How to get a char to replace parts in a string (java)

Question

I am currently doing a array project in my intro CS class and I have to successfully make a game of hangman. I think I am starting it off fairly right, but I can't seem to grasp how to replace a char into a string. I have to have a method to create a random word, so that's why I have a method in my code. Check out what I have so far:

import java.util.Random;
import java.util.Scanner;
public class ProjectNum2 {
//creator's name

    public static void main(String[] args) {

        System.out.println("Welcome to the Hangman Word game!");

        String[] wordKey = {
                            "loop",
                            "for",
                            "while",
                            "java",
                            "switch",
                            "scanner",
                            "else",
                            "double",
                            "integer",
                            "public",
                            "static",
                            "method",
                            "return",
                            "null",
                            "void",
                            "true",
                            "false",
                            "import",
                            "string",
                            "character"
                           };
        String[] wordSpace = {
                              "_ _ _ _",
                              "_ _ _",
                              "_ _ _ _ _",
                              "_ _ _ _",
                              "_ _ _ _ _ _",
                              "_ _ _ _ _ _ _",
                              "_ _ _ _",
                              "_ _ _ _ _ _",
                              "_ _ _ _ _ _ _",
                              "_ _ _ _ _ _",
                              "_ _ _ _ _ _",
                              "_ _ _ _ _ _",
                              "r _ _ _ _ _",
                              "_ _ _ _",
                              "_ _ _ _",
                              "_ _ _ _",
                              "_ _ _ _ _",
                              "_ _ _ _ _ _",
                              "_ _ _ _ _ _",
                              "_ _ _ _ _ _ _ _ _"
                             };
        char[] letters = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};

        final int guesses = 6;

        int index = 0;
        index=(int) randomGen(wordKey, wordSpace);

        Scanner keyboard = new Scanner(System.in);
        System.out.println();
        System.out.print("Choose a letter or enter zero to guess the word: ");
        char letter = keyboard.nextLine().charAt(0);
    }

    private static Object randomGen(String[] wordKey, String[] wordSpace) {
        String gameWord;
        Random randIndex = new Random();
        int index = randIndex.nextInt(wordKey.length);
        gameWord = wordSpace[index];
        System.out.print(gameWord);
        return (index);

    }

}

Answer 1

To get you on the right track while not spoiling all the fun, here's a method that from the string "loop" and the char 'o' derives and prints the string _oo_:

class Test {                                                                 
  public static void main(String[] args) {                                   
    String word="loop";         

    char[] array = new char[word.length()];                                  
    for(int i=0; i<array.length; i++) {                                      
      array[i] = '_';                                                        
    }                                                                        

    char guess = 'o';                                                        
    for(int i=0; i<word.length(); i++) {                                     
      if(word.charAt(i) == guess) {                                          
        array[i] = guess;                                                    
      }                                                                      
    }                                                                        

    System.out.println(new String(array));                                   
  }                                                                          
}         

Answer 2

Strings in Java are immutable, meaning that you cant change it. You have to build a new String object, using in your case for example the String.replace(...) method.

Answer 3

String are immutable which means once you create them you cannot change them. Use a StringBuilder class instead. Here is a simple example to get you started

    StringBuilder sb = new StringBuilder(30);
    sb.append("Hello Matthew");

    // append a character
    char c = '!';
    sb.append(c);