为什么默认的布尔值会影响我的测试结果

Question

I'm learning Java and while completing exercises I stumbled upon an issue in CodingBats sameStarChar program.

I know this is a simple exercise but the logic behind the different outcome is really bugging me.

When I write :

public boolean sameStarChar(String str) {

    for (int i = 1; i < str.length() - 1; i++) {
        if (str.charAt(i) == '*') {
            if (str.charAt(i - 1) != str.charAt(i + 1))
                return false;
        }
    }
    return true;
}

All results are OK.

But when I change the code and invert the condition in the if block and return false as default return value, the code does not work anymore and some test fail:

public boolean sameStarChar(String str) {

    for (int i = 1; i < str.length() - 1; i++) {
        if (str.charAt(i) == '*') {
            if (str.charAt(i - 1) == str.charAt(i + 1))
                return true;
        }
    }
    return false;
}

Can you please tell me why are the outcomes different? I can`t seem to find an exact explanation for this in any book.

Answer 1

Pay close attention to what the code is doing, in English:

• Looping across all characters of a string, starting at 1 and going until 1 before its end
  • If the character at a given position i is * :
    • If the characters a position before and a position after are equal:
      • Return false.
• Return true. Assume other scenario has its case exhausted.

The reason you get completely different results is because you completely flip the logic of the program. Here's your code, in English again:

• Looping across all characters of a string, starting at 1 and going until 1 before its end
  • If the character at a given position i is * :
    • If the characters a position before and a position after are not equal:
      • Return true.
• Return false. Assume other scenario has its case exhausted.

You haven't made false the default return option; you've inverted the entire program's logic. Consider the empty string, which is a valid test case. Your code said that this is invalid, when there's no asterisk to be had in the string (which would be a strange false positive).

Answer 2

The 1st case works because it returns true only if the * is preceded and followed by the same character or if the string doesn't contain a * at all.

The 2nd case doesn't work because it returns true if it contains at-least one * and first instance of * is preceded and followed by the same characters regardless of next instances of * . So if a blank string is passed it should return true but it instead returns false because it doesn't contain * . If another string *xa*a*b is passed the second program will return true because the instance of * follows the convention. The 2nd Program will return true right away, ignoring all the * after the it's first appearance.