Unit 8: Class Methods

Learning Objectives

Students should

• understand the differences between instance methods and class methods.
• be able to define and use a class method.
• know that the main method is the entry point to a Java program.
• the modifies and parameters required for a main method.

Static Methods

Let's suppose that, in our program, we wish to assign a unique integer identifier to every Circle object ever created. Without using class fields, we might attempt the following.

class Circle {
  private double x;  // x-coordinate of the center
  private double y;  // y-coordinate of the center
  private double r;  // the length of the radius
  private final int id; // identifier

  public Circle(double x, double y, double r, int id) {
    this.x = x;
    this.y = y;
    this.r = r;
    this.id = id;
  }

    : // code omitted
}

Then, on every instantiation of circle, we will have to explicitly supply the latest value for id.

Circle c = new Circle(x, y, lastId);

Of course this can be tedious and prone to error especially if we may instantiate Circle in multiple places. Then we will have to ensure that the latest value of id is communicated on these places correctly.

As an improvement, we may actually just let the value of the latest id be stored in the class Circle itself as a class field.

class Circle {
  private double x;  // x-coordinate of the center
  private double y;  // y-coordinate of the center
  private double r;  // the length of the radius
  private final int id; // identifier
  public static int lastId = 0; // the id of the latest circle instance

  public Circle(double x, double y, double r) {
    this.x = x;
    this.y = y;
    this.r = r;
    this.id = id;
  }

    : // code omitted
}

Then, each time we instantiate a circle, we have to: (i) retrieve the value of lastId and (ii) update the value of lastId.

Circle c = new Circle(x, y, Circle.lastId);
Circle.lastId++;

While better, this is still prone to error as it relies on the fact that the client should not forget to use and update. Furthermore, it requires the client to update it correctly and not just set the value of lastId to some random value. But this can be done better if this is the responsibility of the implementer instead. This can be done with the use of class method.

class Circle {
  private double x;  // x-coordinate of the center
  private double y;  // y-coordinate of the center
  private double r;  // the length of the radius
  private final int id; // identifier
  private static int lastId = 0; // the id of the latest circle instance

  /**
   * Create a circle centered on (x, y) with a given radius
   */
  public Circle(double x, double y, double r) {
    this.x = x;
    this.y = y;
    this.r = r;
    this.id = Circle.lastId;
    Circle.lastId += 1;
  }

  /**
   * Return how many circles have ever existed.
   */
  public static int getNumOfCircles() {
    return Circle.lastId;
  }
}

• On Line 5, we added a new instance field id to store the identifier of the circle. Note that, since the identifier of a circle should not change once it is created, we use the keyword final here.
• On Line 6, we added a new class field lastId to remember that the lastId of the latest circle instance. This field is maintained as part of the class Circle and is initialized to 0. Now this class field is private instead of public because it is only accessed and updated within the constructor.
• On Line 15 and 16, as part of the constructor, we initialize id to lastId and increment lastId. We explicitly access lastId through Circle to make it clear that lastId is a class field.

Note that all of the above are done privately beneath the abstraction barrier. In other words, the class field that we have added is actually set to private. Additionally, now it does not matter what the client does, the value of lastId will correctly reflect the number of circle that has been created. The client need not even know about the existence of this class field.

Since lastId is incremented by one every time a circle is created, we can also interpret lastId as the number of circles created so far. On Line 22-24, we added a method getNumOfCircles to return its value.

The interesting thing here is that we declare getNumOfCircles with a static keyword. Similar to a static field, a static method is associated with a class, not to an instance of the class. Such method is called a class method. A class method is always invoked without being attached to an instance, and so it cannot access its instance fields or call other of its instance methods. The reference this has no meaning within a class method. Furthermore, just like a class field, a class method should be accessed through the class. For example, Circle.getNumOfCircles().

Other examples of class methods include the methods provided in java.lang.Math: sqrt, min, etc. These methods can be invoked through the Math class: e.g., Math.sqrt(x).

Non-Static from Static

Recap that for static fields (i.e., class fields), we only have exactly one instance of it throughout the lifetime of the program. More generally, a field or method with modifier static belongs to the class rather than the specific instance. In other words, they can be accessed/updated (for fields, assuming proper access modifier) or invoked (for methods, assuming proper access modifier) without even instantiating the class.

Think for a moment about the consequence of this. If we have not instantiate the class, it means that there is no instance. The keyword this captures the current instance. If there is no instance, the keyword this cannot be referring to any instance. Therefore, it is not reasonable to assign a specific value to this keyword.

Since we have no reasonable value for this within the context of static method, Java actually prevents the use of this from any method with static modifier.

  public static int getLastId() {
    return this.id;  // what is "this" if no Circle has been created?  what if there are 100 circles?
  }

Try it out, you will get the following error.

_.java:_: error: non-static variable this cannot be referenced from a static context
    return this.id;
               ^

The opposite is not true. We can access class fields from non-static methods.

The main method

The most common class method you will use is probably the main method.

Every Java program has a class method called main, which serves as the entry point to the program. To run a Java program, we need to tell the JVM the class whose main method should be invoked first. In the example that we have seen,

java Hello

will invoke the main method defined within the class Hello to kick start the execution of the program.

The main method must be defined in the following way:

public static final void main(String[] args) {
}

You have learned what public and static means. The return type void indicates that main must not return a value. We have discussed what final means on a field, but are not ready to explain what final means on a method yet. In fact, this is an optional modifier for the main method.

The main method takes in an array ([]) of strings as parameters. These are the command-line arguments that we can pass in when invoking java. String (or java.lang.String) is another class provided by the Java library that encapsulates a sequence of characters.

Class Diagram (Part 3)

Static fields and/or methods are represented by underline in class diagram. The current version of Circle has the following class diagram.

If the number of constructs for class diagram is getting too much, do not worry, we will have a unit added to the Java Guide specifically to revise class diagram with application on designing solutions to problems.