// APS101, Winter 2009: Lecture 4 (Jan. 15)
//
// Review: last time we learned all about
// primitive types, and variables.
// ex. boolean (true, false)
true && ! true || !(true && !false) || (false || true)
// ex. character values
char c; // variable declaration
c // gets initialized to a default value
c = 'a'; // variable assignment
// we also talked about typecasting:
int i = (int) 2.9f
double x = 2.9f // float is "promoted" to double
float f = 2.9 // error! can't assign a double to a float
//
// Today will be an introduction to Strings,
// objects, and method calls.
//
// what if we want to store more than just 1 character?
// i.e., strings of characters (words and sentences).
'abc'
// need to use Strings: non-primitive type
// also called object type
"abc"
String = "abc";
String myString = "abc";
myString
String s;
s // objects are automatically initialized to null!
"hello" + "world" // concatenation
// concatenation operator: +
"hello" + " world"
"hello" + " " + "world"
"hello" + '!' // can combine Strings with chars!
'a' + "bc"
"12" + 3
"This lecture begins at " + 10 + "am."
10 + " is when this lecture begins"
"this is true or " + false
myString.length()
// length() is a method in the String class.
// how do we know this? Let's take a look at the Java API...
// API = Application Programming Interface
myString.charAt(0)
myString.charAt(1)
myString.charAt(2)
myString.charAt(3) // error: index out of bounds!
// Method calls: 
// objectName.methodName(args)
// args = arguments (0 or more)
// method length() has takes no arguments
// method charAt(int index) takes one int argument
// (we'll look at Strings in more detail later.)
String s = "abc"
s.length()
length("abc") // doesn't work - incorrect syntax!
"abc".length()
"abc ".length() // spaces (and all other characters) count
//
// Another type of object: Date
// ex. Halloween: October 31, 2009 -> 10/31/09
// this is a way of representing information
Date d; // can't find the class
import java.util.Date; // need to import it first
Date d;
d // initialized to null! like all other objects.
d = 10/21/09
d = new Date()
d
// how to declare an object variable:
// <ClassName> <objectName>;
// how to initialize an object variable:
// <objectName> = new <ClassName>(args)
Date d = new Date(); // can do it all on one line
d
d.getYear()
d.getMonth()
Date d1 = new Date(2008, 9, 31)
d1
Date d1 = new Date(08, 9, 31)
Date d1 = new Date(108, 9, 31)
d1
d1.getMinutes()
d1
Date d2 = new Date(109, 9, 31)
d2
d2.after(Date d1)
d2.after(d1)
d1.after(d2)
d2.compareTo(d1)
d1.compareTo(d2)
// comparison operator: ==
d1 == d2
d1
Date d2 = new Date(108, 9, 31)
d1
d2
d1 == d2 // false!
d1.equals(d2) // use the "equals" method to compare objects!
"abc".equals("abc")
"abc" == "abc"
d1 = d2
d1 == d2 // the "==" checks if it's the SAME object
// the "equals" compares the VALUES of the objects