CS 112
Spring 2005
Ileana Streinu

Java Data Tyes

• Reading: From on-line notes of Rebecca Hasti, Lessons -> Lesson 1 -> Java Types.
• Primitive types: boolean, char, byte, short, int, long, float, double.
• Reference types: arrays, classes. They are really pointers.
• Wrapper classes: Boolean, Integer etc. When an object is really needed. E.g., the Java Stack class is a stack of Objects. To use it for integers, you must convert int to Integer (which is a type of Object). Read from Hasti -> Lesson 2 (Classes) -> Some useful built-in classes (Integer, etc.).

Inheritance: extending existing classes

• How complex a class definition may be? Let's look at the class Point in the java.awt package (what you import in the beginning of any Java program that uses graphics).
  
  How does it compare with the class myPoint defined by you during last week's lab?
• What if we wanted to take advantage of all the methods implemented in the java.awt.Point class, but also include other desirable features (such as shape and color, as in last week's lab)? The solution provided by Java is class inheritance.
  
  public class myPoint extends Point
  {
  int shape;
  int color;
  ...
  }
  
  Notice that:
  
  
  int x;
  int y;
  no longer appear explicitly as class variables of myPoint. But they are present, and are inherited from the Point class.
  
  
• Example of the new class myPoint and how to use it.
• Overriding methods. A class which extends another class inherits the parent's methods. But it can also override them. This means, defining a method with exactly the same name as a method from the parent.
  
  For example, the java.awt class Point has a method to test when two points are equal. This is done by comparing the corresponding x and y coordinates. We can override this method in the class myPoint and have equals also test for equality of the shape and color of a point.
  
  boolean equals(myPoint p)
  {
  if (x == p.x && y == p.y && shape == p.shape && color == p.color)
  return true;
  
  else return false;
  }
  

More on Inheritance and Extending Objects

• Use of super in constructor of a class which extends another class.


• When you are using paint, mousePressed, actionPerformed, etc., you are overriding some method from the class that you are extending.

Overview of new Hwk 4

• Insertion Sort. Applet, from Dominique Thiebaut's web page.
  
  Input: an array A of integers, of size n
  
  Output: in the same array A, the numbers in increasing order.
  
  Algorithm:
  The algorithm works in n-1 iterations by scanning the array from left to right. At iteration i, the ith element is considered and inserted in the part of the array that has already been sorted (from 1 to i-1). For this, the algorithm first searches the array from 1 through i-1 to find where the ith element would fit, then shifts to the right those elements larger than it to make room for its insertion in the array.
  
  
• Developing the program by stepwise refinement. We will develop the program in stages, by stepwise refinement, first on paper, as pseudo-code, then implement it. The pseudo-code development should roughly follow the same structure as the one I gave you for Selection Sort, starting with a general description in English, then refining it to include functions. All parameters to the functions should be clearly identified as IN, OUT or IN/OUT.
  In the stepwise refinement of your program for insertion sort, you should have separate functions for searching and for shifting, invoked when inserting the current element. The main program (applet) should be similar to the one for Selection Sort: get input via mouse clicks, sort, then print and draw the result.