Convert a String Value to ASCII

I want to take the value of a ASCII value(Saved as a string) and convert it to the character to reveal a message. I tried this and it keeps throwing an index out of bound at the declaration of the int b.It also shows that str and b do not have a value

String value = "104 101 108 108 111";

    char[] ch = new char[value.length()];
    for (int i = 0; i < value.length(); i++) {
        ch[i] = value.charAt(i);
    }
    System.out.println(ch.length);
    String ans = "";
    int i = 0;
    while (i+2 < ch.length) {
       
        
        int b= ch[i]+ch[i++]+ch[i+2];
        String str = new Character((char) b).toString();
        System.out.println(str);
        System.out.println(b);
    ans = ans+str;
    i=i+3;
    }

Using string split function

	String value = "104 101 108 108 111";

	String[] arrOfStr = value.split(" "); 
	
	String ans = "";

	for(String str : arrOfStr) {
		 String str1 = Character.toString((char)Integer.parseInt(str));
         ans += str1;
	}
	
	System.out.println(ans); // output: hello

We can switch the Imperative code to Declarative code using Java 8 Streams.

Key points to observe:

1. Declarative style is more readable and easy to write.
2. String Joiner is faster than simple String Concatenation.
3. No need to write an iterator.

import java.util.Arrays;

public class Main {

    public static void main(String[] args) {
        String value = "104 101 108 108 111";

        Arrays.stream(value.split(" ")) // Starting a stream of String[]
            .mapToInt(Integer::parseInt) // mapping String to int
            .mapToObj(Character::toChars) // finding ASCII char from int
            .forEach(System.out::print); // printing each character
    }

}

If you wish to store the result and then print it, this is how is done.

import java.util.Arrays;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        String value = "104 101 108 108 111";

        String result = Arrays.stream(value.split(" ")) // Starting a stream of String[]
                            .mapToInt(Integer::parseInt) // mapping String to int
                            .mapToObj(Character::toChars) // finding ASCII char from int
                            .map(String::new) // convert char to String
                            .collect(Collectors.joining()); // combining individual result using String Joiner

        System.out.println(result);
    }

}

Comments to code:

There is a built-in method for for getting a char[] with the characters of a string, so the following two blocks of code are the same:

// Code from question
char[] ch = new char[value.length()];
for (int i = 0; i < value.length(); i++) {
    ch[i] = value.charAt(i);
}

// Using built-in method
char[] ch = value.toCharArray();

It is better to use a for loop when increment a value while looping. The following two ways of writing the loop behave the same, but the for loop keeps the loop logic together:

// Code from question
int i = 0;
while (i+2 < ch.length) {
    // some code here
    i=i+3;
}

// Using for loop
for (int i = 0; i + 2 < ch.length; i=i+3) {
    // some code here
}

The following line of code is entirely wrong:

int b= ch[i]+ch[i++]+ch[i+2];

1. i++ increments the value is i, but it is the value before the increment that is used in the expression, which means that if i = 0 before the line, the result is the same as this code:

   int b = ch[0] + ch[0] + ch[2];
   i = i + 1;
   

   You need to replace i++ with i + 1, and realize that those are not the same.

2. Since you no longer increment the value of i by 1 in that statement, the loop much be changed from i=i+3 to i = i + 4, to correctly skip the spaces in the input.

3. The value of ch[i] is a char value, which is widened to an int value by the use of the + operator. The int value of a char is the Unicode Code Point value, which for your text is also the same as the ASCII code for the character.

   This means that if i = 0, the expression would (after fixing issue #1) evaluate as:

   int b = ch[0] + ch[1] + ch[2];
   
   int b = `1` + `0` + `4`;
   
   int b = 49 + 48 + 52;
   
   int b = 149;
   

   That matches the output from running the code is in question, where the second printed number is 149 (after fixing issue #1).

What you really wanted was to get the substring "104" and convert that to a number, then cast that ASCII code value to a char, like this:

String numberStr = value.substring(i, i + 3); // E.g. "104"
int number = Integer.parseInt(numberStr);     // E.g. 104
String str = String.valueOf((char) number);   // E.g. "h"

With that, you no longer need the char[], so the final code would be:

String value = "104 101 108 108 111";

String ans = "";
for (int i = 0; i + 2 < value.length(); i += 4) {
	String numberStr = value.substring(i, i + 3);
	int number = Integer.parseInt(numberStr);
	String str = String.valueOf((char) number);
	ans = ans + str;
}
System.out.println(ans);

Output

hello