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I'd like to create a few immutable objects for my codebase. What's the best way to really deliver the message that a given class is intended to be immutable? Should I make all of my fields final, and initialize during object construction? (This seems really awkward...) Should I create some Immutable interface, and have objects implement it? (Since Java doesn't have some standard interface behind this, I thought they had some other way of dealing with it.) What's the standard way this is dealt with? (If it's simply done by adding a bunch of comments around the fields exclaiming that they shouldn't be modified once initialized, that's fine too.)
Should I make all of my fields final, and initialize during object construction?
Yes. And ensure that those types are themselves immutable, or that you create copies when you return values from getter methods. And make the class itself final. (Otherwise your class on its own may be immutable, but that doesn't mean that any instance of your class would be immutable - because it could be an instance of a mutable subclass.)
(This seems really awkward...)
It's hard to know what to suggest without knowing how you find it to be awkward - but the builder pattern can often be useful. I usually use a nested static class for that, often with a static factory method. So you end up with:
Foo foo = Foo.newBuilder()
.setName("asd")
.setPoints(10)
.setOtherThings("whatever")
.build();
Yes and no. Making all fields final is not a guarantee in and of itself. If you'd like to get really in-depth with this there are a number of chapters in Effective Java by Joshua Bloch dealing with immutability and the considerations involved. Item 15 in Effective Java covers the bulk of it and references the other items in question.
He offers these five steps:
Don’t provide any methods that modify the object’s state (known as muta-
tors).
Ensure that the class can’t be extended.
Make all fields final.
Make all fields private.
Ensure exclusive access to any mutable components.
One way to learn how to do all of this is to see how the language designers make classes immutable by reviewing the source for classes like String which are immutable (for example see http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/String.java).
Write a unit test that will fail if your coworkers make the class mutable.
Using Mutability Detector, you can write a test like this:
import static org.mutabilitydetector.unittesting.MutabilityAssert.assertImmutable;
#Test public void isImmutable() {
assertImmutable(MyImmutableThing.class)
}
If a coworker comes along, and, for example, adds a setter method to your class, the test will fail. Your use case is one of the core purposes of Mutability Detector.
Disclaimer: I wrote it.
I was asked this question in an interview recently:
Can you name any class in the Java API that is final that shouldn't be or one that isn't and should be'?
I couldn't think of any. The question implies that I should know all the API classes like the back of my hand, which I personally wouldn't expect any Java developer to know.
If anyone knows any such classes, please provide examples.
java.awt.Dimension isn't final or immutable and should have been. Anything that returns a Dimension (e.g a Window object) needs to make defensive copies to prevent callers from doing nasty things.
The first examples that come to mind are some of the non-final Number subclasses, such as BigDecimal and BigInteger, which should probably have been final.
In particular, all of their methods can be overriden. That enables you to create a broken BigDecimal, for example:
public class BrokenBigDecimal extends BigDecimal {
public BigDecimal add(BigDecimal augend) {
return BigDecimal.ZERO;
}
}
That could create significant issues if you receive BigDecimal from an untrusted code for example.
To paraphrase Effective Java:
Design and document for inheritance or else prohibit it
Classes should be immutable unless there's a very good reason to make them mutable
In my opinion, your reply should have been that it is a matter of taste which classes should be final and which shouldn't.
There are good reasons to make Integer, Double and String all final.
There are good reasons to complain about this.
Then there is BitSet, BitInteger etc. which could be made final.
There are a number of situations where classes are not final, but they also cannot be extended reasonably, so they probably should have been made final.
To pick on a particular class: BitSet. It is not final, yet you cannot extend it to add a bit shift operation. They might as well have made it final then, or allow us to add such functionality.
The Date class leaps out. It is a mutable simple value class (essentially a wrapper around a long), but a good heuristic is that simple value classes should be immutable. Note also its numerous deprecated methods: more evidence that the design was botched. The mutability of the Date is a source of bugs, requiring disciplined defensive copying.
one that isn't and should be
Most final classes in java are designed so due w/ security considerations in mind, overall there are relatively few final ones. For instance java.util.String is final for that very reason. So are many others.
Some classes w/ private c-tor are declared final (Math, StrictMath) but it doesn't matter in such a case.
