removeEldestEntry overriding

Question

How can I override removeEldestEntry method to saving eldest entry to file? Also how to limit the size of a file like I did it in LinkedHashMap. Here is 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 to use 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 have gained 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 throw java.io.IOException

Without IOExecptio compiler asks to handle IOexception and Filenotfoundexception. Maybe another way exists? Pls show me example code, I am new in java and just trying to understand the basic principles of 2 level caching. Thx

Answer 1

You first need to make sure your method properly overrides the parent. You can make some small changes to the signature, such as only throwing a more specific checked exception that is a sub-class of a checked exception declared in the parent. In this case, the parent does not declare any checked exception so you can not refine that further and may not throw any checked exceptions. So you will have to handle the IOException locally. There are several ways you can do that, convert it to a RuntimeException of some kind and/or log it.

If you are concerned about the file size, you probably do not want to keep just the last removed entry but many of them - so you should open the file for append.

You need to return true from the method to actually remove the eldest and you need to decide if the element should be removed.

When working with files you should use try/finally to ensure that you close the resource even if there is an exception. This can get a little ugly - sometimes it's nice to have a utility method to do the close so you don't need the extra try/catch.

Generally you should also use some buffering for file I/O which greatly improves performance; in this case use wrap the file stream in a java.io.BufferedOutputStream and provide that to the ObjectOutputStream.

Here is something that may do what you want:

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

You can't change the method signature when overriding a method. So you need to handle the exception in the overridden method instead of throwing it.

This contains a good explanation on how to use try and catch: http://download.oracle.com/javase/tutorial/essential/exceptions/try.html