Luhn algorithm using Stream API

I cannot help but ask myself if they don't teach you methods anymore today.

Use methods whith meaningful names, use variables with meaningful names, when you do streams use line breaks with one operation per line. Refrain from supposedly "clever tricks" like doing an actual calculation in an if.

Rewrite of your code (whithout testing or understanding, just formally):

String creditCardNumber1 = "4561261212345467";
int sumOfOddNumbers = IntStream.range(0, creditCardNumber1.length())
    .filter(this::isOddNumber)
    .map(creditCardNumber1::charAt)
    .mapToInt(Character::getNumericValue)
    .sum()

int sumOfWhateverThisIs = IntStream.range(0, creditCardNumber1.length())
    .filter(this::isEvenNumber)
    .map(creditCardNumber1::charAt)
    .mapToInt(Character::getNumericValue)
    .map(this::giveThisMethodAGoodNameWhichExplainsWhatItDoes)
    .sum();

int sum = sumOfOddNumbers + sumOfWhateverThisIs;

System.out.println(sum % 10 == 0);
    
private boolean isOddNumber(int number) {
    return i % 2 != 0;
}
    
private boolean isEvenNumber(int number) {
    return i % 2 == 0;
}

private int giveThisMethodAGoodNameWhichExplainsWhatItDoes(int number) {
    int newValue = number * 2;
    if (newValue > 9) {
        newValue = newValue - 9;
    }
    return newValue;
}

To only have one stream, push down the decision of separating odd and even values:

int sum = IntStream.range(0, creditCardNumber1.length())
    .map(i -> {
        int value = Character.getNumericValue(creditCardNumber1.charAt(i));
        if (i % 2 == 0) {
            value = value * 2;
        }
        return value;
    })
    .map(value -> value > 9 ? value - 9 : value)
    .sum();
System.out.println(sum % 10 == 0);

It can be done with one stream like this:

public static int calculateLuhnChecksum(String creditCardNumber) {
  int luhnSum = IntStream.range(0, creditCardNumber.length())
    .mapToInt(i -> {
      char c = creditCardNumber.charAt(i);
      int value = Character.getNumericValue(c);
    
      if(isEven(i)) { // i % 2 != 0 - remember, index starts with 0, so you have "even" numbers at odd indices
        value *= 2;
      }

      return sumOfDigits(value);
    })
    .sum();

  return (luhnSum * 9) % 10;
}

private static int sumOfDigits(int value) {
  if(value > 9) {
    return value - 9;
  }

  return value;
}
```