第三方图形库中的NotSerializableException

Question

I have created a clone of Atari Breakout game using the ACM graphics library and just finished adding a highscore interface and functionality. The player's name and score should be displayed on the GUI window (it is successfully) and also be written to a .dat binary file.

However, when the code attempts to load the existing file I get the following error.

writing aborted; java.io.NotSerializableException: acm.graphics.GCanvasListener

I've researched this error online and it seems it can be solved by editing the class to implement Serializable . However, the class throwing this error is not one of my own but rather a class that belongs to the third-party ACM graphics library . How do I solve this?

I'm not even sure why this error is being caused in the first place since the data I'm attempting to serialize is only a name and scor e, I'm not trying to serialize a canvas of objects or anything like that.

Main class (called Breakout)

public class Breakout extends GraphicsProgram {
    ... // game variables
    public void run() {
        ... // this if clause runs when game ends
        if (brickCounter > 0) {
                removeAll(); // clears screen
                printGameOver(); // displays game over message
                HighscoreManager hm = new HighscoreManager();
                String name = getHighScoreName();
                hm.addScore(name, score);
                hm.displayHighscores();
        }
    }
    ... // game functionality methods
    private String getHighScoreName(){
        IODialog dialog = new IODialog();
        String name = dialog.readLine("Enter your name: ");
        return name;
    }

Score class

private class Score implements Serializable {
    private int score;
    private String name;

    public Score(String name, int score) {
        this.score = score;
        this.name = name;
    }

    public int getScore() { return score; }
    public String getName() { return name; }
}

ScoreComparator class

private class ScoreComparator implements Comparator<Score> {
    public int compare(Score score1, Score score2) {

        int sc1 = score1.getScore();
        int sc2 = score2.getScore();

        if (sc1 > sc2) {
            return -1;
        } else if (sc1 < sc2) {
            return 1;
        } else {
            return 0;
        }
    }
}

HighscoreManager class

private class HighscoreManager {
    private ArrayList<Score> scores;
    private static final String HIGHSCORE_FILE = ".//bin//scores.dat";
    ObjectOutputStream outputStream = null;
    ObjectInputStream inputStream = null;

    public HighscoreManager() {
        scores = new ArrayList<Score>(10);
    }

    public ArrayList<Score> getScores() {
        loadScoreFile();
        sort();
        return scores;
    }

    private void sort() {
        ScoreComparator comparator = new ScoreComparator();
        Collections.sort(scores, comparator);
    }

    public void addScore(String name, int score) {
        loadScoreFile();
        scores.add(new Score(name, score));
        updateScoreFile();
    }

    public void loadScoreFile() {
        try {
            inputStream = new ObjectInputStream(new FileInputStream(HIGHSCORE_FILE));
            scores = (ArrayList<Score>) inputStream.readObject();
        }
        catch (FileNotFoundException e) {
            System.out.println("[Load] File Not Found Error: " + e.getMessage());
        }
        catch (IOException e) {
            System.out.println("[Load] Input/Output Error: " + e.getMessage());
        }
        catch (ClassNotFoundException e) {
            throw new RuntimeException("[Load] Class Not Found Error: " + e.getMessage());
        }
        finally {
            try {
                if (outputStream != null) {
                    outputStream.flush();
                    outputStream.close();
                }
            } catch (IOException e) {
                System.out.println("[Load] Input/Output Error: " + e.getMessage());
            }
        }
    }

    public void updateScoreFile() {
        try {
            outputStream = new ObjectOutputStream(new FileOutputStream(HIGHSCORE_FILE));
            outputStream.writeObject(scores);
        }
        catch (FileNotFoundException e) {
            System.out.println("[Update] File Not Found Error: " + e.getMessage());
        }
        catch (IOException e) {
            System.out.println("[Update] Input/Output Error: " + e.getMessage());
        }
        finally {
            try {
                if (outputStream != null) {
                    outputStream.flush();
                    outputStream.close();
                }
            } catch (IOException e) {
                System.out.println("[Update] Input/Output Error: " + e.getMessage());
            }
        }
    }

    public void displayHighscores() {
        int max = 10;
        ArrayList<Score> scores;
        scores = getScores();
        int x = scores.size();

        if (x > max) {
            x = max;
        }

        removeAll(); // clears screen
        int npos = 160;
        int spos = 160;

        for (int i = 0; i < x; i++) {
            GLabel showName = new GLabel(scores.get(i).getName(), (getWidth() / 2.0) - 100, (getHeight() / 2.0) - npos);
            showName.move(-showName.getWidth() / 2, -showName.getHeight());
            showName.setColor(Color.WHITE);
            add(showName);
            npos -= 40;
        }

        for (int i = 0; i < x; i++) {
            GLabel showScore = new GLabel(Integer.toString(scores.get(i).getScore()), (getWidth() / 2.0) + 100, (getHeight() / 2.0) - spos);
            showScore.move(-showScore.getWidth() / 2, -showScore.getHeight());
            showScore.setColor(Color.WHITE);
            add(showScore);
            spos -= 40;
        }
    }

After running the application:

[Load] Input/Output Error: writing aborted; java.io.NotSerializableException: acm.graphics.GCanvasListener
[Update] Input/Output Error: acm.graphics.GCanvasListener
[Load] Input/Output Error: writing aborted; java.io.NotSerializableException: acm.graphics.GCanvasListener

Answer 1

You should go to the class where the fields name and score are, and add for example public class nameclass implements Serializable . I hope it works for you.

Answer 2

Your task will be to find a hidden reference from your name and score structure to the UI components. Many GUI applications use a lot of inner classes, and this might be the missing link.

When you have a class something like this:

class MyGame {
    private SomeUIWidget widget;
    class TopScore implements Serializable {
        String name;
        int score;
        ...
    }
    ...
}

There is a hidden member in TopScore that references the "enclosing instance" of MyGame , including its SomeUIWidget member. When you try to serialize a TopScore instance, all the rest gets dragged in with it.

You could simply declare TopScore as a static nested class. This means that there is no enclosing instance, and serves only to hide the TopScore class from other code. But, I would suggest just making TopScore a top-level class, in its own file, because it's likely that other objects will want to use those objects in different ways—that is, it seems like a likely candidate for part of your public API.

This is an educated guess, in the absence of any actual code. To get a better answer, reduce your code to the minimum required to demonstrate the problem, and include that in your question.