“异常：java.lang.StringIndexOutOfBoundsException：字符串索引超出范围”

Question

Here is the problem: Return true if the string "cat" and "dog" appear the same number of times in the given string. Examples: catDog("catdog") → true; catDog("catcat") → false; catDog("1cat1cadodog") → true

  public boolean catDog(String str) {
  int countCat=0;
  int countDog=0;
  for (int i=0; i<str.length();i++)
  {
     if (str.charAt(i)== 'c'&& str.length()>=3)
       {
          if (str.substring(i,i+3).equals("cat"))
          countCat++;
       }
  }
  for (int i=0; i<str.length();i++)
  {
     if (str.charAt(i)== 'd' && str.length()>=3)
       {
          if (str.substring(i,i+3).equals("dog"))
          countDog++;
       }
  }
  if (countCat == countDog)
      return true;
  else
     return false;
}

Answer 1

str.substring(i,i+3).equals("cat")

i可能是最后一个，而i+3会给出错误

Answer 2

In your for loops conditions you are checking if entire String has length greater or equal 3 instead of checking only part from i till end. Try maybe with

str.length() - i >= 3

instead of

str.length() >= 3

Answer 3

Why don't you simply use StringUtils#countMatches ?

StringUtils.countMatches(myStr, "cat") == StringUtils.countMatches(myStr, "dog");

Don't get lost with the indexes. However, if you don't want to use this method, debugging your code is the best thing you can do.

Answer 4

Okay, this is what I might do:

The problem was with your check str.length() >= 3 . It should have been i + str.length() .

I also suggest some changes to your code to get rid of duplication. Here I extracted the part that counts the number of appearances of a substring and moved it to its own method. The part that checks if count of cat equals count of dog now calls said method twice.

public static void main(String[] args) {
    System.out.println(catDog("catdog"));
    System.out.println(catDog("catcat"));
    System.out.println(catDog("1cat1cadodog"));
    System.out.println(catDog("catdogcatc"));//Would previously throw error.
}

public static boolean catDog(String str) {
    int countCat = countAppearances(str, "cat");
    int countDog = countAppearances(str, "dog");
    return countCat == countDog;
}

private static int countAppearances(String str, String key) {
    int count = 0;
    for (int i = 0; i < str.length(); i++) {
        if (str.charAt(i) == key.charAt(0) && i + key.length() <= str.length()) {
            if (str.substring(i, i + key.length()).equals(key)) {
                count++;
            }
        }
    }
    return count;
}

Answer 5

You need to update your first condition before spiting you string like:

if (str.charAt(i)== 'c' && (str.length() - i) >= 3)
{
    if (str.substring(i,i+3).equals("cat"))
        countCat++;
}

Answer 6

    public boolean catDog(String str) {
     int catCount = 0, dogCount = 0;
//run a for loop to check cat count
//run loop till 2nd last character
     for (int i = 0; i < str.length() - 2; i++) {
//now check if the charaters at positions matches "cat"
//if matches then increment cat count
      if (str.charAt(i) == 'c' && str.charAt(i + 1) == 'a' && str.charAt(i + 2) == 't') {
       catCount++;
      } else if (str.charAt(i) == 'd' && str.charAt(i + 1) == 'o' && str.charAt(i + 2) == 'g') {
//else check if the charaters at positions matches "dog"
//if matches then increment dog count
       dogCount++;
      }
     }

//check cat count and dog count
     if (catCount == dogCount) {
      return true;
     } else {
      return false;
     }
    }