使用Java中的indexOf查找字符串中的空格

Question

So I am trying to write a Method that will return to me the number of occurrences of a String in Another String. In this case it's finding the number of spaces in a String. It's as if indexOf() is not recognizing the spaces.

Here is my Method:

public int getNumberCardsDealt()
{
    int count = 0;
    int len2 = dealtCards.length();

    int foundIndex = " ".indexOf(dealtCards);

    if (foundIndex != -1)
    {
        count++;
        foundIndex = " ".indexOf(dealtCards, foundIndex + len2);
    }

    return count;
}

Here is my application:

public class TestDeck
{
public static void main(String [] args)
{
    Deck deck1 = new Deck();

    int cards = 52;
    for(int i = 0; i <= cards; i++)
    {
        Card card1 = deck1.deal();
        Card card2 = deck1.deal();
    }

    System.out.println(deck1.cardsDealtList()); 
    System.out.println(deck1.getNumberCardsDealt());
}
}

Note that I already have a Card Class and the deal method works.

Answer 1

Check the documentation of the indexOf method. You are using it wrong.

You should change the invocation

" ".indexOf(dealtCards);

To

dealtCards.indexOf(" ");

That is, invoking the method on the concerned string and passing to it the character you are looking for, not the other way around.

---

Moreover, your method would not calculate it correctly anyway, you should change it to something like:

public int getNumberCardsDealt() {
    int count = 0;
    int foundIndex = -1; // prevent missing the first space if the string starts by a space, as fixed below (in comments) by Andy Turner

    while ((foundIndex = dealtCards.indexOf(" ", foundIndex + 1)) != -1) {
        count++;
    }

    return count;
}

Answer 2

@A.DiMatteo's answer gives you the reason why your indexOf doesn't work currently.

Internally, String.indexOf is basically just iterating through the characters. If you're always just looking for a single character, you can trivially do this iteration yourself to do the counting:

int count = 0;
for (int i = 0; i < dealtCards.length(); ++i) {
  if (dealtCards.charAt(i) == ' ') {
    ++count;
  }
}