Java Reference
In-Depth Information
There are times when we want a class simply to aggregate a few values, and
we should not hesitate to construct a class with public fields if that suits the pur-
pose. However, more usually, a class is developed for the behavior of its
instances: the fields are private, and all interaction with an instance is through
public methods. Classes URL , JFrame , Date , and Employee are examples of this;
they represent the more popular case.
In Fig. 3.6, we show a use of class Coordinates . Field cent of class Circle
contains the center of a circle. There are four methods, which give the usual
properties of a circle: center, radius, diameter, and area. We have not used the
convention for getter-method names in naming these methods. In this case, the
more conventional names seem more appropriate. We leave the writing of a
toString method for this class to you.
Self-review exercises
SR1. Write a declaration (with no initialization) of two variables: one of class-
type String and another of class-type Employee.
SR2. Write down the three steps in evaluating a new-expression (it is impor-
tant for future work that you memorize these three steps).
SR3. Below is a new-expression. Evaluate it. In executing the constructor call,
step over the call, i.e. execute it as an indivisible action based on what the spec-
ification of the constructor says. See Fig.3.1. What is the value of the call?
new Employee("Clinton", 1996)
Figure 3.6 contains a class Circle . What is wrong with this expression?
/** An instance is a point (x, y) in the plane */
public class Coordinates {
/** The point is (x, y) */
public int x;
public int y;
/** Constructor: an instance for point (x,y) */
public Coordinates( int x, int y) {
this .x= x; this .y= y;
/** = the String "(x, y)" */
public String toString() {
return "(" + x + ", " + y + ")";
Figure 3.5:
Class Coordinates
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