StringIndexOutOfBoundsException: string index out of range

/**
 * Program selects the first word from the sentence "Hello, my dear!" with
 * the String Classes's methods this.indexOf() and this.substring()  
 * 
 *
 * Last modify: 29th October 2015
 */

public class TakeSubstringWanted {

    public static void main(String[] args) {

        String sentence = "Hello, my dear!";
        String reference = "Hello";

        System.out.println("The sentence is: " + sentence);
        System.out.println("You want to take the first word of that sentence");
        System.out.println("Give me a second...");

        int firstReference = sentence.indexOf(reference);
        int firstLength = reference.length();
        System.out.println("The first reference of the searched word is " + firstReference);
        System.out.println("The first word of the sentence " + sentence + " is: " + """ + sentence.substring(firstReference, firstLength) + """);
        int secondReference = sentence.indexOf("my");
        System.out.println("The second reference of the searched word is " + secondReference);
        int secondLength = "my".length();
        System.out.println(secondLength);
        System.out.println(sentence);
        System.out.println("The second word of the sentence " + sentence + " is: " + """ + sentence.substring(secondReference, secondLength) + """);
    }

}

sentence.substring(secondReference, secondLength)

sentence.substring(startIndex, endIndex);