Java/Android SoftReference, WeakReference, PhantomReference

基本上是和Garbage Collection有關的物件引用等級

Class Definition on Android
http://developer.android.com/reference/java/lang/ref/WeakReference.html
http://developer.android.com/reference/java/lang/ref/SoftReference.html
http://developer.android.com/reference/java/lang/ref/PhantomReference.html

Examples on the Web
StrongReference、SoftReference、WeakReference、PhantomReference  <--不用程式碼就解釋得很清楚
对SoftReference、WeakReference和PhantomRefrence的理解