Java/Android - Passing objects to methods with resources open

Question

Supposing I have a class that holds a reference to an InputStream that is reading from a file, or an OutputStream that is writing to a file (or another example, if you're an Android developer, a MediaPlayer instance).

Knowing that Java passes objects by value, what happens when you have something like the following pseudocode?

class MyClass  {


  InputStream is;

  public void read() {
    // initialize inputstream here (assume it's reading from a file that contains the alphabet from A-z)

     is.read();

 }
 public MyClass() {
    this.read();
  }
}


class OtherClass {
  MyClass mine = new MyClass();
  public OtherClass() {
     mine.read();
     LastClass lastClass = new LastClass(mine);

  }
}


class LastClass {
   MyClass his;

   public LastClass(MyClass mc) {
       his = mc;
        his.read();
    }
}

My question is, since we're passing objects by value, how are they able to maintain a reference to the open resource? Also, just to be clear, by the time everything is executed, how many instances of MyClass exist?

Answer 1

While it is true that Java is pass-by-value, you must keep in mind that for objects, it is the value of the object's reference that is passed.

Although there may not seem to be a difference between pass-by-reference and passing the value of a reference, their are subtle distinctions. See is-java-pass-by-reference for a more in-dept discussion on the subject.

So to answer your question, in your example you will have one instance of MyClass created for each instance of OtherClass you create.

As for the input stream, since it is initialized in MyClass.read() rather than in the MyClass constructor, then we can assume that the file will be opened each time read() is called and one byte will be read from the file. The InputStream will stay opened but the next time read() is called then a reference to a new InputStream will be assigned to the is member variable and the old one would be release for garbage collection. It is hard to say more that that without more code.

When you create an OtherClass object, the following happen:

• A new MyClass object is created in the heap and a reference to this object is stored in the mine member variable.
  • The MyClass constructor calls the read() method. So a new InputStream is initialize and the first byte is read from the file.
• The OtherClass constructor calls mine.read() . So a new InputStream is initialize and the first byte is read from the file.
• A new LastClass object is created. The value of the reference to the MyClass object stored in variable mine is passed to the constructor.
  • This copy of the MyClass reference is saved in the his member variable of the new LastClass object.
  • The LastClass constructor calls mine.read() . So a new InputStream is initialize and the first byte is read from the file.