Basically unless there are security issues involved I don't care if the class is final, yet you can always use non-public c-tor w/ some factory, effectively limiting the ability to subclass. Usually that's my preferred way as it allows package-private subclassing.
In short: I can't think of a final class that should not be, however there are some that could potentially have been. For instance java.lang.Thread being final might have not needed to protect vs malicious clone().
I believe java.util.Arrays and java.util.Collections should be declared final.
Here is why:
They contain only static members and a private constructor.
The private constructor prevents those classes from being extended.
So, those classes cannot be extended, but this fact is not visible in their public interface. Declaring them final would expose it and clarify intent.
Additionally, java.lang.Math (another so-called utility class) has the same structure and it is also declared final.
Check the String class which is final and probably should had been your answer in the interview.
Check the docs.
http://docs.oracle.com/javase/7/docs/api/java/lang/String.html
Could you please clarify that why final keyword is required before class when we are making it an immutable one.
I mean, if we declare all of it's attributes as private and final, then also it is an immutable class, isn't it?
Sorry if the question seems easy, but i am truly confused about it. Help me out.
Editted:
I know that a class declared final can't be subclassed.. But if each attribute is private and final then what difference does that make?
As stacker says, final makes sure the class isn't subclassed. That's important so that any code which is relying on its immutability can do so safely.
For example, immutable types (where each field is also of an immutable type) can be freely used between threads without worrying about data races etc. Now consider:
public class Person {
private final String name;
public Person(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
That looks like you can share Person instances freely across threads with no problem. But what about when the object you're sharing is actually a mutable subclass:
public class Employee extends Person {
private String company;
public Employee(String name, String company) {
super(name);
this.company = company;
}
public void setCompany(String company) {
this.company = company;
}
public String getCompany() {
return company;
}
}
Now instances of Employee aren't safe to share between threads, because they're not immutable. But the code doing the sharing may only know about them as instances of Person... leading them into a false sense of security.
The same goes for caching - it should be safe to cache and reuse immutable types, right? Well, it is safe to cache instances which are genuinely of an immutable type - but if you're dealing with a type which itself doesn't allow mutation, but does allow subclasses, it's suddenly not safe any more.
Think about java.lang.Object. It doesn't have any mutable fields, but it's clearly a bad idea to treat every Object reference as if it's a reference to an immutable type. Basically it depends on whether you think about immutability as a property of the type or of objects. A truly immutable type declares "any time you see a reference of this type, you can treat it as immutable" - whereas a type which allows arbitrary subclassing can't make that claim.
As an aside, there's a half-way house: if you can limit the subclassing to only "trusted" places, you can ensure that everything's immutable, but still allow that subclassing. The access in Java makes that tricky, but in C# for example you could have a public class which only allowed subclassing within the same assembly - giving a public API which is nice and strong in terms of immutability, while still allowing for the benefits of polymorphism.
A class that is declared final cannot be subclassed. See also http://docs.oracle.com/javase/tutorial/java/IandI/final.html
The different semantics of all uses of the final keyword are described in the The Java Language Specification
4.12.4 final Variables Page 80
8.1.1.2 final Classes Page 184
8.3.1.2 final Fields Page 209
8.4.3.3 final Methods Page 223
You don't strictly need final to make an immutable class. i.e. you can make an immutable class without it being final.
However, if you don't make it final, then it is possible for someone to extend a class and create a subclass that is mutable (either by adding new mutable fields, or overriding methods in a way that enables you to mutate protected fields of the original immutable class). This is a potential problem - it violates the Liskov Substitution Principle, in the sense that you would expect the property of immutablity to be preserved by all subtypes.
Hence, it is usually good practice to make immutable classes final to avoid this risk.
'final' as the keyword's name suggest means that the attribute to which final keyword is attached can't be changed(in terms of value) in other words it behaves like a constant.
As per your question if all members of the class is made private and final but the class is not made final then the same class can be inherited but the super class member are immutable as final keyword is attached to them.
An immutable object is an object which state is guaranteed to stay identical over its entire lifetime. While it is perfectly possible to implement immutability without final, its use makes that purpose explicit, to the human (the software developer) and the machine (the compiler).
