CSCI 162 - Lecture 3 - Java Classes and Information Hiding (continued), Multidimensional Arrays

Instructor's Class Notes

1. Java Classes and Information Hiding - Chapter 2
• Location Class
1. Source is here
2. Documentation is here
3. Parts of the class:
• Constructor - Constructor that accepts as parameters the inital coordinates of the location object.
• shift method - Modification method that shifts the point a specified amount alont the x and y axis.
• rotate90 method - Modification method that rotates the location 90 degrees in a clockwise direction.
• getX and getY methods - Two accessor methods that get the x and y coordinates, respectively.
• distance method - A static method that computes and returns the distance between two supplied Location objects.
• midpoint method - A static method that computes and returns the point midway between tow supplied Location objects.
• equals method - Accepts an object and returns true if that object is equal to the Location object.
• toString method - Returns a string representation of the object and its current coordinates.
• clone method - A method that returns an exact copy of the Location object.
4. Key Topics:
• Parameters that are Objects - Objects can often be useful parameters, especially for static methods like distance and midpoint.
• Methods May Access Private Instance Variables - The accessor and modificaiton methods access the private instance methods to return them or to modify them. No direct access from private callers (code that has created a Location object) can directly access the private instance variables. Again, this is "information hiding."
• Return Value of a Method may be an Object - It is often necessary for certain methods to return objects, in particular objects of the same class. Examples are the clone method.
• Java's Object Type - Java has a generic object type that can be used to pass any specific object type as a parameter, very useful and pretty much required for implementing the equals method.
• Using and Implementing an Equals Method - The equals method is a method common to the object world. Programmers come to expect a well written class will include the equals method.

• public boolean equals(Object obj)
  {
      if (obj instanceof Location)
      {
          Location candidate = (Location) obj;   // Cast to a Location object
          // Return true if instance states are the same
          return (candidate.x == x && candidate.y == y);   
      }
      else
      {
          return false;  // Not a Location object
      }
  }                           

• Using and Implementing a Clone Method - A clone method is expected by programmers when dealing with object classes so that they can conveniently make an exact copy of an object.
• Modify Class Head - The class header must be updated to included the implmenets Cloneable keywords
  

  public class Location implements Cloneable                            
                              

• Use super.clone to make a copy - This is important as the object itself has a clone method (the one being implemented) and if you don't specific super.clone then your implementation will be stuck in an infinite loop of the clone method calling itself repeatedly.
• Actual code
  

     public Location clone( )
     {  // Clone a Location object.
        Location answer;
        
        try
        {
           answer = (Location) super.clone( );
        }
        catch (CloneNotSupportedException e)
        {  // This exception should not occur. But if it does, it would probably
           // indicate a programming error that made super.clone unavailable.
           // The most common error would be forgetting the "Implements Cloneable"
           // clause at the start of this class.
           throw new RuntimeException
              ("This class does not implement Cloneable.");
        }
        
        return answer;
     }                            
                              

2. Multidimensional Arrays - Chapter 3
• Multidimensional Array - An array of arrays, the elements of which are accessed with multiple integer indexes (one per dimension).
• All dimensions must be arrays of the same data type.
• All dimensions have arrays that are zero-based.
• Easy to think of in two dimensions (think of a sheet with rows and columns), especially Rectangular Two-Dimensional Arrays.
  

                  
  double[][] temps = new double[3][4];   // 3 rows by 4 columns                
                  
                  

• Harder to visualize in three or more dimensions, but a bit easier if you think of multiple sheets, with each sheet a different "plane", with general recommendation to think of first dimension as the plane #, followed by row, followed by column.
  

                  
  int[][][] numbers = new int[numPlanes][numRows][numColumns];