TestC.java

package imbir;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Date;

public class TestC {

    private final static File outputFile = new File("plik.dat");

    public static void main(String[] args) throws Exception {

        SomeData date = new SomeData(12, "abc", new Date());
        saveTofile(outputFile, date);

        System.out.println("Zapisany.");
        SomeData restoredData = readFromFile(outputFile);

        System.out.println(restoredData);
    }

    static SomeData readFromFile(File file) throws IOException, ClassNotFoundException {
        try (ObjectInputStream objectIn = new ObjectInputStream(new FileInputStream(file))) {
            Object obj = objectIn.readObject();
            return (SomeData) obj;
        }
    }

    static void saveTofile(File file, Serializable object) throws IOException {
        try (ObjectOutputStream objectOut = new ObjectOutputStream(new FileOutputStream(file))) {
            objectOut.writeObject(object);
            objectOut.flush();
            objectOut.close();
        }
    }
}

class SomeData implements Serializable {

    private static final long serialVersionUID = 13423L;

    final int x;
    final String napis;
    final Date data;

    public SomeData(int x, String napis, Date data) {
        this.x = x;
        this.napis = napis;
        this.data = data;
    }

    @Override
    public String toString() {
        return "SomeData{" +
                "x=" + x +
                ", napis='" + napis + '\'' +
                ", data=" + data +
                '}';
    }
}