Immutable objects carry some very desirable characteristics:
they are simple to understand and easy to use
they are inherently thread-safe: they require no synchronization
they make great building blocks for other objects
Clearly final is going to help us define immutable objects. First in labelling our object as immutable, which makes it simple to use and understand by other programmers. Second in guaranteeing that the object's state never changes, which enable the thread-safe property: thread concurrency issues are relevant when one thread can change data while another thread is reading the same data. Because an immutable object never changes its data, synchronizing access to it is not needed.
Create an immutable class by meeting all of the following conditions:
Declare all fields private final.
Set all fields in the constructor.
Don't provide any methods that modify the state of the object; provide only getter methods (no setters).
Declare the class final, so that no methods may be overridden.
Ensure exclusive access to any mutable components, e.g. by returning copies.
A class declared final cannot be sub classed. Other classes cannot extend final class. It provides some benefit to security and thread safety.
If all public and protected methods are final and none of them allows modifying private fields, and all public and protected fields are both final and immutable, then I guess it could be said class is semi-immutable, or sort of constant.
But things break down when you create a subclass and need to override equals and hashcode. And can not because you made them final... So the whole thing is broken, so just make the whole class final to prevent programmer from being a fool by accident.
As an alternative to doing this kind of bastardized version immutability, you have several options.
If you want to attach extra data to immutable instance, use Map. Like if you wanted to add age to name, you would not do class NameAge extends String... :-)
If you want to add methods, create a class of static utility functions. That is a bit klunky, but it is the current Java way, Apache commons for example is full of such classes.
If you want to add extra methods and data, create a wrapper class with delegate methods to methods of the immutable class. Anybody needing to use the extra methods needs to be aware of them anyway, and there is not much practical difference in casting to derived non-immutable class or doing something like new MyWrapper(myImmutableObj) for many use cases.
When you really have to have reference to original imutable object (like storing it in existing class you can not change), but need the extra data somewhere, you need to use the Map approach to keep the extra data around, or something like that.
If an immutable class Foo is sealed ("final"), then anyone who receives a reference to a Foo may be assured that if Foo was implemented correctly, the referenced instance will in fact be immutable. If an immutable class is not sealed, then someone who receives a reference to a Foo may be assured that if the actual class of of the referenced object (which may be Foo or some derivative type implemented by some arbitrary unknown person) was implemented correctly, the instance will be immutable. Leaving Foo unsealed means that anyone who relies upon Foo to be immutable will have to trust that everyone who writes a class that derives from Foo will implement it correctly. If one wants to be certain that every reference to a Foo will in fact target an immutable instance without having to rely upon the authors of derivative classes to abide by the contract, making Foo final can aid in such assurance.
On the other hand, the possibility that a class might derive from Foo but violate its immutability isn't terribly different from the possibility that a class which derives from any other class might violate the contracts of its parent class. Any code which accepts a reference of any type which can be subclasssed by outside code might be given an instance of a subclass which violates its parent's contract.
The fundamental question when deciding whether an immutable class should be sealed is the same as for any other class: whether the benefits of leaving the type unsealed outweigh any dangers that would be posed by doing so. In some cases, it may make sense to have an extensible immutable class, or even an abstract class or interface whose concrete implementations are all contractually obligated to be immutable; for example, a drawing package might have an ImmutableShape class with some concrete fields, properties, and methods to define 2D transformations, but an abstract Draw method, allowing for the definition of derivative types ImmutablePolygon, ImmutableTextObject, ImmutableBezierCurve, etc. If someone implements an ImmutableGradientFilledEllipse class but fails to have that type make its own copy of a mutable GradientColorSelector, the colors of gradient-filled polygons might change unexpectedly, but that would be a fault of the ImmutableGradientFilledEllipse class, and not the consuming code. Despite the possibility of a broken implementation failing to uphold the "immutability" contract, an extensible ImmutableShape class would be much more versatile than a sealed one.
I want to make sure that a given group of objects is immutable.
I was thinking about something along the lines of:
check if every field is private final
check if class is final
check for mutable members
So I guess my question is: is 3. possible ?
I can check recursively whether every member of a class has its fields private final, but this is not enough since a class can have e method named getHaha(param) which adds the given param to an array for instance.
So is there a good way to check if an object is immutable or is it even possible ?
