如何递归使用 replaceAll() function？

Question

The program should allow a user to enter a string, a substring they wish to find and another string with which they wish to replace the found substring.

The output of your program should be similar to the output given below:

Please enter a string: Hello world

Please enter the substring you wish to find: llo

Please enter a string to replace the given substring: @@

Your new string is: he@@ world

I am new to Java and cant find and so far this is what I have:

import java.util.Scanner;

public class searchReplace 
{
    static String word, substring, newWord;
    static String output = "";
    

    public static void main(String[] args) 
    {
        Scanner input = new Scanner(System.in);
        
        System.out.println("Please enter a string: ");
        word = input.next();
        
        System.out.println("Please enter the substring you wish to find: ");
        substring = input.next();
        
        System.out.println("Please enter a string to replace the given substring: ");
        newWord = input.next();
        
        replace(substring,newWord);
        
        
        
        input.close();


    }


    private static void replace(String substring, String newWord) 
    {
        if(output.contains(newWord))
        {
            System.out.println(output);
        }
        
        else
        {
            output = word.replaceAll("substring","newWord");
            replace(substring,newWord);
        }
        
    }

}

I Get The Error:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.contains(java.lang.CharSequence)" because "searchReplace.output" is null at searchReplace.replace(searchReplace.java:33) at searchReplace.main(searchReplace.java:21)

Answer 1

For your goal, you need to use just:

output = word.replace(substring, newWord);

instead of:

word.replaceAll("substring", "newWord");

You don't need to make any recursion. Replace function will replace your substring inside word with the newWord for each occurence.

Answer 2

Recursion is not needed:

public static void main(String [] args) {
    try (Scanner scanner = new Scanner(System.in)) {
        System.out.println("Enter String: ");
        String input = scanner.nextLine();
        System.out.println("String to replace: ");
        String toReplace = scanner.nextLine();
        System.out.println("Replace with: ");
        String replaceWith = scanner.nextLine();
        
        System.out.println(input.replaceAll(toReplace, replaceWith));
    }
}