writeObject(this) cannot write object, java

I am using ObjectOutputStream to save object, but when I use .writeObject(this) to save it as a file, the material cannot be saved. The class I defined is already serializable.

public class LanguageModel implements Serializable {


private static LanguageModel lm_;

/* ******************************* */
//word -> count(w)
public static Dictionary unigramDict = new Dictionary();
//word_pair -> count(wi,wi+1)
public static Dictionary bigramDict = new Dictionary();

private static int wordIdCounter = 0;
/* ***************************** */


// Do not call constructor directly since this is a Singleton
private LanguageModel(String corpusFilePath) throws Exception {
    constructDictionaries(corpusFilePath);
}


public void constructDictionaries(String corpusFilePath)
        throws Exception {

    ...
    }

// Saves the object (and all associated data) to disk
public void save() throws Exception{
    FileOutputStream saveFile = new FileOutputStream(Config.languageModelFile);
    ObjectOutputStream save = new ObjectOutputStream(saveFile);
    save.writeObject(this);
    save.close();
}

// Creates a new lm object from a corpus
public static LanguageModel create(String corpusFilePath) throws Exception {
    if(lm_ == null ){
        lm_ = new LanguageModel(corpusFilePath);
    }
    return lm_;
}

}

The class I defined is as follows:

import java.io.Serializable;

import java.util.HashMap;

public class Dictionary implements Serializable {

private int termCount;
private HashMap<String, Integer> map;

public int termCount() {
    return termCount;
}

public Dictionary() {
    termCount = 0;
    map = new HashMap<String, Integer>();
}

...
}

When I try save.writeObject(unigramDict), it can save this variable properly. Since it is a large variable, I can simply check the size of the file. It is 5MB. When I switch to save.writeObject(this), the size of the file is only 53 Bytes.