Thanks,
You may want to check out this project:
Mutability Detector
This library attempts to analyse the bytecode of a particular class, to discover if it is immutable or not. It allows testing for this condition in a unit test, as demonstrated in a video available here. It is certainly not perfect (a String field will be considered mutable, and your array example is not handled well) but it's more sophisticated than what FindBugs offers (i.e. only checking that every field is final).
Disclaimer: I wrote it ;-)
If you generate your data model and all its code, you can ensure the possible Data Value objects you create will be immutable to meet your needs.
The problem you have is that there is different forms of immutability. Even String would fail your test Are String, Date, Method immutable? You can prove that a class is strictly immutable this way, but you are likely to be better off generating your data model.
Yes, you can write an immutability detector.
First of all, you are not going to be just writing a method which determines whether a class is immutable; instead, you will need to write an immutability detector class, because it is going to have to maintain some state. The state of the detector will be the detected immutability of all classes which it has examined so far. This is not only useful for performance, but it is actually necessary because a class may contain a circular reference, which would cause a simplistic immutability detector to fall into infinite recursion.
The immutability of a class has four possible values: Unknown, Mutable, Immutable, and Calculating. You will probably want to have a map which associates each class that you have encountered so far to an immutability value. Of course, Unknown does not actually need to be implemented, since it will be the implied state of any class which is not yet in the map.
So, when you begin examining a class, you associate it with a Calculating value in the map, and when you are done, you replace Calculating with either Immutable or Mutable.
For each class, you only need to check the field members, not the code. The idea of checking bytecode is rather misguided.
First of all, you should not check whether a class is final; The finality of a class does not affect its immutability. Instead, a method which expects an immutable parameter should first of all invoke the immutability detector to assert the immutability of the class of the actual object that was passed. This test can be omitted if the type of the parameter is a final class, so finality is good for performance, but strictly speaking not necessary. Also, as you will see further down, a field whose type is of a non-final class will cause the declaring class to be considered as mutable, but still, that's a problem of the declaring class, not the problem of the non-final immutable member class. It is perfectly fine to have a tall hierarchy of immutable classes, in which all the non-leaf nodes must of course be non-final.
You should not check whether a field is private; it is perfectly fine for a class to have a public field, and the visibility of the field does not affect the immutability of the declaring class in any way, shape, or form. You only need to check whether the field is final and its type is immutable.
When examining a class, what you want to do first of all is to recurse to determine the immutability of its super class. If the super is mutable, then the descendant is by definition mutable too.
Then, you only need to check the declared fields of the class, not all fields.
If a field is non-final, then your class is mutable.
If a field is final, but the type of the field is mutable, then your class is mutable. (Arrays are by definition mutable.)
If a field is final, and the type of the field is Calculating, then ignore it and proceed to the next field. If all fields are either immutable or Calculating, then your class is immutable.
If the type of the field is an interface, or an abstract class, or a non-final class, then it is to be considered as mutable, since you have absolutely no control over what the actual implementation may do. This might seem like an insurmountable problem, because it means that wrapping a modifiable collection inside an UnmodifiableCollection will still fail the immutability test, but it is actually fine, and it can be handled with the following workaround.
Some classes may contain non-final fields and still be effectively immutable. An example of this is the String class. Other classes which fall into this category are classes which contain non-final members purely for performance monitoring purposes (invocation counters, etc.), classes which implement popsicle immutability (look it up), and classes which contain members that are interfaces which are known to not cause any side effects. Also, if a class contains bona fide mutable fields but promises not to take them into account when computing hashCode() and equals(), then the class is of course unsafe when it comes to multi-threading, but it can still be considered as immutable for the purpose of using it as a key in a map. So, all these cases can be handled in one of two ways:
Manually adding classes (and interfaces) to your immutability detector. If you know that a certain class is effectively immutable despite the fact that the immutability test for it fails, you can manually add an entry to your detector which associates it with Immutable. This way, the detector will never attempt to check whether it is immutable, it will always just say 'yes, it is.'
Introducing an #ImmutabilityOverride annotation. Your immutability detector can check for the presence of this annotation on a field, and if present, it may treat the field as immutable despite the fact that the field may be non-final or its type may be mutable. The detector may also check for the presence of this annotation on the class, thus treating the class as immutable without even bothering to check its fields.
I hope this helps future generations.
