX(double x) {
this.x = x;
}
public void setY(double y) {
this.y = y;
}
}
This would make calculating the distance between two points very painful to write.
Point a = new Point(5.0, 4.0);
Point b = new Point(4.0, 9.0);
double distance = Math.sqrt(Math.pow(b.getX() - a.getX(), 2) + Math.pow(b.getY() - a.getY(), 2));
The class does not have any behavior other than plain getters and setters. It is acceptable to use public fields when the class represents just a data structure, and does not have, and never will have behavior (thin getters and setters is not considered behavior here). It can be written better this way:
class Point {
public double x;
public double y;
public Point(double x, double y) {
this.x = x;
this.y = y;
}
}
Point a = new Point(5.0, 4.0);
Point b = new Point(4.0, 9.0);
double distance = Math.sqrt(Math.pow(b.x - a.x, 2) + Math.pow(b.y - a.y, 2));
Clean!
But remember: Not only your class must be absent of behavior, but it should also have no reason to have behavior in the future as well.
(This is exactly what this answer describes. To quote "Code Conventions for the Java Programming Language: 10. Programming Practices":
One example of appropriate public instance variables is the case where the class is essentially a data structure, with no behavior. In other words, if you would have used a
structinstead of a class (if Java supportedstruct), then it's appropriate to make the class's instance variables public.
So the official documentation also accepts this practice.)
Also, if you're extra sure that members of above Point class should be immutable, then you could add final keyword to enforce it:
public final double x;
public final double y;