Coding Bat Java热身题：stringE方法错误原因排查求助

Let's break down the critical issues in your code that prevent it from passing the test cases:

1. substring Returns an Empty String Every Time #

In your loop, you're using str.substring(i,i++). The substring(beginIndex, endIndex) method returns a string that starts at beginIndex and ends before endIndex. When you pass the same value for both indices (substring(i,i)), this will always return an empty string (""), which can never match "e". This means your count variable will never increment—so every test case will return false because count >= 1 will never be true.

2. Loop Variable Gets Skipped Due to Misused i++ #

Even if the substring logic was fixed, using i++ inside the substring call breaks your loop iteration. Here's what happens in each loop cycle:

• First, the substring method uses the current value of i for both start and end indices
• Then, the post-increment i++ increases i by 1
• After the loop body finishes, the for loop's header will increment i again (from the i++ in for (int i=0; i<str.length(); i++))

This means you're skipping every other character in the string. For example, with "Hello", you'd check index 0, then jump straight to index 2, then 4—completely missing the 'e' at index 1.

How the Correct Code Fixes This #

The working solution uses str.charAt(i) == 'e' to directly check each character, which is efficient and avoids substring mistakes. If you prefer using substring, you'd need to write str.substring(i, i+1) to get the single character at position i, and keep the loop's i increment only in the loop header (no extra i++ inside the loop body).

Let's walk through your code with "Hello" to see the failure in action:

• Initial i=0: substring(0,0) returns "", no match; i becomes 1 (from post-increment), then loop header increments to 2
• Next iteration i=2: substring(2,2) returns ""; i becomes 3, then loop header increments to 4
• Next iteration i=4: substring(4,4) returns ""; i becomes 5, then loop header increments to 6 (exceeds str.length()), loop ends
• count stays 0, returns false—which fails the first test case.

内容的提问来源于stack exchange，提问作者Andrew Yang