Where the hell is the error coming from. . . ?

[Yunus Einsteinium]

import java.util.Scanner;
public class CharOccurence {
  public static void main(String[] arg){
    Scanner input = new Scanner(System.in);
    
    System.out.println("Enter a sentence to count the occurence of each letter");
    String words=input.nextLine();
    
    int[] count = letterOccurence(words.toLowerCase());
    
    for(int i=0;i<words.length();i++){
      if(count[words.charAt(i) - 'a'] == 1)
          System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " time");
      else
          System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " times");
    }
    
    
 }
    
    
    public static int[] letterOccurence(String sentence){
        int[] count = new int[26];
        
        for(int i=0;i<sentence.length();i++){
            if(Character.isLetter(sentence.charAt(i)))
                count[sentence.charAt(i) - 'a']++;
        }
        
      return count;
    }
 
} 

[Voodoo]

man, Einsteinium, you commit a basic error and wonder why you're killed the poison is

int[] count = letterOccurence(words.toLowerCase());
for(int i=0;i<words.length();i++){
..
}

to detox

int[] count = letterOccurence(words.toLowerCase());
for(int i=0;i<count.length;i++){
..
}

[Yunus Einsteinium]

man, when i do that another error pops out instructing me to change it back to length()

[andreas90]

import java.util.Scanner;
public class CharOccurence {
  public static void main(String[] arg){
    Scanner input = new Scanner(System.in);
    
    System.out.println("Enter a sentence to count the occurence of each letter");
    String words=input.nextLine();
    
    int[] count = letterOccurence(words.toLowerCase());
    
    for(int i=0;i<words.length();i++){
      if(count[words.charAt(i) - 'a'] == 1)
          System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " time");
      else
          System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " times");
    }
    
    
 }
    
    
    public static int[] letterOccurence(String sentence){
        int[] count = new int[26];
        
        for(int i=0;i<sentence.length();i++){
            if(Character.isLetter(sentence.charAt(i)))
                count[sentence.charAt(i) - 'a']++;
        }
        
      return count;
    }
 
} 

Hello Yunus Einsteinium!
For future posts, it would help if you explained what are you trying to do with your code and posted the full error messages and the code that produces them.
I tried your code and it causes an ArrayIndexOutOfBoundsException. This happens when you enter a sentence that has characters which are NOT letters.
You can avoid that by checking every character (in the for loop) and see whether it is a letter or not.
Hope this helps.

[Voodoo]

Einsteinium, your 2nd poison vial is:

 
        for(int i=0;i<sentence.length();i++){
            if(Character.isLetter(sentence.charAt(i)))
                count[sentence.charAt(i) - 'a']++;
        }

Where? the term sentence.charAt(i) - 'a' can be bigger than 26 or a negative value if sentence.charAt(i) is an uppercase character A, B,...
to detox:

        String tmp = sentence.toLowerCase();
        for(int i=0;i<sentence.length();i++){
            if(Character.isLetter(sentence.charAt(i)))
                count[tmp.charAt(i) - 'a']++;
        }

[Voodoo]

and the 3rd poison bottle you have is:

 
            Scanner input = new Scanner(System.in);
 
            System.out.println("Enter a sentence to count the occurence of each letter");
            String words=input.nextLine();
	    int[] count = letterOccurence(words.toLowerCase());
 
	    for(int i=0;i<count.length;i++){
	      if(count[words.charAt(i) - 'a'] == 1)
		  System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " time");
	      else
		  System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " times");
	    }

What happens if words == null ?
to detox

 
    Scanner input = new Scanner(System.in);
 
    System.out.println("Enter a sentence to count the occurence of each letter");
    String words=input.nextLine();
    if (words != null) {
	    int[] count = letterOccurence(words.toLowerCase());
 
	    for(int i=0;i<count.length;i++){
	      if(count[words.charAt(i) - 'a'] == 1)
		  System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " time");
	      else
		  System.out.println(words.charAt(i)+ " occurence: "+ count[words.charAt(i) - 'a']+ " times");
	    }
 
    }