调用Collections.sort()后PostingsEntry列表未排序，求问题排查

Ah, I’ve run into this exact issue before—let’s break down why your list isn’t sorting by the score property and how to fix it! Here are the most common causes to check:

1. PostingsEntry Doesn’t Implement Comparable<PostingsEntry> #

If you’re using the default Collections.sort() method without a custom comparator, the JVM has no idea how to compare two PostingsEntry objects. You need to explicitly define the sorting logic by implementing the Comparable interface:

public class PostingsEntry implements Comparable<PostingsEntry> {
    private double score;
    // Other fields, constructors, and methods

    @Override
    public int compareTo(PostingsEntry other) {
        // Sort in descending order (higher score first)
        return Double.compare(other.score, this.score);
        // For ascending order, use: return Double.compare(this.score, other.score);
    }
}

Once implemented, calling Collections.sort(yourArrayList) will automatically sort by the score property as defined.

2. Your Custom sort() Method in PostingsList Has a Bug #

If you wrote your own sorting logic instead of leveraging Java’s built-in Collections.sort(), there’s likely an error in your implementation. Common mistakes include:

• Incorrect comparison logic (e.g., comparing the wrong fields, returning the wrong integer value for ordering)
• Not modifying the original list (e.g., sorting a copy but never updating the stored list)
• Using a flawed sorting algorithm (like an incomplete bubble sort)

Fix: Replace your custom sort with Java’s reliable built-in method. If you need custom ordering, pass a comparator:

public class PostingsList {
    private ArrayList<PostingsEntry> entries;

    public void sort() {
        // Sort descending by score using a lambda comparator
        entries.sort((entry1, entry2) -> Double.compare(entry2.getScore(), entry1.getScore()));
    }
}

3. You’re Using a Comparator That’s Not Correctly Comparing Scores #

If you’re passing a comparator to your sort method, double-check that it’s accessing the score property correctly and returning the right ordering value. For example, a common mistake is swapping the order of comparison (leading to ascending instead of descending, or vice versa).

Example of a correct comparator for ascending order:

Collections.sort(entries, Comparator.comparingDouble(PostingsEntry::getScore));

For descending order:

Collections.sort(entries, Comparator.comparingDouble(PostingsEntry::getScore).reversed());

4. You’re Printing the Wrong List or Modifying It After Sorting #

Double-check that:

• You’re printing the same list instance that you sorted (not a copy or an older version)
• No code is adding/removing entries or updating score values after the sort operation runs

Once you address whichever of these issues applies to your code, your list should sort correctly by the score property.

内容的提问来源于stack exchange，提问作者Sahand