Why I cannot read List object from my file (Java IO)

Question

I have a method writing List<String> into file like this:

public static void main(String[] args) {
    List<String> data = Arrays.asList("1", "2", "3", "4", "5");
    writeObjectToFile(data, "demo.dat");
    
    List<String> result = null;
    readObjectFromFile(result, "demo.dat");
    System.out.println(result);
}

with the writeObjectToFile method:

public static <T> void writeObjectToFile(List<T> obj, String fileName) {
    File file = new File(fileName);
    FileOutputStream fos = null;
    ObjectOutputStream oos = null;

    try {
        fos = new FileOutputStream(file);
        oos = new ObjectOutputStream(fos);
        oos.writeObject(obj);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        // close fos, oos, ... 
    }
}

and readObjectFromFile method:

public static <T> void readObjectFromFile(List<T> obj, String fileName) {
    File file = new File(fileName);
    FileInputStream fis = null;
    ObjectInputStream ois = null;

    if (file.exists()) {
        try {
            fis = new FileInputStream(file);
            ois = new ObjectInputStream(fis);
            Object readObject = ois.readObject();
            obj = (List<T>) readObject;
            System.out.println(obj);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            // close fis, ois,...
        }
    }
}

The writing data to file is OK, but the reading step is not working as expected. I got the result list is null:

// console output
[1, 2, 3, 4, 5] // --> this is result of System.out.println(obj);
null            // --> this is result of System.out.println(result);

But when I change the reading method to return a list:

public static <T> List<T> readObjectFromFile(String fileName) {
    File file = new File(fileName);
    FileInputStream fis = null;
    ObjectInputStream ois = null;

    if (file.exists()) {
        try {
            fis = new FileInputStream(file);
            ois = new ObjectInputStream(fis);
            Object readObject = ois.readObject();
            List<T> obj = (List<T>) readObject;
            System.out.println(obj);
            return obj;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            // close fis, ois,...
        }
    }
    return null;
}

Then I get the expected result:

// console output
[1, 2, 3, 4, 5] // --> this is result of System.out.println(obj);
[1, 2, 3, 4, 5] // --> this is result of System.out.println(result);

I don't know why is it. Why when I passed the result list as a parameter, I could not get the expected result? Please help me.

Answer 1

You are changing it reference value, Java is pass by value, when you pass an object to a method as a parameter, java pass a variable that keep the reference value, you can't change that value ( correctly, you can change that value just inside method only )

Imagine that it point to null before you pass it into method

Inside method, you are trying to point it to another object ( assign new reference value ) It will be discard when method end, it still point to null