I doubt you can do this with unit tests. The best way would be to be careful during writing the class or looking into the code. Precisely because of the problem that methods on the object can mutate its state which you might not see from the outside. Just because it's discouraged doesn't mean it doesn't happen :-)
Pretty sure it is impossible. Consider this function:
public void doSomething() {
if (System.currentTimeMillis() % 100000 == 0) {
this.innerMember.changeState();
}
}
First, you won't be able to detect it by running every class function, as this function changes the state of object precisely only once in 100 seconds.
Second, you won't be able to detect it by parsing code, as you do not know if changeState() function changes the state of innerMember or not.
This thread can help How do I identify immutable objects in Java. Take a look at the second popular answer, it might be possible to check for any immutability problems with FindBugs. If you run it on every commit then you can call it a unit test :)
EDIT
It seems that FindBugs only check for final, that's not much. You could implement your own rule according to you patterns and classes which you use in the code.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Java: Rationale of the Object class not being declared abstract
Why is the Object class, which is base class of 'em all in Java, not abstract?
I've had this question for a really really long time and it is asked here purely out of curiosity, that's all. Nothing in my code or anybody's code is breaking because it is not abstract, but I was wondering why they made it concrete?
Why would anyone want an "instance" (and not its presence a.k.a. Reference) of this Object class? One case is a poor synchronization code which uses the instance of an Object for locking (at least I used it this way once.. my bad).
Is there any practical use of an "instance" of an Object class? And how does its instantiation fit in OOP? What would have happened if they had marked it abstract (of course after providing implementations to its methods)?
Without the designers of java.lang.Object telling us, we have to base our answers on opinion. There's a few questions which can be asked which may help clear it up.
Would any of the methods of Object benefit from being abstract?
It could be argued that some of the methods would benefit from this. Take hashCode() and equals() for instance, there would probably have been a lot less frustration around the complexities of these two if they had both been made abstract. This would require developers to figure out how they should be implementing them, making it more obvious that they should be consistent (see Effective Java). However, I'm more of the opinion that hashCode(), equals() and clone() belong on separate, opt-in abstractions (i.e. interfaces). The other methods, wait(), notify(), finalize(), etc. are sufficiently complicated and/or are native, so it's best they're already implemented, and would not benefit from being abstracted.
So I'd guess the answer would be no, none of the methods of Object would benefit from being abstract.
Would it be a benefit to mark the Object class as abstract?
Assuming all the methods are implemented, the only effect of marking Object abstract is that it cannot be constructed (i.e. new Object() is a compile error). Would this have a benefit? I'm of the opinion that the term "object" is itself abstract (can you find anything around you which can be totally described as "an object"?), so it would fit with the object-oriented paradigm. It is however, on the purist side. It could be argued that forcing developers to pick a name for any concrete subclass, even empty ones, will result in code which better expresses their intent. I think, to be totally correct in terms of the paradigm, Object should be marked abstract, but when it comes down to it, there's no real benefit, it's a matter of design preference (pragmatism vs. purity).
Is the practice of using a plain Object for synchronisation a good enough reason for it to be concrete?
Many of the other answers talk about constructing a plain object to use in the synchronized() operation. While this may have been a common and accepted practice, I don't believe it would be a good enough reason to prevent Object being abstract if the designers wanted it to be. Other answers have mentioned how we would have to declare a single, empty subclass of Object any time we wanted to synchronise on a certain object, but this doesn't stand up - an empty subclass could have been provided in the SDK (java.lang.Lock or whatever), which could be constructed any time we wanted to synchronise. Doing this would have the added benefit of creating a stronger statement of intent.
Are there any other factors which could have been adversely affected by making Object abstract?
There are several areas, separate from a pure design standpoint, which may have influenced the choice. Unfortunately, I do not know enough about them to expand on them. However, it would not suprise me if any of these had an impact on the decision:
Performance
Security
Simplicity of implementation of the JVM
Could there be other reasons?
It's been mentioned that it may be in relation to reflection. However, reflection was introduced after Object was designed. So whether it affects reflection or not is moot - it's not the reason. The same for generics.
