Recitation 1: the week of 3 September

Go over type char and class String, because these are used in fairly often.
First, type char.  

1. Note that "A" is of type String, while 'A' is of primitive type char

2. Literals of type char are written with single quotes: 'A', '3', '#', etc.
Here are possible escape sequences:
  
  			'\\'  backslash \           '\''  single quote '
            '\"'  double quote "        '\n'  new line
            '\r'  carriage return       '\t'  tab
            '\f'  form feed             '\b'  backspace
            
3. The old ASCII representation of characters used 1 bybte per character. Java
uses instead the new Unicode representation of chars: 16 bits. It allows just
about all the languages of the world to be encoded. Here's the representation of
standard chars, which turns out to be the same as for ASCII.

They are given in hexadecimal (explain if necessary). This means that 'A' is
hexademical 41 (decimal 65), and 'b' is hexadecimal 61 (decimal 97).
   
UNICODE FOR THE STANDARD CHARACTERS

Java    Char   Java    Char  Java    Char
\u0020         \u0040  @     \u0060  `
\u0021  !      \u0041  A     \u0061  a
\u0022  "      \u0042  B     \u0062  b
\u0023  #      \u0043  C     \u0063  c
\u0024  $      \u0044  D     \u0064  d
\u0025  %      \u0045  E     \u0065  e
\u0026  ;      \u0046  F     \u0066  f
\u0027  '      \u0047  G     \u0067  g
\u0028  (      \u0048  H     \u0068  h
\u0029  )      \u0049  I     \u0069  i
\u002A  *      \u004A  J     \u006A  j
\u002B  +      \u004B  K     \u006B  k
\u002C  ,      \u004C  L     \u006C  l
\u002D  -      \u004D  M     \u006D  m
\u002E  .      \u004E  N     \u006E  n
\u002F  /      \u004F  O     \u006F  o
\u0030  0      \u0050  P     \u0070  p
\u0031  1      \u0051  Q     \u0071  q
\u0032  2      \u0052  R     \u0072  r
\u0033  3      \u0053  S     \u0073  s
\u0034  4      \u0054  T     \u0074  t
\u0035  5      \u0055  U     \u0075  u
\u0036  6      \u0056  V     \u0076  v
\u0037  7      \u0057  W     \u0077  w
\u0038  8      \u0058  X     \u0078  x
\u0039  9      \u0059  Y     \u0079  y
\u003A  :      \u005A  Z     \u007A  z
\u003B  ;      \u005B  [     \u007B  {
\u003C  <      \u005C  \     \u007C  |
\u003D  =      \u005D  ]     \u007D  }
\u003E  >      \u005E  ^     \u007E  ~
\u003F  ?      \u005F  _  
  

4. Type char is an integeral type. If you execute 
	
    int i= 'A';
    
i will contain the integer representation of 'A', or the decimal integer 65.
You can also go the other way:

    char c= 65;
    
System.out.println(c) will result in: A

You can put in explicit casts, if you wish, but they are not needed:

    int i= (int) 'A';
    char c= (char) 65;
    
Below is a loop that prints AaBbCc ... Zz. Important things to note.

	1. comparison of characters: c <= 'Z'.
	2. character arithmetic: c++,  'a' - 'A'
	3. The arithmetic 'a' - 'A' and c+diff actually converts the chars
	   to integer and then does the arithmeteic operation. The cast
	       (char) (c+diff)
	   is necessary; otherwise, the integer representation of the char,
	   and not the char, will be printed.
	4. Note the use of the characters themselves, 'a' and 'A', in computing
	   the difference between them. It is TERRIBLE programming practice to
	   use their representations (e.g. 97-65). DON'T DO THAT!
    
// Print AaBbCc ... Zz
   int diff= 'a' - 'A';
   // invariant: Cap. letter c is next letter to be printed
   for (char c= 'A'; c <= 'Z'; c++) {
        System.out.print(c);
        System.out.print( (char)(c+diff) );
   }
   
   
Strings
Some of the students don't know much about classes yet; take this into account
when discussing Strings.

1.  String s=  "abc";   stores in s the String "abc". String literals are
written with double quotes. 'A' is of primitive type char. "A" is a String that
contains a single character, 'A'.

2. "" is the empty string.

3. "...".length() is the number of characters in String "...".

4. You can use the escape sequences in Strings. Consider the following:

   System.out.println("ab\ncd\n345\".");
   
This prints:

   ab
   cd
   345".
   
Remind them that Systen.out.println(arg) prints its argument on the Java console
and then does a "new line". There's also System.out.print(arg).

5. Catenation.  If either s1 or s2 is a String, then s1 + s2 denotes String
catenation. "ab" + "cd" is the String "abcd". If one operand is not a String, it
will be converted to a String. Make a point about the last one!!!

   System.out.println(2+"bc");        prints:  2bc
   System.out.println(2 + 3 + "bc");  prints:  5bc
   System.out.println((2+3) + "bc");  prints:  5bc
   System.out.println("bc" + (2+3));  prints:  bc5
   System.out.println("bc" + 2 + 3);  prints:  bc23

6. Suppose a method p requires a String as a parameter and you want to
give it char c as a parameter.  Use the following:

   char c= 'A';
   ...
   p("" + c);

Just p(c) would yield a type error --the argument is not a String. But with
argument "" + c, the char is first converted to String and then catenated to the
empty String.

7. Checking for equality of Strings s and t

USE  s.equals(t)  and NOT s == t.

We explain it more when we get to classes.

8. Useful methods on a String s. Remember, chars are nubmered 0, 1, 2, ...
s.length().

s.charAt(i): the char at position i.
s.substring(i). The substring s[i..s.length()-1].
s.substring(i,j). THe substring s[i..j-1].
s.trim()          s with whitespace removed from front and rear