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And then, finally, we can all go home and rest in peace. So, colloquially speaking, this is what the Soft reference and the Weak reference tell us about themselves. I am a soft reference. Ill take your shit as long as the JVM has patience.
Generalized algebraic data types and object-oriented programming
When it begins running out of patience i. Your object will be gone. And then you simply have to create a new object. I am even cooler than the WeakReference. Coz I wont take your shit at all. The moment you lose me, if the JVM detects that you're no longer around, am gonna punch you in the face and run away! Marked for garbage collection. Can you dig it sucka! As you see, our two awesome friends, Softy and Weaky, certainly have some ego there. But they are pretty useful, at crucial times.
Before I proceed any further, there is one more puny lil thing that you might need to know. Did i just say 'might'. I meant, you should know. And that is ReferenceQueues. ReferenceQueues are some sort of a queue where the JVM can store objects of type reference once it has decided to take some action on the objects to which they refer.
What I mean to say is, let us suppose you have a weak reference which points to an object in the heap. And that object has been left lonely and desolate by the rest of the application. No strong references to it. And the garbage collector detects this object during its garbage collection cycle.
Since this object only has a weak reference to it, the garbage collector will mark it for garbage collection.
Generalized algebraic data types and object-oriented programming - Semantic Scholar
But at the same time, it looks if there is a reference queue associated with the weak reference that points to this object. If yes, it puts this weak reference in the reference queue to indicate that the object has been marked for garbage collection.
The subtle point to be noted here is that even though the object has been marked for garbage collection, garbage collection may not have happened yet. This may be because the object has a finalize method, which the JVM needs to execute before reclaiming memory. This also means that you can, but should not,unless deemed necessary, resurrect an object in the finalize method and create a strong reference to it.
But when you do that, the weak reference still remains en-queued in the ReferenceQueue. Overriding the finalize method to resurrect an object is a rare case, but since it is one of the options that the JVM supports, it is therefore something that needs to be considered. Nevertheless, when you do such things, its almost equivalent to artificially manipulating the life of an object. That's because the second time when the object becomes eligible for garbage collection, the finalize method wont run, which is a good thing because if you run it again, its simply gonna revive itself.
So, practically speaking, an object in the JVM has only one spare life. You get just one medical kit at the max.
- Java in a Nutshell, 6th Edition by David Flanagan, Benjamin J Evans
- Reference Types In Java - Part 1
- Subtyping vs. Parametrization for a Complex Domain
You screw up again, and ur doomed. Your object will be on mars, having a boss fight with the garbage collecting thread of the JVM, which will eventually win, and reclaim the memory of the object.
The same facts about the reference queue hold true for Soft references as well.
In order to associate a weak or a soft reference with a reference queue, you can use the 2 argument constructor as shown below. Yes of course it is. Then again, you haven't met the Phantom yet, have ya? The phantom reference is a place where it gets all the more interesting. Phantom References Phantom references tell a long tale themselves and its a topic that warrants a blog post of its own. However, in this blog post, ill give a brief idea about what they are.
Phantom references are quite similar to Strong and Weak references in the sense that the garbage collector will collect the referent object if the only reference to it is the phantom reference. But that's where the similarity ends. What makes Phantom references are unique is the way in which they are used along with a reference queue. A phantom reference is always associated with a references queue during construction time. This is because phantom references are enqueued in the queue only when the the object is about to be garbage collected, after the finalize method if any has been executed on it.
Calling a get on the Phantom reference always returns null. And that is quite appropriate because the finalize function has already run on the referent object.
So, there should be no 'legal' way of resurrecting the object resurrecting i. This may at first seem to make no sense, since, what use is a phantom reference if we cant extract the referent from it?
But on giving it a deep thought, you would realize that this is not the reason why phantom references are meant to be useful in the first place. It is the time at which the JVM puts a phantom reference in the reference queue that makes its use so intuitively amazing. That's something that I will be discussing in more detail in the next post.
So stay tuned, because, it might just turn out to be a weird crazy experiment. And of course, its possible that some so called 'laws' will be violated. I am still working on it right now. Ill put it up sooner or later.
So folks, see you on the other side! That's what phantom references say too. Default Methods With the advent of Java 8, it is possible to include methods in interfaces that include an implementation. Backward compatibility The Java platform has always been very concerned with backwards compatibility.
This means that code that was written or even compiled for an earlier version of the platform must continue to keep working with later releases of the platform. This principle allows development groups to have a high degree of confidence that an upgrade of their JDK or JRE will not break currently working applications.
Backward compatibility is a great strength of the Java platform, but in order to achieve it, some constraints are placed on the platform. One of them is that interfaces may not have new mandatory methods added to them in a new release of the interface. Note You can see this effect quite easily in your own code. Compile a class file that depends on an interface. Then add a new mandatory method to the interface, and try to run the program with the new version of the interface, together with your old class file.
You should see the program crash with a NoClassDefError. This limitation was a concern for the designers of Java 8—as one of their goals was to be able to upgrade the core Java Collections libraries, and introduce methods that made use of lambda expressions. To solve this problem, a new mechanism was needed, essentially to allow interfaces to evolve by allowing new, optional methods to be added to interfaces without breaking backward compatibility.
