Question

I have map collection like this:

Map<File,Boolean> status = new HashMap<File,Boolean>();

Wrote it to file using serializable then i must edit collection(status) by adding new element
or remove one or more element.

Serializable edit (write changed thing just) to file by add and remove element or empties it and fill it again as object?
and how (if i can) to add or remove elements from file??

for any thing not clear ask me plz

Answer 1

It is not possible remove only the part of file. Every time when You make changes in the map, You must to overwrite the whole file. See my example:

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;

public class SerializationProgram {

    public static void main(String[] args) throws Exception {
        emptyTest();
        addAndRemoveTest();
    }

    private static void emptyTest() throws Exception {
        ObjectSerializer serializer = new ObjectSerializer();
        serializer.serialize(new FileStatus());
        FileStatus persisted = serializer.deserialize(FileStatus.class);
        test(persisted, new FileStatus());
    }

    private static void addAndRemoveTest() throws Exception {
        ObjectSerializer serializer = new ObjectSerializer();
        FileStatus toPersist = new FileStatus();
        FileStatus mirror = new FileStatus();

        toPersist = serializeDeserializeAndTest(serializer, toPersist, mirror);

        File file = new File("/test/test.txt");
        Boolean status = Boolean.TRUE;

        // Add file status
        toPersist.addOrChangeStatus(file, status);
        mirror.addOrChangeStatus(file, status);

        toPersist = serializeDeserializeAndTest(serializer, toPersist, mirror);

        // remove file status
        toPersist.removeStatus(file);
        mirror.removeStatus(file);

        toPersist = serializeDeserializeAndTest(serializer, toPersist, mirror);
        toPersist = serializeDeserializeAndTest(serializer, toPersist, mirror);
    }

    private static FileStatus serializeDeserializeAndTest(
            ObjectSerializer serializer, FileStatus toPersist, FileStatus mirror)
            throws Exception {
        serializer.serialize(toPersist);
        toPersist = serializer.deserialize(FileStatus.class);
        test(toPersist, mirror);
        return toPersist;
    }

    private static void test(FileStatus given, FileStatus expected) {
        if (given.equals(expected)) {
            System.out.println("everything is OK");
        } else {
            throw new IllegalArgumentException("Are not the same!");
        }
    }
}

class ObjectSerializer {

    private static final boolean OVERWRITE_MODE = false;

    public <T> void serialize(T instance) throws IOException {
        OutputStream overwriteStream = new FileOutputStream(getFile(instance.getClass()), OVERWRITE_MODE);
        OutputStream bufferedStream = new BufferedOutputStream(overwriteStream);
        ObjectOutputStream objectstream = new ObjectOutputStream(bufferedStream);
        objectstream.writeObject(instance);
        objectstream.close();
    }

    @SuppressWarnings("unchecked")
    public <T> T deserialize(Class<T> clazz) throws Exception {
        InputStream input = new BufferedInputStream(new FileInputStream(getFile(clazz)));
        ObjectInputStream objectStream = new ObjectInputStream(input);
        T instance = (T) objectStream.readObject();
        objectStream.close();
        return instance;
    }

    private <T> File getFile(Class<T> clazz) {
        return new File(clazz.getName());
    }
}

class FileStatus implements Serializable {

    private static final long serialVersionUID = 1L;

    Map<File, Boolean> statusMap = new HashMap<File, Boolean>();

    public void addOrChangeStatus(File file, boolean status) {
        statusMap.put(file, status);
    }

    public void removeStatus(File file) {
        statusMap.remove(file);
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result
                + ((statusMap == null) ? 0 : statusMap.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        FileStatus other = (FileStatus) obj;
        if (statusMap == null) {
            if (other.statusMap != null)
                return false;
        } else if (!statusMap.equals(other.statusMap))
            return false;
        return true;
    }
}