05-Writing Classes

Objects

• Attribute / Field / State: basic characteristics that define the object
• Behaviors: activities of the object

Class variables (static fields)

certain states that belong to the class

• if you change a class variable anywhere, you change it everywhere, that is, for every object in the class

Instance variables (non-static fields)

certain states that belong to the object

• only an object may have a value for instance variables
• changing the value of one object does NOT affect other objects

Classes

the blueprint from which an object is created

• an object is also called an instance of the class
• classes define attributes and available behaviors of its objects
• members:
  • instance variables: attributes of objects
  • constructors: procedures for constructing a new object of the class and initializing its fields
  • methods: behaviors
  • e.g.

   public class Student{
	   //instance variables
	   private String name;
	   private double gpa;
	   
	   //constructor
	   public Student(String newName, double newGpa){
		   name = newName;
		   gpa = newGpa;
	   }
	   
	   //methods
	   public String getName(){
		   return name;
	   }
	   
	   public void setGpa(double newGpa){
		   gpa = newGpa;
	   }
   }

Constructors

• The name of the constructor is always the same as the name of the class
• Constructors have no return type (not even void)

Methods

• Methods that don’t need an object, that can be called from the class are called static methods (can access only class variables)
• Methods that can only be accessed from an object are called non-static methods (can access class variables or instance variables)

Creating an object

• use the keyword “new”: e.g. in a Lightbulb class, creating an object looks like this (assuming the constructor has no parameters): Lightbulb x = new Lightbulb();

• Different ways of creating an object have different effects, below is an example using String objects

  • String y = "Hello"; identical string objects created this way points to the same location in memory
  • String z = new String ("Hello"); identical string objects created this way create new strings each time in memory

• In sum, use == to compare primitive data types. Some objects can be accurately compared using the .equals() method.