CSC152 2005F, Class 14: Conditionals (1): "Lecture"

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Overview:
* Class fields and methods
* Three kinds of classes
* Boolean values and expressions
* Conditionals
* Recursion

Review:
* Five parts of a class:
  Fields, Constructors, Methods, Class Fields, Class Methods

Class fields and class methods
* Just like regular fields and methods EXCEPT
  * Preceded by the keyword "static" (appears after protection and before the type)
  * May not directly reference object fields or methods
* Example (I hope): Create from polar coords
  (See Vec2D.java and TestVec2D.java)
* Side note: If you provide a toString method, Java calls it when you try to print the object, EVEN IF YOU DO NOT CALL IT EXPLICITLY

Observation: The Java we've seen to date suggests three ways to use classes
* Main classes: Define a main method (and little more)
  Control the working of the program
* Template classes: Define objects (fields, methods, constructors)
  Used by main classes to define the "players" within the program
* Utility classes: Provide primarily class/static methods which manipulate other objects  (e.g., Math)

/On to conditionals (whee!)/

* Used to make decisions in program
* Past evidence suggests that programs that don't make many decisions are, well, BORING
* Decisions made with the conditional statement
if (TEST) {
  STUFF-TO-DO-IF-TEST-SUCCEEDS
}
* Were we to create an "absolute value" method for vectors
	double resultx = this.xcoord;
	if (resultx < 0) {
	  resultx = -resultx;
	}
* Detour: A semicolon by itself represents the empty statement.
* Note
	if (TEST) ; means "run the test and ignore the result"
* Usually provide alternatives
	if (TEST) {
	  STUFF-TO-DO-IF-TEST-SUCCEEDS
	}
	else {
	  STUFF-TO-DO-IF-TEST-FAILS
	}
  * Include semicolons after each command in both succeeds and fails
* No equivalent to Scheme's "cond": To do lots of tests, use lots of ifs.  
	if (TEST1) {
	  STUFF-TO-DO-IF-TEST1-SUCCEEDS
	}
	else if (TEST2) {
	  STUFF-TO-DO-IF-TEST1-FAILS-AND-TEST2-SUCCEEDSs
	} 
	else {
	  STUFF-TO-DO-IF-BOTH-TESTS-FAIL
	}
* If you don't include the first else, it treats the conditionals as separate
	if (TEST1) {
	  STUFF-TO-DO-IF-TEST1-SUCCEEDS
	}
	if (TEST2) {
	  STUFF-TO-DO-IF-TEST2-SUCCEEDS_NO_MATTER_WHAT_TEST1_DID
	} 
	else {
	  STUFF-TO-DO-IF-TEST2-FAILS
	}

/How do we write the tests?/

* Arithmetical: ==, <, <=, >, >=, !=
  * All infix 
  	if (a op b)
* Boolean: and (&&), or (||), not (!)
  * && and || are infix
  	if (bool1 && bool2)
  * not is prefix
  	if (!(a op b))
* Boolean methods (normally written as combinations of the above)
	public boolean inFirstQuadrant()
	{
		return (x > 0) && (y > 0);
		//if ((x > 0) && (y > 0)) {
		//	return true;
		//}
		//else {
		//	return false;
		//}

	}
* Expecation that many classes provide an int compareTo(Object other	)
  method
  	returns a negative number if this object naturally precedes the param object
	returns 0 if the two objects are naturally the same
	returns a positive number if this object naturally follows the param

	BigInteger ba;
	BigInteger bb;
	...
	if (ba.compareTo(bb) < 0) {
		pen.print(ba);
		pen.print(" appears to be smaller than ");
		pen.println(bb);
	}