In Java, as soon as an object no longer has any references, it becomes eligible for deletion, but the JVM decides when the object is actually deleted. To use Objective-C terminology, all Java references are inherently "strong". However, in Objective-C, if an object no longer has any strong references, the object is deleted immediately. Why isn't this the case in Java?
Java Garbage Collection – Why Java Objects Aren’t Deleted Immediately
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Best Answer
First of all, Java has weak references and another best-effort category called soft references. Weak vs. strong references is a completely separate issue from reference counting vs. garbage collection.
Second, there are patterns in memory usage that can make garbage collection more efficient in time by sacrificing space. For example, newer objects are much more likely to be deleted than older objects. So if you wait a bit between sweeps, you can delete most of the new generation of memory, while moving the few survivors to longer-term storage. That longer term storage can be scanned much less frequently. Immediate deletion via manual memory management or reference counting is much more prone to fragmentation.
It's sort of like the difference between going grocery shopping once per paycheck, and going every day to get just enough food for one day. Your one large trip will take a lot longer than an individual small trip, but overall you end up saving time and probably money.