There's also the unforgettable point that java.lang.Object was designed by humans: they may have made a mistake, they may not have considered the question. There is no language without flaws, and this may be one of them, but if it is, it's hardly a big one. And I think I can safely say, without lack of ambition, that I'm very unlikely to be involved in designing a key part of such a widely used technology, especially one that's lasted 15(?) years and still going strong, so this shouldn't be considered a criticism.
Having said that, I would have made it abstract ;-p
Summary
Basically, as far as I see it, the answer to both questions "Why is java.lang.Object concrete?" or (if it were so) "Why is java.lang.Object abstract?" is... "Why not?".
Plain instances of java.lang.Object are typically used in locking/syncronization scenarios and that's accepted practice.
Also - what would be the reason for it to be abstract? Because it's not fully functional in its own right as an instance? Could it really do with some abstract members? Don't think so. So the argument for making it abstract in the first place is non-existent. So it isn't.
Take the classic hierarchy of animals, where you have an abstract class Animal, the reasoning to make the Animal class abstract is because an instance of Animal is effectively an 'invalid' -by lack of a better word- animal (even if all its methods provide a base implementation). With Object, that is simply not the case. There is no overwhelming case to make it abstract in the first place.
From everything I've read, it seems that Object does not need to be concrete, and in fact should have been abstract.
Not only is there no need for it to be concrete, but after some more reading I am convinced that Object not being abstract is in conflict with the basic inheritance model - we should not be allowing abstract subclasses of a concrete class, since subclasses should only add functionality.
Clearly this is not the case in Java, where we have abstract subclasses of Object.
I can think of several cases where instances of Object are useful:
Locking and synchronization, like you and other commenters mention. It is probably a code smell, but I have seen Object instances used this way all the time.
As Null Objects, because equals will always return false, except on the instance itself.
In test code, especially when testing collection classes. Sometimes it's easiest to fill a collection or array with dummy objects rather than nulls.
As the base instance for anonymous classes. For example:
Object o = new Object() {...code here...}
I think it probably should have been declared abstract, but once it is done and released it is very hard to undo without causing a lot of pain - see Java Language Spec 13.4.1:
"If a class that was not abstract is changed to be declared abstract, then preexisting binaries that attempt to create new instances of that class will throw either an InstantiationError at link time, or (if a reflective method is used) an InstantiationException at run time; such a change is therefore not recommended for widely distributed classes."
From time to time you need a plain Object that has no state of its own. Although such objects seem useless at first sight, they still have utility since each one has different identity. Tnis is useful in several scenarios, most important of which is locking: You want to coordinate two threads. In Java you do that by using an object that will be used as a lock. The object need not have any state its mere existence is enough for it to become a lock:
class MyThread extends Thread {
private Object lock;
public MyThread(Object l) { lock = l; }
public void run() {
doSomething();
synchronized(lock) {
doSomethingElse();
}
}
}
Object lock = new Object();
new MyThread(lock).start();
new MyThread(lock).start();
In this example we used a lock to prevent the two threads from concurrently executing doSomethingElse()
If Object were abstract and we needed a lock we'd have to subclass it without adding any method nor fields just so that we can instantiate lock.
Coming to think about it, here's a dual question to yours: Suppose Object were abstract, will it define any abstract methods? I guess the answer is No. In such circumstances there is not much value to defining the class as abstract.
I don't understand why most seem to believe that making a fully functional class, which implements all of its methods in a use full way abstract would be a good idea.
I would rather ask why make it abstract? Does it do something it shouldn't? is it missing some functionality it should have? Both those questions can be answered with no, it is a fully working class on its own, making it abstract just leads to people implementing empty classes.
public class UseableObject extends AbstractObject{}
UseableObject inherits from abstract Object and surprise it can be implemented, it does not add any functionality and its only reason to exist is to allow access to the methods exposed by Object.
Also I have to disagree with the use in "poor" synchronisation. Using private Objects to synchronize access is safer than using synchronize(this) and safer as well as easier to use than the Lock classes from java util concurrent.
Seems to me there's a simple question of practicality here. Making a class abstract takes away the programmer's ability to do something, namely, to instantiate it. There is nothing you can do with an abstract class that you cannot do with a concrete class. (Well, you can declare abstract functions in it, but in this case we have no need to have abstract functions.) So by making it concrete, you make it more flexible.
