How do I pass an object in Java from Class B to Class C via Class A without duplication

Question

Let's say I have 3 classes.

• Class A (extends Activity)
  Class B
  Class C (extends View)

A creates a B object that holds a Bitmap. A also invalidates C. C's onDraw draws a Bitmap to its Canvas.

How do I draw the Bitmap that B holds without creating another Bitmap object in C? Note it is important for me to keep the same "class structure."

Here is the pseudo code. You can see this puts 2 Bitmap objects in memory. Thanks!

public class A extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        B b = new B();
        C c = (C) findViewById(R.id.c);

        c.passTheBitmap(b.bitmap);
        c.postInvalidate();
    }       
}

public class B {
    public Bitmap bitmap;
    public B(){
        bitmap = BitmapFactory.decodeResource(ActivityAContext.getResources(), R.drawable.fooBitmap);
    }
}

public class C extends View {
    public Bitmap bitmap;

    public C(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public passTheBitmap(Bitmap b) {
        bitmap = b;
    }
    @Override
    public void onDraw(Canvas canvas) {
        if (bitmap != null) {
            canvas.drawBitmap(bitmap, 0, 0, null);
        }
    }
}

Answer 1

I assume you think that the following line:

c.passTheBitmap(b.bitmap);

Would cause b.bitmap to get copied. It doesn't, so long as b.bitmap is not a primitive.

In your code, the only way for there to be a second copy is if you use new to create a new object intentionally.

E.g. in class C

public passTheBitmap(Bitmap b) {
    bitmap = new Bitmap(b);
}

If you do as you have done:

public passTheBitmap(Bitmap b) {
    bitmap = b;
}

... then A.b.bitmap and C.bitmap will reference the same object in memory.

If this was C++, then you have a valid concern - but in Java, what you are doing does not make a copy of the object being passed.

HTH

Answer 2

Well, by having the instance variable declared in C, you're not exactly duplicating the Bitmap object in C. You're just having another reference to the original Bitmap object that gets created in the constructor of B.