Classes

• Inside a Java class declaration, programmers define the behavior that objects of the class will implement. Instance variables appear within most class declarations. If a class contains one or more instance variables, each object instance of the class will have its own copy of these. The following sample code is a typical example of an instance variable appearing within a class declaration:
  
  String myName;
  
  The constructor method for the class could instantiate this variable as follows:
  
  public Person(String name) {
  
  myName = name;
  
  }
  
  In this case the class, named "Person," has a constructor function which takes a string parameter representing the name for an individual instance of the class. To create an object of the class, programs could use the following code:
  
  Person aPerson = new Person("Mary");
  
  

Variables

• Unlike instance variables, static variables belong to a class, rather than to specific instances of it. This means that there is only one copy of a static variable, which is accessible from all members of the class, as well as from external "customer" code via objects of the class. For example, a static variable could keep track of a property within the application which remains the same for all class members. The following sample code demonstrates declaring a static variable inside a class declaration:
  
  private static int numWomen = 0;
  
  Within the class constructor or another method, the variable can be accessed and updated as follows:
  
  numWomen++;
  
  

Methods

• Class declarations can include static methods. As with variables, static methods provide some functionality that is the same across all object instances of a class. Static methods commonly carry out processing that involves static variables. The following sample static method returns the value of a static variable within a class declaration:
  
  public static int getNumWomen() {
  
  return numWomen;
  
  }
  
  

Access

• Java code does not need to create an instance of a class to call static methods. For example, the following syntax demonstrates calling the method on the class itself:
  
  Person.getNumWomen();
  
  Many programmers first experience this technique when using classes of the Java language, rather than their own classes, as in the following example:
  
  System.out.println("Hello");
  
  This code uses the System class to write a text string to the output console. Programs can access both static variables and methods using the class name rather than the name of an object instance of the class.