Java如何避免字符串索引超出范围

Question

I have the following task to do:

Return true if the string "cat" and "dog" appear the same number of times in the given string.

catDog("catdog") → true catDog("catcat") → false catDog("1cat1cadodog") → true

my code:

public boolean catDog(String str) {
int catC = 0;
int dogC = 0;
if(str.length() < 3) return true;
for(int i = 0; i < str.length(); i++){
   if(str.charAt(i) == 'd' && str.charAt(i+1) == 'o' && str.charAt(i+2)  == 'g'){
     dogC++;
   }else if(str.charAt(i) == 'c' && str.charAt(i+1) == 'a' && 
                                       str.charAt(i+2) == 't'){
    catC++;
  }
}

if(catC == dogC) return true;
return false;
}

But for catDog("catxdogxdogxca") → false I'm getting a StringIndexOutOfBoundsException . I know it's caused by the if clause when it tries to check if the charAt(i+2) equals t. How can I avoid this? Thanks in regards :)

Answer 1

for(int i = 0; i < str.length(); i++){ // you problem lies here
   if(str.charAt(i) == 'd' && str.charAt(i+1) == 'o' && str.charAt(i+2)  == 'g')

You are using the i < str.length() as the loop termination condition but you are using str.charAt(i+1) and str.charAt(i+2)

Since you need to access i+2 , then you should limit the range by i < str.length() - 2 instead.

for(int i = 0, len = str.length - 2; i < len; i++) 
// avoid calculating each time by using len in initialising phase;

Answer 2

There is a problem with logic, the conditional statements are trying to access character which is out of string size.

Input: catxdogxdogxca have ca at the end that's why else block is executed and which tries to get character present at i+3 which does not exist in the input. That's why you are seeing java.lang.StringIndexOutOfBoundsException .