The String method indexOf()
One of the more useful String instance methods (so each instance of a String has its own version of this method) is the indexOf method. This method looks for certain chars or Strings in another String. That is:
s1.indexOf( s2 )
is looking for the first occurrence of s1 within s2. In this example, s1 must be a String, and s2 can be either a String or a char.
The indexOf method returns an int. The value returned is the index of the first occurrence of s2 within s1 (if s2 is a String, its the index of the starting character in s2 that is returned). When s2 does not appear in s1, the returned value is -1. Here are some illustrative examples. Suppose that s1 is the String "hi there dude, how are you?". This String is printed out below, with the indexes of its characters printed above.
index
 0
 1
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26

h
 i
  
 t
 h
 e
 r
 e
  
 d
 u
 d
 e
 ,
  
 h
 o
 w
  
 a
 r
 e
  
 y
 o
 u
 ?

s1.indexOf('t') = 3
s1.indexOf("dude") = 9
s1.indexOf("the") = 3
s1.indexOf("Hi") = -1
s1.indexOf('c') = -1
s1.indexOf('e') = 5
 If you want to look for Strings or chars, beyond some index, say from index 5 on, you use s1.indexOf( s2, 5). Here are some of those examples:
To find the second occurrence of 'e': s1.indexOf('e',6)
To find the second "re": s1.indexOf("re", 7) or s1.indexOf("re", 8)
To find the second 'h' in a tricky manner: s1.indexOf('h', s1.indexOf('h') + 1)

It is common to use indexOf as part of a test, relying on the fact that when the index of something is -1, it means that thing does not occur in s1.
To see if the string s1 starts with a capital letter, you could use:
if ("ABCDEFGHIJKLMNOPQRSTUVWXYZ".indexOf(s1.charAt(0)) >= 0)
    outy.printLine("First letter is uppercase");
else
    outy.printLine("First letter is not uppercase");
To see if the letter 'a' appears twice in s1:
int loc = s1.indexOf('a'), loc2 = -1;
if (loc >= 0)
    if (s1.indexOf('a', loc + 1) >= 0 )
        outy.printLine("The letter a appears at least twice.");
    else
        outy.printLine("The letter appears exactly once.");
else
    outy.printLine("The letter a does not appear.");