RemoveEldestEntry - Override

Question

How can I override the removeEldestEntry method to save an older record to a file? Also how to limit the file size, as I did it in LinkedHashMap. Here is the code:

import java.util.*;

public class level1 {
private static final int max_cache = 50;
private Map cache = new LinkedHashMap(max_cache, .75F, true) {
protected boolean removeEldestEntry(Map.Entry eldest) {
    return size() > max_cache;
}
};


public level1() {
for (int i = 1; i < 52; i++) {
    String string = String.valueOf(i);
    cache.put(string, string);
    System.out.println("\rCache size = " + cache.size() +
                       "\tRecent value = " + i + " \tLast value = " +
                       cache.get(string) + "\tValues in cache=" +
                       cache.values());

}

I tried using FileOutPutSTream:

    private Map cache = new LinkedHashMap(max_cache, .75F, true) {
    protected boolean removeEldestEntry(Map.Entry eldest) throws IOException {
        boolean removed = super.removeEldestEntry(eldest);
        if (removed) {
            FileOutputStream fos = new FileOutputStream("t.tmp");
            ObjectOutputStream oos = new ObjectOutputStream(fos);

            oos.writeObject(eldest.getValue());

            oos.close();
        }
        return removed;
    }

But I got an error

Error (15,27): removeEldestEntry (java.util.Map.Entry) in cannot override removeEldestEntry (java.util.Map.Entry) in java.util.LinkedHashMap; overridden method does not call java.io.IOException

Without a compiler, IOExecptio requests the handling of IOexception and Filenotfoundexception. Maybe there is another way? Pls shows me some sample code, I'm new to java and just trying to understand the basic principles of level 2 caching. Thanks

Answer 1

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, :

private static final int MAX_ENTRIES_ALLOWED = 100;
private static final long MAX_FILE_SIZE = 1L * 1024 * 1024; // 1 MB

protected boolean removeEldestEntry(Map.Entry eldest) {
    if (size() <= MAX_ENTRIES_ALLOWED) {
        return false;
    }

    File objFile = new File("t.tmp");
    if (objFile.length() > MAX_FILE_SIZE) {
        // Do something here to manage the file size, such as renaming the file
        // You won't be able to easily remove an object from the file without a more
        // advanced file structure since you are writing arbitrary sized serialized
        // objects. You would need to do some kind of tagging of each entry or include
        // a record length before each one. Then you would have to scan and rebuild
        // a new file. You cannot easily just delete bytes earlier in the file without
        // even more advanced structures (like having an index, fixed size records and
        // free space lists, or even a database).
    }

    FileOutputStream fos = null;
    try {
        fos = new FileOutputStream(objFile, true); // Open for append
        ObjectOutputStream oos = new ObjectOutputStream(new BufferedOutputStream(fos));

        oos.writeObject(eldest.getValue());
        oos.close(); // Close the object stream to flush remaining generated data (if any).
        return true;
    } catch (IOException e) {
        // Log error here or....
        throw new RuntimeException(e.getMessage(), e); // Convert to RuntimeException
    } finally {
        if (fos != null) {
            try {
                fos.close();
            } catch (IOException e2) {
                // Log failure - no need to throw though
            }
        }
    }
}

Answer 2

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, try catch: http://download.oracle.com/javase/tutorial/essential/exceptions/try.html