javaLogic errorEasy

StringBuffer synchronization overhead in loop

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BugFind the bug and fix the code

What it should do

Join strings with delimiter. Example: buildDelimitedString(["a", "b"], ",") -> "a,b"

What happens instead

Performance degradation from synchronized StringBuffer in thread-local string construction

import java.util.List;

public class Main {
    public static String buildDelimitedString(List<String> items, String delim) {
        // StringBuffer synchronizes every method call, causing unnecessary overhead
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < items.size(); i++) {
            sb.append(items.get(i));
            if (i < items.size() - 1) sb.append(delim);
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        System.out.println(buildDelimitedString(List.of("a", "b"), ","));
    }
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the logic errors pattern

The wrong operator, an inverted condition, a formula the right way round but backwards.

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import java.util.List; public class Main { public static String buildDelimitedString(List<String> items, String delim) { // StringBuffer synchronizes every method call, causing unnecessary overhead StringBuffer sb = new StringBuffer(); for (int i = 0; i < items.size(); i++) { sb.append(items.get(i)); if (i < items.size() - 1) sb.append(delim); } return sb.toString(); } public static void main(String[] args) { System.out.println(buildDelimitedString(List.of("a", "b"), ",")); } }

StringBuffer synchronization overhead in loop

What it should do

What happens instead

More logic errors to practise

StringBuffer synchronization overhead in loop

What it should do

What happens instead

More logic errors to practise