Replace while loop with lambda

Question

I'm learning lambdas and I have an exercise to rewrite the while loop in this code using lambdas. This method gets encoded user input and returns decoded. I faced this problem and can't understand what I'm supposed to do. (I know that it's not hard, but I just can't get the concept.) I didn't find any similar questions here.

p.s. Also, one more qustion - can this while loop( or maybe whole method) be reworked with method reference?

public String decode(String input) {
    StringBuilder letters = new StringBuilder();
    input = input.toUpperCase();
    int i = 0, j = 0;
    while (i < input.length()) {
        char symbol = input.charAt(i);
        char keySymbol = KEY.charAt(j);
        int newIndex = (ALPHABET.indexOf(symbol) - ALPHABET.indexOf(keySymbol)) % ALPHABET.length();
        char newSymbol = ALPHABET.charAt(newIndex);
        letters.append(newSymbol);
        j = ++j % KEY.length();
        i++;
    }
    return letters.toString().toLowerCase();
}

How do you want the lambda to be? forEeachIndexed(()->{})? — Mahdi-Malv
– Mahdi-Malv, Commented Nov 24, 2019 at 5:16

AndyMan · Accepted Answer · 2019-11-24 04:44:34Z

The code sample below includes the original decode method, and a new decodeLamda method.

The decodeLambda method replaces the iteration over 'input'. Running the sample will show they both have the same output. It is possible to change the loop to a method reference.

public class Main {

public static void main(String[] args) {
    Main main = new Main();
    System.out.println(main.decode("SECRET MESSAGE"));
    System.out.println(main.decodeLambda("SECRET MESSAGE"));
}

public static String KEY = "HOPSCOTCH";
public static String ALPHABET = "ABCDEFGHIJKLMONOPQRSTUVWXYZ";

public String decode(String input) {
    StringBuilder letters = new StringBuilder();
    input = input.toUpperCase();
    int i = 0, j = 0;
    while (i < input.length()) {
        char symbol = input.charAt(i);
        char keySymbol = KEY.charAt(j);
        int newIndex = Math.abs(ALPHABET.indexOf(symbol) - ALPHABET.indexOf(keySymbol)) % ALPHABET.length();
        char newSymbol = ALPHABET.charAt(newIndex);
        letters.append(newSymbol);
        j = ++j % KEY.length();
        i++;
    }
    return letters.toString().toLowerCase();
}

public String decodeLambda(String input) {
    StringBuilder letters = new StringBuilder();
    input = input.toUpperCase();
    var ref = new Object() {
        int j = 0;
    };

    input.chars()
            .forEach( symbol -> {
                char keySymbol = KEY.charAt(ref.j);
                int newIndex = Math.abs(ALPHABET.indexOf(symbol) - ALPHABET.indexOf(keySymbol)) % ALPHABET.length();
                char newSymbol = ALPHABET.charAt(newIndex);
                letters.append(newSymbol);
                ref.j = ++ref.j % KEY.length();
            });

    return letters.toString().toLowerCase();
}

}

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