Classy Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 141

private final String color;
ColorPoint(int x, int y, String color) {
super(x, y);
this.color = color;
}
protected String makeName() {
return super.makeName() + ":" + color;
}
public static void main(String[] args) {
System.out.println(new ColorPoint(4, 2, "purple"));
}
}
Solution 51: What's the Point?
The main method creates and prints a ColorPoint instance. The println method invokes the
toString method of the ColorPoint instance, which is defined in Point . The toString method
simply returns the value of the name field, which is initialized in the Point constructor by calling
the makeName method. For a Point instance, the makeName method returns a string of the form
[x,y] . For a ColorPoint instance, makeName is overridden to return a string of the form
[x,y]:color . In this case, x is 4, y is 2, and the color is purple, so the program prints
[4,2]:purple , right? No. If you ran the program, you found that it prints [4,2]:null . What is the
matter with the program?
The program suffers from a problem with the order of instance initialization. To understand the
problem, we will trace the program execution in detail. Here is an annotated program listing to
guide us:
class Point {