...Section: Class

A class is something that encapsulates functions as well as data, e.g.,

public class SomeClass {
  
  // Example of data:
  int i;

  // Example of a constructor
  public SomeClass(int i) {
    this.i = i;  
    // this.i refers to the instance variable i as opposed to the argument i
  }

  // Example of a function:
  public void printStuff ()
  {
    System.out.println (i);
  }

  // Main function.  Not all classes have a main
  public static void main (String[] argv)
  {
    SomeClass sc = new SomeClass(42)
    sc.printStuff (); 
  }

} // End of class SomeClass

Note three conventions that java programmers tend to follow:

1. Class names begin with a Capital letter and continue in lower case (subject to (3))
2. Variable names begin with a lower case letter and continue that way (subject to (3))
3. If a name consists of many words, the start of a new word inside the name is signified with a Capital letter

You can have multiple classes in a single file, but only one may be public. The public class name must match the file name. For example the above class SomeClass should live in a file called SomeClass.java. If you have a main method, it should be in the public class if it is to run.

Here are some charming facts about methods:

• You can have as many methods as you like.
• Methods do not nest. (You cannot declare a method inside another.) Same is true in C. But not in many other languages, including Pascal.
• Methods must declare a return value. (The return type is void if nothing is returned).
• All of a class's methods are at the same level, like printStuff and main above
• static methods can be accessed without creating an object of that class. main has to be static because it is called before the JVM has had a chance to create any objects. Other than that, it is not good Object Oriented (OO) style to declare static methods without a darn good reason.
• Constructor methods have the same name as the class public ClassName(..) and can have whatever arguments you like. You may have several constructors as long as each has a different signature. By default, Java provides every class with a trivial constructor taking no arguments. You can replace Java's default constructor with your own (as we do in SomeClass below.

Modifiers like public are used to indicate the visibility of class members. We use the dot-operator to access methods (or data) from another class, e.g,

public class SomeClass {
  
  // Example of data:
  int i;

  // Examples of constructors
  public SomeClass() {
    i = 0;             // provide a default value if called with no args
  }
  public SomeClass(int i) {
    this.i = i;  
    // this.i refers to the instance variable i as opposed to the argument i
  }

  // Example of a function:
  public void printStuff ()
  {
    System.out.println (i);
  }

  // Main function.  Not all classes have a main
  public static void main (String[] argv)
  {
    AnotherClass sc = new AnotherClass(42)
    sc.printStuff (); 
  }

} // End of class SomeClass

Even though there is a printStuff method in SomeClass, the programmer has decided to call a printStuff method belonging to another class (presumably because it is better for current purposes.

Exercise 1

class AnotherClass {
  public void printStuff (int i)
  {

  }
} 

The integer value is: 42
and thank you for using AnotherClass