Gagallium : Gallium submissions to the ML Workshop
Implementation of default methods To add new methods to an interface without breaking backward compatability requires that some implementation must be provided for the older implementations of the interface so that they can continue to work.
This mechanism is a default method, and it was first added to the platform in JDK 8. Note A default method sometimes called an optional method can be added to any interface.
This must include an implementation, called the default implementation, which is written inline in the interface definition. The basic behavior of default methods is: An implementing class may but is not required to implement the default method.
If an implementing class implements the default method, then the implementation in the class is used. If no other implementation can be found, then the default implementation is used. An example default method is the sort method. List in JDK 8, and is defined as: As the return type is void, we might expect that this is an in-place sort, and this is indeed the case. Marker Interfaces Sometimes it is useful to define an interface that is entirely empty.
A class can implement this interface simply by naming it in its implements clause without having to implement any methods.
In this case, any instances of the class become valid instances of the interface. Java code can check whether an object is an instance of the interface using the instanceof operator, so this technique is a useful way to provide additional information about an object.
Serializable interface is a marker interface of this sort. RandomAccess is another example: List implementations implement this interface to advertise that they provide fast random access to the elements of the list. Algorithms that care about the performance of random-access operations can test for RandomAccess like this: A marker interface is a great example of this—it has nothing at all except a name. Java Generics One of the great strengths of the Java platform is the standard library that it ships.
It provides a great deal of useful functionality—and in particular robust implementations of common data structures. These implementations are relatively simple to develop with and are well documented. Although they were still very useful, the earliest versions of the collections had a fairly major limitation, however. This limitation was that the data structure often called the container essentially hid the type of the data being stored in it.
Note Data hiding and encapsulation is a great principle of object-oriented programming, but in this case, the opaque nature of the container caused a lot of problems for the developer. Introduction to Generics If we want to build a collection of Shape instances, we can use a List to hold them, like this: What we really want is a form of List that understands what type it contains.
Then, javac could detect when an illegal argument was passed to the methods of List and cause a compilation error, rather than deferring the issue to runtime.
Java provides syntax to cater for this—to indicate that a type is a container that holds instances of another reference type we enclose the payload type that the container holds within angle brackets: This is, of course, the whole point of static type systems—to use compile-time knowledge to help eliminate whole swathes of runtime problems. Container types are usually called generic types—and they are declared like this: This should convey the sense that the container type e.
When we define a type that has parameters, we need to do so in a way that does not make assumptions about the type parameters. Tip Type parameters always stand in for reference types. It is not possible to use a primitive type as a value for a type parameter. The type parameter can be used in the signatures and bodies of methods as though it is a real type, for example: Diamond Syntax When creating an instance of a generic type, the right-hand side of the assignment statement repeats the value of the type parameter.
This is usually unnecessary, as the compiler can infer the values of the type parameters. In modern versions of Java, we can leave out the repeated type values in what is called diamond syntax. The addition of generics in Java 5 was another example of where backwards compatibility was an issue for a new language feature. The central question was how to make a type system that allowed older, nongeneric collection classes to be used along with newer, generic collections.
The design decision was to achieve this by the use of casts: Java achieves this compatibility by type erasure. This means that generic type parameters are only visible at compile time—they are stripped out by javac and are not reflected in the bytecode. It is still perfectly legal Java to work with the raw form of types—even for types that are now generic.
This is almost always a sign of poor quality code, however. Type erasure also prohibits some other definitions, which would otherwise seem legal.BlueJ Chapter 8 Part 3 Polymorphic Variables, Subtyping, Casting
In this code, we want to count the orders as represented in two slightly different data structures: The issue is that although the two methods seem like normal overloads, after type erasure, the signature of both methods becomes: The runtime would be unable to distinguish between the methods by signature, and so the language specification makes this syntax illegal. It is only when we provide a concrete value for the type parameter, e.
This poses a problem if the type that we want to work with is unknown at compile time. Fortunately, the Java type system is able to accommodate this concept. We can write expressions that involve the unknown type: When working with the unknown type, there are some limitations on its use in user code.
For example, this code will not compile: The only value that we know we can always insert into a container is null—as we know that null is a possible value for any reference type.
However, consider the following code: However, as the actual instance holds strings, then trying to add an Object would not be compatible, and so this would fail at runtime. The resolution for this is to realize that although this is legal because String inherits from Object: If we want to have a subtyping relationship for containers, then we need to use the unknown type: For example, the return type of get is now effectively Object. This is the ability to restrict the types that can be used as the value of a type parameter.
This provides a useful lifeline to the programmer—instead of being restricted to the totally unknown type, she knows that at least the capabilities of the type bound are available. Warning The extends keyword is always used, regardless of whether the constraining type is a class or interface type. This is an example of a concept called type variance, which is the general theory of how inheritance between container types relates to the inheritance of their payload types.
Type covariance This means that the container types have the same relationship to each other as the payload types do. This is expressed using the extends keyword. Type contravariance This means that the container types have the inverse relationship to each other as the payload types.
This is expressed using the super keyword.