\" inside a string: use \ as escape char
Don't use comma as separator of tokens.
You can use string.charAt(n) to get the nth char.
(I am in a hurry...)

Thanks for your answer!

I'm sorry, my english is very bad. I think I hadn't explain well. I don't want to get a special place of the string. I want to count a special char in the string. The string has got only one line with a lot of chars and I don't know how to cound all the a's for example in a string with "bananas" inside. I tried to use charAt, because I though if I can seperat every single sign from the string to another string by saving every char in a single place, then I can use equals again and count then ... but I couldn't managed even that, like you see in my code at the bottom.

And I don't understand how to use \ as seperate. I wanted this part   String [] woerter = splitTokens(alleZeilen," ,:,.,?,!,-"); including the sign for ", because I using a text. The text have some conversation inside, but I don't need the sign ", so I though I can delete it like the other marks (?, !, - and so on). But I can't write   String [] woerter = splitTokens(alleZeilen," ,:,.,?,",!,-");

My try by using charAt and getting "cannot convert from char to String[]":

void setup() {
 
  size(400, 400);
 
   String text[] = loadStrings("deutsch.txt");

  String alleZeilen = join(text, ":");
 
  String [] woerter = splitTokens(alleZeilen," ,:,.,?,!,-");
 
  String alleBuchstaben = join(woerter,"");
 
//I thought I can save with charAt every single char in an other string
  for( int i = 0; i < alleBuchstaben.length(); i++){
    String [] einzelbuchstaben = alleBuchstaben.charAt(i);
    println(einzelbuchstaben);
  }
}

Now I use an arrayList and saved every single char

  ArrayList einzelbuchstaben;
  einzelbuchstaben = new ArrayList();
  for( int i = 0; i < alleBuchstaben.length(); i++){
    einzelbuchstaben.add(alleBuchstaben.charAt(i));
  }

So you have duplicate letters in the arraylist.

I tried my best and write the following code. But I don't know why, it doesn't count:

int anzahlA=0;
int anzahlI=0;
int anzahlO=0;
int anzahlU=0;
int anzahlE=0;


void setup() {
 
  size(400, 400);
 
   String text[] = loadStrings("deutsch.txt");
   //println(text);
 
  //Absätze sind im String alleZeilen durch ein : gekennzeichnet, bzw sind es die einzelnen Zeilen aus lines
  // die einzelnen Textteile, die in text in mehreren Reihen gespeichert sind, werden mit join
  // zu in alleZeilen in einer Reihe verbunden
  String alleZeilen = join(text, ":");
 
  //String wird auf Wörter reduziert
  //durch splitTokens werden die Satzzeichen entfernt und in woerter gespeichert
  String [] woerter = splitTokens(alleZeilen," ,:,.,?,!,-,\"");
 
  //hier werden die Wörter zu einer Reihe an Informationen gemacht, die nur aus Buchstaben besteht
  //join verbindet hier wieder einen string aus mehreren Reihen zu einem mit nur einer Reihe
  String alleBuchstaben = join(woerter,"");
 
  //Speicherung jedes Buchstabens einzeln in einer Arrayliste
  ArrayList einzelbuchstaben;
  einzelbuchstaben = new ArrayList();
  for( int i = 0; i < alleBuchstaben.length(); i++){
    einzelbuchstaben.add(alleBuchstaben.charAt(i));
    println(einzelbuchstaben.get(i)+" "+einzelbuchstaben.size());
  }

  //Zählung der Einzelbuchstaben
  for( int i = 0; i  < einzelbuchstaben.size(); i++) {
    if(einzelbuchstaben.get(i)=="a"||einzelbuchstaben.get(i)=="A") {
      anzahlA++;
    }
    if(einzelbuchstaben.get(i)=="o"||einzelbuchstaben.get(i)=="O") {
      anzahlO++;
      println("yay");
    }
    if(einzelbuchstaben.get(i)=="i"||einzelbuchstaben.get(i)=="I") {
      anzahlI++;
    }
    if(einzelbuchstaben.get(i)=="u"||einzelbuchstaben.get(i)=="U") {
      anzahlU++;
    }
    if(einzelbuchstaben.get(i)=="e"||einzelbuchstaben.get(i)=="E") {
      anzahlE++;
    }
   
  }
  println(anzahlA+" "+anzahlI+" "+anzahlO+" "+anzahlU+" "+anzahlE);
}

That's Not a good Solution http://processing.org/learning/topics/hashmapclass.html

if you are still fighting with this, try use indexOf().

have a look at the java String reference for more methods:
http://docs.oracle.com/javase/6/docs/api/java/lang/String.html

example, using indexOf() for counting.

1. void setup() {
2.   String text = "hellllo WORLD";
3.   text = text.toLowerCase();
4. 
   
5.   int count1 = countString(text, "o");
6.   int count2 = countString(text, "l");
7.   int count3 = countString(text, "ll");
8.   int count4 = countString(text, "world");
9. 
   
10.   println("count1 = "+count1);
11.   println("count2 = "+count2);
12.   println("count3 = "+count3);
13.   println("count4 = "+count4);
14. }
15. 
    
16. // returns the number of appearances of "c" within "str".
17. int countString(String str, String c) {
18.   int count = 0;
19.   for (int i=-1; (i=str.indexOf(c,++i))!=-1; count++);
20.   return count;
21. }

" I can't write   String [] woerter = splitTokens(alleZeilen," ,:,.,?,",!,-");"
OK, I have a bit more time, let's detail my answer:
- Don't use comma to separate tokens in the list: " ,:,.,?,",!,-" becomes  " :.,?"!-" I kept one comma as I suppose you want it as separator as well.
- Use \" in the string: " :.,?"!-" becomes " :.,?\"!-"
- As said, I pointed to charAt() for testing on the fly. If you want an array, you can just do:
char[] chars = someString.toCharArray();