Scarborough College

   University of Toronto
   Fall 1997

   _________________________________________________________

   CSC A06F: Introduction to Computer Programming

   Lecture notes 3: The elements of programs _ Part 2

   _________________________________________________________



   Reading:


     fflLewis and Loftus, chapters 3 and 4.



Copyright cfl1997 Philip Edmonds. All rights reserved.
CSC A06F LECTURE NOTES 3                                             2




                    The if-else statement



       if (condition)

           statement1;

       else

           statement2;



   Just like the normal if statement, but if the condition is false, state-

   ment2 is executed.


   One or the other is executed. Never both.
CSC A06F LECTURE NOTES 3                                             3




                          Nested ifs


   Can `nest' if-statements within each other to make more complex

   decisions.

       if (age < 19)

          if (selection.equals ("beer"))

              System.out.println("Not legal.");

          else

              System.out.println("OK");

       else

          System.out.println("OK");
CSC A06F LECTURE NOTES 3                                             4




                      Block statements


   The syntax of the if-statement says it only allows one statement.


   So, how do we make it do several things when the condition is true?



   A block statement  gathers a group of statements together. Java

   treats it like a single statement.


   Use fg (brace brackets) to mark the beginning and end of the block.


   E.g.,

       if (selection.equals ("beer")) -

          int years = 19 - age;

          System.out.println("Not legal.");

          System.out.println("Come back in " + years);

       "

       else -

          ...

       "
CSC A06F LECTURE NOTES 3                                             5




                       Flow of control


   Normally the computer executes your program from top to bottom,

   one statement at a time.


   The flow of control is linear .


   The if-statement changes the flow of control:

       a statement may or may not be executed, depending on the

   condition.


   There are other ways to change the flow of control:



     fflloops

     fflcalls to methods
CSC A06F LECTURE NOTES 3                                             6




                    Repetition and loops


   A loop is a way to have the same statements executed many times

   in a row.


   Why would you want this?



   Several kinds of loop: one is the while loop.
CSC A06F LECTURE NOTES 3                                             7




                       The while loop



       while (condition)

           statement;



   Executes statement if condition is true.


   Then goes back and checks the condition again. If it's still true,

   executes statement again.


   Keeps `looping' until the condition is false. E.g.,



       int sum = 0;

       int number = Integer.parseInt (stdin.readLine());



       while ( number > 0 ) -

          sum = sum + number;

          number = Integer.parseInt (stdin.readLine());

       "


   How does the condition become true?
CSC A06F LECTURE NOTES 3                                             8




                      Another example


   Complete this code fragment:

       final int N = 10;

       int sum = 0;



       // this loop sums up all the integers from 1 to N

       while (             ) -



       "



   What happens if the condition never becomes true?
CSC A06F LECTURE NOTES 3                                             9




                           Objects


   Breaking a system up into objects that communicate with one an-

   other is a good way to design a system.



   An object in a system is like a real-world object. It has three parts:



     fflidentity


     fflstate


     fflbehaviour



   Can have many objects that are similar.

       I.e., , that have same behaviour, but different states and identi-

   ties



   A class is used to define an object (or a set of similar objects).


   A class is a blueprint for building lots of similar objects.
CSC A06F LECTURE NOTES 3                                             10




                       Objects in Java


   To use an object in Java, you need:



     ffla variable

     ffla class



   Then, you have to build the object. E.g.,


       String student;



       student = new String("Phil Edmonds");


   Notes:



     fflstudent is a variable that is declared to contain String ob-

       jects.

     fflString is a class that is defined in the Java API.

     fflDifferent from primitive types. Must use new to build and ini-

       tialize an object.
CSC A06F LECTURE NOTES 3                                             11




                  Assignment with objects


   Can also use assignment with objects:


       String student2;

       student2 = student;


   Although it looks the same as regular assignment, it works a little

   differently.


   This makes student2 contain the exact same object as student

   does. (Same identity, same everything).



   Whereas assignment for primitive data types copies the data value.

   So you have 2 separate, but same, data values.
CSC A06F LECTURE NOTES 3                                             12




            Using an object: calling its methods


   Objects have methods . You can call these to tell the object to do

   something.


   Examples:

       System.out.println("Hello world!");

                   ^^^^^^^^^^^^^^^^^^^^^^^



       token = stdin.readLine();

                       ^^^^^^^^^^



       if (selection.equals("beer") -"

                       ^^^^^^^^^^^^^^


   You use the dot operator (.) to call a method of an object.


   Syntax:



       object-reference.method-name (parameters);
CSC A06F LECTURE NOTES 3                                             13




                 How calls to methods work


   This makes the flow of control change. What happens?



     fflthe computer remembers this spot in the code;

     fflit then starts running the statements in the method;

     fflwhen finished these, it `returns' to the spot, and continues on

       from there.
CSC A06F LECTURE NOTES 3                                             14




                  Some methods of String



   length()



   equals(Object obj)



   compareTo(String str)



   indexOf (char ch)



   charAt(int index)