Please explain soft/weak references to me!

Question

What I would like to do is have following class:
abstract class ImageContainer
  private Image img;
  public Image getImage()
    if (img==null)
      renderImage();
    return img;   
  public void lowMemory()
     img=null;
     System.gc();
  abstract protected void renderImage();
}I think that you get my point, but this doesn't really work, since there's noplace where from call lowMemory() (and I mean low RAM memory, not low swapping memory, since this is to remove swapping during gameplay). I need somehow to use soft references or something like that so I was wondering if you could point me to right direction... I was unable to understand completily the API for package java.lang.ref

Answer 1

abstract class ImageContainer
private SoftReference imgRef = new SoftReference();
public Image getImage()
   Image img = (Image)imgRef.get();
   if (img==null)
      img = renderImage();
      imgRef = new SoftReference(img);
   return img;
abstract protected Image renderImage();
}Reference objects "wrap" a regular hard reference, passed into the constructor. The "get" method returns that reference, or null if the object has been collected. (After a GC cycle for WeakReference, after a low-memory condition for SoftReference.) Therefore, upon fetch, call "get" on the reference class and check for null. If null, recreate and create a new reference class instance. Return the object either re-fetched or just created.