Of course if there was some active harm that was done by making it concrete, that "flexibility" would be a drawback. But I can't think of any active harm done by making Object instantiable. (Is "instantiable" a word? Whatever.) We could debate whether any given use that someone has made of a raw Object instance is a good idea. But even if you could convince me that every use that I have ever seen of a raw Object instance was a bad idea, that still wouldn't prove that there might not be good uses out there. So if it doesn't hurt anything, and it might help, even if we can't think of a way that it would actually help at the moment, why prohibit it?
I think all of the answers so far forget what it was like with Java 1.0. In Java 1.0, you could not make an anonymous class, so if you just wanted an object for some purpose (synchronization or a null placeholder) you would have to go declare a class for that purpose, and then a whole bunch of code would have these extra classes for this purpose. Much more straight forward to just allow direct instantiation of Object.
Sure, if you were designing Java today you might say that everyone should do:
Object NULL_OBJECT = new Object(){};
But that was not an option in 1.0.
I suspect the designers did not know in which way people may use an Object may be used in the future, and therefore didn't want to limit programmers by enforcing them to create an additional class where not necessary, eg for things like mutexes, keys etc.
It also means that it can be instantiated in an array. In the pre-1.5 days, this would allow you to have generic data structures. This could still be true on some platforms (I'm thinking J2ME, but I'm not sure)
Reasons why Object needs to be concrete.
reflection
see Object.getClass()
generic use (pre Java 5)
comparison/output
see Object.toString(), Object.equals(), Object.hashCode(), etc.
syncronization
see Object.wait(), Object.notify(), etc.
Even though a couple of areas have been replaced/deprecated, there was still a need for a concrete parent class to provide these features to every Java class.
The Object class is used in reflection so code can call methods on instances of indeterminate type, i.e. 'Object.class.getDeclaredMethods()'. If Object were to be Abstract then code that wanted to participate would have to implement all abstract methods before client code could use reflection on them.
According to Sun, An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed. This also means you can't call methods or access public fields of an abstract class.
Example of an abstract root class:
abstract public class AbstractBaseClass
{
public Class clazz;
public AbstractBaseClass(Class clazz)
{
super();
this.clazz = clazz;
}
}
A child of our AbstractBaseClass:
public class ReflectedClass extends AbstractBaseClass
{
public ReflectedClass()
{
super(this);
}
public static void main(String[] args)
{
ReflectedClass me = new ReflectedClass();
}
}
This will not compile because it's invalid to reference 'this' in a constructor unless its to call another constructor in the same class. I can get it to compile if I change it to:
public ReflectedClass()
{
super(ReflectedClass.class);
}
but that only works because ReflectedClass has a parent ("Object") which is 1) concrete and 2) has a field to store the type for its children.
A example more typical of reflection would be in a non-static member function:
public void foo()
{
Class localClass = AbstractBaseClass.clazz;
}
This fails unless you change the field 'clazz' to be static. For the class field of Object this wouldn't work because it is supposed to be instance specific. It would make no sense for Object to have a static class field.
Now, I did try the following change and it works but is a bit misleading. It still requires the base class to be extended to work.
public void genericPrint(AbstractBaseClass c)
{
Class localClass = c.clazz;
System.out.println("Class is: " + localClass);
}
public static void main(String[] args)
{
ReflectedClass me = new ReflectedClass();
ReflectedClass meTwo = new ReflectedClass();
me.genericPrint(meTwo);
}
Pre-Java5 generics (like with arrays) would have been impossible
Object[] array = new Object[100];
array[0] = me;
array[1] = meTwo;
Instances need to be constructed to serve as placeholders until the actual objects are received.
I suspect the short answer is that the collection classes lost type information in the days before Java generics. If a collection is not generic, then it must return a concrete Object (and be downcast at runtime to whatever type it was previously).
Since making a concrete class into an abstract class would break binary compatibility (as noted upthread), the concrete Object class was kept. I would like to point out that in no case was it created for the sole purpose of sychronization; dummy classes work just as well.
The design flaw is not including generics from the beginning. A lot of design criticism is aimed at that decision and its consequences. [oh, and the array subtyping rule.]
Its not abstract because whenever we create a new class it extends Object class then if it was abstract you need to implement all the methods of Object class which is overhead... There are already methods implemented in that class...