CS 112
Spring 2005
Ileana Streinu

User-defined Objects and Classes in Java

• Object oriented programming comes with a distinct programming philosophy. The way you think about your program is different.
• You design your program from the top, thinking about what you want to do and organizing the code into functional units that act on objects.
• You may want to think of objects as being highly structured data with some prescribed functionality. For example, in Java you have seen Button's, Fonts etc.
• In C, you have seen so far only one type of non-elementary data structure, the array. An array is only one of the many possible ways of grouping data. One of its characteristics, for instance, is that it is suitable for the use of iterative for loops to access all of its elements.
• In object oriented programming you group the data by functional criteria and at the same time specify the methods that are relevant for manipulating the objects in that class. For instance, a Button can be created (constructed) with a specific label; it can be added on to the applet, etc.

• New type of object: myPoint (a sort of point...).
  
  class myPoint{
  {
  ...
  }
  
• Information grouped together in a single object of type point: the x and y coordinates of a point on the screen.
  
  class myPoint{
  {
  int x;
  int y;
  ...
  }
  
• Operations done with an object of type point:
  • Create a new point (from a pair of integers giving the coordinates).
  • Draw a point on the screen (for instance, as a small black spot).
  • Print the coordinates of the point on the Java console.
  In Object oriented programming, we refer to these operations as methods applied on objects of type myPoint, rather than operations or functions or procedures.
  
  public class myPoint{
  {
  int x;
  int y;
  myPoint(int a, int b) // constructor
  {
  ...
  }
  
  public void Print() // print method
  {
  ...
  }
  
  public void Draw() // draw method
  {
  ...
  }
  // and perhaps other methods
  ...
  }// end class myPoint
  
  
  
• Applying methods on an object from this new class.
  • Creating a new object with new:
    
    myPoint p1; // declaration of a variable of type myPoint
    ...
    // creating a new object of type myPoint
    p1 = new myPoint(20,30);
    ...
    
    
  • To access the values of the class variables of an existing object:
    
    p1.x
    p1.y
    
    
  • To Print an object of type myPoint:
    
    p1.Print(); // "apply" the method Print to the object p1
    
    
  • To Draw an object of type myPoint:
    
    p1.Draw(); // "apply" the method Draw to the object p1