Defining Classes - Beginning Java - page 230

aPoint.y));
}

// Convert a point to a string
public String toString() {
return Double.toString(x) + ", " + y; // As "x, y"

}

Directory "Try Geometry"

You should save this as Point.java in the directory Try Geometry .
How It Works
This is a simple class that has just two instance variables, x and y , which are the coordinates of the Point
object. At the moment you have two constructors. One creates a Point object from a coordinate pair
passed as arguments of type double , and the other creates a new Point object from an existing one.
Three methods are included in the class. You have the move() method, which moves a Point to another
position by adding an increment to each of the coordinates. You also have the distance() method,
which calculates the distance from the current Point object to the Point object passed as the argument.
This uses the Pythagorean theorem to compute the distance, as shown in Figure 5-7 .

FIGURE 5-7

Finally, you have a toString() method, which returns a string representation of the coordinates of the
current point. If a class defines the toString() method and an object of that class is used as an operand
of the string concatenation operator + , the method is called to represent the object as a string. The com-
piler automatically inserts a call to toString() when necessary. For example, suppose thePoint is an
object of type Point , and you write the statement:
System.out.println("The point is at " + thePoint);

The toString() method is automatically invoked to convert the object referenced by the variable
thePoint to a String , and the resultant string is appended to the String literal. You have specified the
toString() method as public , as this is essential here for the class to compile. I will defer explanations
as to why this is necessary until a little later in this chapter.