A)
String str = "a string";
int length = str.length( ); // length == 8
http://download.oracle.com/javase/7/docs/api/java/lang/String.html#length%28%29
edit
If you want to count the number of a specific type of characters in a String, then a simple method is to iterate through the String checking each index against your test case.
int charCount = 0;
char temp;
for( int i = 0; i < str.length( ); i++ )
{
temp = str.charAt( i );
if( temp.TestCase )
charCount++;
}
where TestCase can be isLetter( ), isDigit( ), etc.
Or if you just want to count everything but spaces, then do a check in the if like temp != ' '
B)
String str = "a string";
char atPos0 = str.charAt( 0 ); // atPos0 == 'a'
http://download.oracle.com/javase/7/docs/api/java/lang/String.html#charAt%28int%29