Question

So I run into a casting problem with storing HashMap properties to a Properties file because the key of the HashMap is an object that cannot be cast to java.lang.String because properties must be (String, String) and my HashMap is (Object, Long).

I just want to save the contents of the HashMap to a file so I can load those contents back into the HashMap the next time the program is ran.

I have gone through an excessive amount of worthless attempts to try to refill HashMaps with objects they contained before they were saved. I tried to think of an easy way to convert the object to a string which I can do, but since Properties and HashMaps are not indexed, I can not change those strings back to the object they need to be. How to achieve this?

Here is what I am trying to do:

    public File savedHashMaps = new File("SavedHashMaps.list");
// in my case, the object is 'Location' from an implemented API
public Map<Location, Long> map = new HashMap<Location, Long>();

public void saveMaps() {
    Properties prop = new Properties();
    try {
        if (!map.isEmpty()) {
            prop.load(new FileInputStream(savedHashMaps));
            prop.putAll(map);
            prop.store(new FileOutputStream(savedHashMaps), null);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

public void loadMaps() {
    Properties prop = new Properties();
    try {
        prop.load(new FileInputStream(savedHashMaps));
        if (!prop.isEmpty()) {
            map.putAll((Map)prop);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Answer 1

Using an ObjectInput/OutputStream

public File savedHashMaps = new File("SavedHashMaps.list");
public Map<Location, Long> map = new HashMap<Location, Long>();

public void saveMaps() {
    try {
        ObjectOutputStream oos = new ObjectOutputStream (new FileOutputStream(savedHashMaps));
        oos.writeObject(map);
        oos.close();
    } catch (Exception e) {
        // Catch exceptions
    }
}

public void loadMaps() {
    try {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(savedHashMaps));
        Object readMap = ois.readObject();
        if(readMap != null && readMap instanceof HashMap) {
            map.putAll((HashMap) readMap);
        }
        ois.close();
    } catch (Exception e) {
        // Catch exceptions
    }
}

This is from memory, so sorry for errors, however this will save and reload all your Location, Long mappings. Also as a note, the Location class must implement Serializable (or have a super-class that implements it)(It’s a marker interface (See java.dzone.com/articles/marker-interfaces-java) so you just add implements Serializable).