CSC153, Class 33: Conditionals in Java

Overview:
* Review of Java to date
* Boolean expressions
* The if statement
* The switch statement
* Lab: J4.2 and J4.4

Notes:
* Exam 1 to be returned tomorrow.
* Latest homework still not graded.
* Meet in Saints Rest sometime next week?  (Probably next
  Tuesday)
* Read about loops (J5).
* Looking ahead: This week is Java stuff.  Next week we start
  talking about data structures.  (After that, I may rearrange
  things.)

----------------------------------------

Review: What do you remember about Java?
* Forgotten: How do I edit, compile, and all that stuff.
  + Edit: Either (1) open a terminal window and type gedit
  + To compile: In the terminal window, type
      javac asdasdfasd.java
    produces asdasdfasd.class
  + To run, 
      java asdasdfasd
    (Don't type .class or .java)
  + If this doesn't work, learn the mystical command
* Remember: Hatred (it's not DDR or Scheme)
  To create program, you need to create one or more classes.
  One of those classes must include a method called main
public static void main(String[] args)
{
} // main(String[])
  To create a class
public class ClassName
{
  // Other functions, which are typically not called main
  // Fields, which can be accessed by those functions
  // Constructors, special functions which tell how to
  //   set up a new object
} // class ClassName
  Sam will fail you if you don't document well

You can write tests in Java

if (some-silly-expression-that-tests-something) {
  stuff-to-do-if-test-holds
}
else {
  stuff-to-do-if-test-doesn't-hold
}
stuff-to-do-always

* The else is optional.
* Is there an equivalent to Scheme's cond?  No.  Write
  "nested ifs"
if (some-silly-expression-that-tests-something) {
  stuff-to-do-if-test-holds
}
else if (some-other-expression) {
  other-stuff-to-do
}
 ...
else {
  stuff-to-do-if-test-doesn't-hold
}
stuff-to-do-always
* Java and C++ have the less-than-amazing switch statement
switch (expression) {
  case val1:
    stuff-to-do-if-expression-matches-val1;
    break;
  case val2:
    stuff-to-do-if-expression-matches-val2;
    break;
  ...
}

Observation: If you don't write a break, the control
  continues into the next case

Restrictions: 
  Can you write 
    case "Evan":
    case "Katherine":  No.
  You can only use "enumerated" types: integers (and thier
    variants), Booleans, chars, ...
  No doubles or floats (approximated, so what does it mean
    to be equal?)
  Can you write:
    case func(adfas): xxx
  No.  The values must be computable at *compile* time.

Why?  Possible to implement really efficiently.  (Take
Compilers or CSC195/CSC201 or both for more details.)

----

What kinds of things can you test in the if?

if (test)
* Basic numeric tests: (a < b), (a > b), (a <= b)
  (a == b), (a != b)
* Boolean combinations
  And: && (infix)
  Or: || (infix)
  Not: ! (prefix)
* Many classes have an equals(Object other) method
* Many classes provide a wide variety of predicates
  (which, unfortunately, cannot end in a question mark)

* In Scheme, if it's not false, it's true.
* In Java, if it's not true and not false, it's an error

* How is question mark used in Java?
  ( a ? b : c)
  If a holds, then use the value of b, otherwise use the
  value of c (if expression rather than an if statement)
  max = (a > b ? a : b);

Lab J4.2, J4.4