CSC153 2004S, Class 32: Conditionals in Java

Admin:
* This week: Conclude Java language issues (I hope)
* Read "Loops in Java" for Wednesday
* I hope you had a great break
* Add the following line to the end of your .bashrc (e.g., with "gedit .bashrc")
	export PATH="$PATH:/usr/java/j2sdk1.4.1_03/bin"

Overview:
* Review of Java to date
* Boolean expressions
* Boolean methods
* Simple conditionals: if statements
* Complex conditionals: cond statements
* Lab

What do you know about Java?
* An object-oriented programming language
* Compile to "computer-independent" language which runs on a "virtual machine"
* Running a Java program on our computers requires setting the PATH variable
* Programs in Java look a lot like programs in C (similar syntax)
* Need a main method which has the signature
	public static void main(String[] args)
	{
	  ...
	}
* Objects are described with classes, with three key parts
  * Fields (data)
  * Constructors (how to build them)
  * Methods (things you can do with built objects)
* You know how to assign values, to write methods, and to call methods.
* You need to know 
  * How to test for conditions (debatable)
  * How to repeat things [Wed]
  * How to deal with errors in Java (Exceptions) [Thu]

Conditionals are based on "Boolean" expressions: Values true or false
* boolean type
* values: true and false
* Compare two values with <, <=, >, >=, ==, !=
* Join two boolean expressions with && (and), || (or)
* Invert boolean exp with !
	(a < b) && (b < c)
* Observation: Infix operators, not prefix
* In Java, boolean is not the same as integer
	while (1) NOT ALLOWED
	while (true) ALLOWED
  * Java is your nanny

Boolean expressions are most typically used in if statements

if (BOOL_EXP) {
  STUFF-TO-DO-IF-EXP-HOLDS
}
else {
  STUFF-TO-DO-IF-EXP-DOESNTHOLD
}

* The else part is optional
* The parens are probably optional
* The braces are optional if you have only one statement in the body

There is no COND!

if (BOOL_EXP) {
  STUFF-TO-DO-IF-EXP-HOLDS
}
else if (BOOL_EXP) {
  ETC
}
else if (BOOL_EXP) {
  ETC
}
else {
  ETC
}

if (BOOL_EXP) {
  STUFF-TO-DO-IF-EXP-HOLDS
} else if (BOOL_EXP) {
  ETC
} else if (BOOL_EXP) {
  ETC
} else {
  ETC
}

Java also has switch statements
	switch (EXP) {
	  case VAL1: CODE
	  case VAL2: CODE
	  case VAL3: CODE
        }
* Compute the value of EXP
* Find the first matching value
* Execute code following the corresponding case.  
* Keep executing until you find a break or the end of the switch.
	switch (val) of {
	  case 1: System.out.println("One");
	  case 2: System.out.println("Two");
	  case 3: System.out.println("Three");
        }
  * If val is 1, "One\nTwo\nTree\n" gets printed
* Why is case better than nested ifs?
  * Perhaps more readable
  * Less typing of (val == VAL), but extra breaks
  * Faster 
    * In a naive implementation, a case statement is just a series of nested ifs O(n)
    * In a moderately smart implementation, you can have a "binary search tree" of possible values O(logn)
    * In a really smart implementation, this is just an indexed goto O(1)
  * Sometimes the "fall through" behavior is useful
    * However, generally it breaks when programmers forget their breaks.

Do the lab!

Programming puzzle
	Why write the test as
		1 == this.month
	rather than
		this.month == 1
	?
* C programming trick: If you accidentally forget an = sign, the first will lead to an error, the second won't (except in nannyish Java)