CS100J, Fall 2001
Tues, 10/16
Lecture 14
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Announcements:
+ reading:
  Strings&Chars on Thurs: pp57-58,66-67,79-89,270-278,320-322
  $for$ on Thurs and beyond: 167-175
  arrays on Thurs-next week: 6.1-6.3, 6.5 (6.4 happens the following week)
+ P5 posted by Mon or Tues
+ T2 on 10/23
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Topics:
+ a bit more on static
+ method overloading
+ "constructor overloading"
+ strings&chars
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class Stuff { public static int k; }

public class static_basics2 {
    public static void main(String args[]) {
        
        // You may access class vars and methods DIRECTLY from class
        // as Class.field:
        System.out.println( ______________ );
        
        // $static$ also means all objects from same class "know"
        // the static variable... setting it one object sets it for
        // all objects:
        
        Stuff A = new Stuff();
        Stuff B = new Stuff();
        ++A.k;
        System.out.println(B.k);
        ++B.k;
        System.out.println(A.k);
	}
} 

// output? (0,1,2)

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Method overloading:
+ A method may have the same name as another method if you change
  order of arguments, types of arguments, number of arguments, or any 
  combination of these. 
+ Note: The following do NOT constitute method overloading:
  - Changing *just* the return type
  - Changing *just* the names of the formal parameters
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Constructor overloading:
+ no such term, but makes sense if you think of constructor as a method!
+ follows same rules as method overloading
+ Big difference: when you write a constructor, Java removes the 
  default (or empty) constructor.






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class Square {
    private int x;

    public Square() { }

    public Square(int x) {
	this.x=x;
    }

    public Square(int x, int y) {
	if (x!=y)
	    System.out.println("Error!Using 1st input");
	this.x = x;
    }

    public int getPerim() { return 4*x; }

    // public void getPerim() { } // won't work!

    public void getPerim(String s) { 
	System.out.println(s+getPerim());
    }
}

public class squares {

    public static void main(String[] args) {

	Square s1 = new Square();
	Square s2 = new Square(1);
	Square s3 = new Square(2,1);
	System.out.println(s2.getPerim());
	s3.getPerim("The perimeter is ");
    }
}

/* output:
   Error! Using first input.
   4
   The perimeter is 8
*/




















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Characters:
+ primitive type (pg 57)
+ stores integer codes for 65000+ characters (UNICODE)
+ deal mostly with ASCII (original 128 characters) (pg 992)
  - characters on keyboard
  - non-printing characters 
    + escape, backspace, bell, new line
    + see \n, \t (pg 88)
+ syntax of type:
  char var = 'integer'
  - default value of char instance variable is NUL character (ASCII 0)
  - note: $''$ is not NUL character -- it's illegal
+ interesting tricks:
   'A' to 'Z' has ASCII range 65 to 90
   'a' to 'z' has ASCII range 97 to 122
   --> 'a'-'A' is 32
   --> 'Q'+'a'-'A' gives 'q'
+ arithmetic
  - character arithmetic gives integers!
  - can mix ints and chars
  - can typecast with char to int and vice versa
  example)
  System.out.println('Q'+ 'a'-'A') gives 113
  To fix and get q?
  -->  System.out.println((char) ('Q'+ 'a'-'A')) 
  -->  char tmp = 'Q'+ 'a'-'A';  System.out.println(tmp)
+ see Character class in API (
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Strings:
+ collections of characters
+ objects in Java
+ string literal: "stuff" (saves hassle of calling a constructor)

+ constructors?
  String s1 = new String();        // create empty string
  String s2 = new String("stuff"); // create string of "stuff"
  char[] tmp = {'a','b','c'};      // uses an array -- will see later!
  String s3 = new String(tmp);     // create string from chars

+ so, what is a string literal?
  an instance of class String -- namely, a shortcut from calling a constructor!

+ immutable -- once created, cannot change!
  see StringBuffer class for mutable strings

+ Resemblence of Strings to arrays:
  - index of characters starts count at ZERO
  - find number of characters with $length()$ (not $length$)
  ex) string:   ABCD
      indicies: 0123

+ String methods... (see lecture example)

+ $equals$
  - to compare contents of 2 Strings $s1$ and $s2$: $s1.equals(s2)$
  - why not $==$?
    $==$ tests equality of references, not contents!
    so, $s1==s2$ test if $s1$ and $s2$ refer to the same object
    $==$ will check contents when comparing 2 String literals only

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