Why we need reference in java?

Question

Hi
I faced one problem while I was doing heapdump analysis.I had to find memory leak suspect.
Somewhere in parent node I have found soft reference.
My question is that why we need reference in java?
and why IBM jdk1.4.2 and above have inbulit class like
softreference
weakreference
phatomreference

Answer 1

These Reference objects have special ways of interacting with the garbage collecor. A normal reference is, in comparison, called a "hard" reference. If an object is only reachable through a special reference it is said to be "softly reachable" or the like, and the garbage collector can collect it. When this happens the reference object is cleared, and will return null if you try to acccess the object through it.
SoftReference tells the garbage collector "On the whole I'd like you to hang on to this, but feal free to delete it if you're short of space."
Weakly and Phantom reachable objects are collected as normal.
A typical use of a WeakReference is when you want to hold onto a pointer to an object as long as that object exists, but allow it to be collected when there are no other references. For example, some object might listen for events on some window, but if you abandon the object you don't want the listener to prevent the object being disposed of.
A typical use of a SoftReference is when you've arrived at some sizable data object through a moderately expensive process e.g. retrieving it from a database. You can rebuild it if it's lost, but you can save time if it's still there when you want it again.
A PhantomReference is used instead of finalize(). You can arrange for references to be added to a queue when cleared, and you can have a background thread removing references from the queue and doing what you like with them. By extending the Reference type you can store information needed for a clean-up.