This is a small program that intend to print "p" between every two vowels. ae, oi, eu, ect.

import java.lang.*;

void setup(){
 String word = "aeiou";
 char le_1, le_2;

 for(int i=1;i<word.length();i++){
   le_1 = toLowerCase(word.charAt(i-1));
   le_2 = toLowerCase(word.charAt(i));
   print(le_1);

   if( le_1.equals("a") &&
       le_1.equals("e") &&
       le_1.equals("i") &&
       le_1.equals("o") &&
       le_1.equals("u") ){
       
       if( le_2.equals("a") &&
           le_2.equals("e") &&
           le_2.equals("i") &&
           le_2.equals("o") &&
           le_2.equals("u") ){
       
           print("p");
       }
   }
 }
}

After running this, I got message that the function toLowerCase (char) doesn't exist.

for the lowercase Problem, you have to write

Character.toLowerCase()

You have to choose between char and String, they are different in Java/Processing.
With String:
Code:

void setup(){
 String word = "aeiou";
 String le_1, le_2;

 for(int i=1;i<word.length();i++){
   le_1 = word.toLowerCase().substring(i-1, i);
   le_2 = word.toLowerCase().substring(i, i+1);
   print(le_1);

   if( le_1.equals("a") ||
	 le_1.equals("e") ||
	 le_1.equals("i") ||
	 le_1.equals("o") ||
	 le_1.equals("u") ){
	 
	 if( le_2.equals("a") ||
	     le_2.equals("e") ||
	     le_2.equals("i") ||
	     le_2.equals("o") ||
	     le_2.equals("u") ){
	 
	     print("p");
	 }
   }
 }
}
 

Note the change from && to ||, a string can't be equals to several different strings...
With char:
Code:

void setup(){
 String word = "aeiou";
 char le_1, le_2;

 for(int i=1;i<word.length();i++){
   le_1 = word.toLowerCase().charAt(i-1);
   le_2 = word.toLowerCase().charAt(i);
   print(le_1);

   if ( ( le_1 == 'a' ||
	    le_1 == 'e' ||
	    le_1 == 'i' ||
	    le_1 == 'o' ||
	    le_1 == 'u' ) &&
	  ( le_2 == 'a' ||
	    le_2 == 'e' ||
	    le_2 == 'i' ||
	    le_2 == 'o' ||
	    le_2 == 'u' ) )
	 
	    print("p");
 }
}
 

with a small variant on the logic... Equality test is simpler, too!

I should make a lowercase copy of the string instead of transforming it two times per loop, but here, it isn't important...

Code:

char[] vowels = {'a', 'e', 'i', 'o', 'u'};

String string = "I should make a lowercase copy of the string instead of transforming it two times per loop, but here, it isn't important... ";
String[] words = splitTokens(string.toLowerCase(), " ,.?!");

String[] wopords = new String[0];

void setup() {
for(int i = 0; i < words.length; i++) {
  String wopord = "";
  for(int j = 0; j < words[i].length(); j++) {
    if(valueExistsInArray(vowels, words[i].charAt(j))) {
	wopord += "op";
    }
    wopord += words[i].charAt(j);
  }
  wopords = append(wopords, wopord);
}

println(join(wopords, " "));
}

boolean valueExistsInArray(char[] arr, char value) {

  for(int i = 0; i < arr.length; i++) {
    if(arr[i] == value) {
	return true;
    }
  }

  return false;
}