Garbage Collection - Beginning Java 8 Language Features - page 495

Table 11-1. Constructors for the SoftReference, WeakReference, and PhantomReference classes

Class Constructors

SoftReference SoftReference(Object referent)

SoftReference(Object referent, ReferenceQueue q)

WeakReference WeakReference(Object referent)

WeakReference(Object referent, ReferenceQueue q)

PhantomReference PhantomReference(Object referent, ReferenceQueue q)

You can create objects of SoftReference and WeakReference classes using an object of any class, or using an
object of any class and an object of the ReferenceQueue class. You must create an object of the PhantomReference
class using an object of any class and an object of the ReferenceQueue class. You can create an object of the
SoftReference class as shown:

Employee john = new Employee ("John Jacobs");
SoftReference sr = new SoftReference(john);

The memory state after executing the above two statements is depicted in Figure 11-4 .

A strong reference

john

John Jacobs

X

sr

A soft reference (or a weak-
reference in general) A SoftReference

object X

A strong reference

Figure 11-4. An example of a soft reference

In Figure 11-4 , there are two strong references and one weak reference. All three weak reference classes have two
instance variables: referent and queue . I will not discuss any other instance variables of these classes here. They are
used to hold the reference of the object and reference queue passed in to the constructors of these classes. A reference
to any object stored in the referent instance variable of any of these three classes is known as a weak reference in
general—and a soft reference, weak reference, or phantom reference in particular, depending on the class being used.
Therefore, the link from a soft reference object to the employee object shown in the figure is a weak reference. To be
specific, I will call it a soft reference because I used an object of the SoftReference class. Any reference that does not
involve the referent instance variable of any of these three classes is a strong reference in Java. Therefore, john and
sr are strong references.