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Java Forum / General / November 2005Ways to instantiate a Class
Hi all, Recently I learned to use "factory method" to instantiate a class. But I am still wondering if there are other ways to instantiate a class besides the stand using way of using "new" and way of using "factory method"? Thanks in advance! > <snip> Well, the factory pattern uses "new"...so I'm not sure what you mean... you can use reflection.  SignatureOf making better designs there is no end, and much refactoring wearies the body. > > <snip> > > Well, the factory pattern uses "new"...so I'm not sure what you mean... Ok, my fault. I am still a java newbie. I mean, the stand way instantiating a Clas s is like "ClassA a = new ClassA()", and the factory way is like "ClassA a = factory.getClassA()". Hope it is clear this time. > you can use reflection. > > -- > Of making better designs there is no end, > and much refactoring wearies the body. "Lian Liming" wrote... >> > <snip> >> [quoted text clipped - 4 lines] > factory way is like "ClassA a = factory.getClassA()". Hope it is clear > this time. It's still as Benji said, the factory method itself uses new, or reflection, so it's no "magic" behind the scenes. The method "getClass()" probably looks something like this: ... public ClassA getClassA() { ClassA newA = new ClassA(); // then it does something with the instance... return newA; } ... ...or it uses reflection... When you're executing... ClassA a = factory.getClassA(); ...it's not the sentence itself that instantiates the object, it's instantiated (with new or with reflection) somewhere inside the factory method. // Bjorn A > Recently I learned to use "factory method" to instantiate a class. >But I am still wondering if there are other ways to instantiate a class >besides the stand using way of using "new" and way of using "factory >method"? there is reflection newInstance. See http://mindprod.com/jgloss/classforname.html A Factory inside usually just does a new. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Roedy Green wrote On 11/01/05 02:22,: >> Recently I learned to use "factory method" to instantiate a class. >>But I am still wondering if there are other ways to instantiate a class >>besides the stand using way of using "new" and way of using "factory >>method"? > > there is reflection newInstance. ... which is just an obscure way to run a constructor. The constructor-less instantiation methods I can think of are clone() and readObject(). SignatureEric.Sosman@sun.com > Roedy Green wrote On 11/01/05 02:22,: >> [quoted text clipped - 8 lines] > The constructor-less instantiation methods I can think of > are clone() and readObject(). clone() is a user implemented create and copy operation, Object#clone() does absolutely nothing. So a call of clone() is just an obscure way to call some other method that performs the real instance creation. And with each object this could be a different one, 'new', reflection, whatever. Am I mislead somewhere? How does readObject() work? Does it simply dump the binary data to some memory and return a reference to this memory space? "Eike Preuss" wrote... >>>> Recently I learned to use "factory method" to instantiate >>>> a class. But I am still wondering if there are other ways [quoted text clipped - 4 lines] >> >> ... which is just an obscure way to run a constructor. But it is still another "way" to instantiate, than to use "new". >> The constructor-less instantiation methods I can think of >> are clone() and readObject(). [quoted text clipped - 7 lines] > How does readObject() work? Does it simply dump the binary data > to some memory and return a reference to this memory space? I became so curious about this, so I tried to read up on the subject. Not that I succeeded to find a definitive answer, but I think I got fairly close. "readObject" relies heavily on its JNI-implementation. It's not quite to "dump it into memory", but close enough to call it different from reflection in Java only. It also resolves references to other serialized objects, etc. To make it short, I would say that deserialization could be considered a third way of "instantiating" objects, though it semantically isn't to instantiate a "new" object, just to restore an "old" one... ;-) We're now up to three ways to "instantiate": - using the "new" keyword - through reflection - through deserialization Anyone up for a fourth... ;-) // Bjorn A On Wed, 02 Nov 2005 13:23:59 +0100, Eike Preuss <usenet@eikepreuss.REMOVE.de> wrote, quoted or indirectly quoted someone who said : >clone() is a user implemented create and copy operation, Object#clone() >does absolutely nothing. So a call of clone() is just an obscure way to >call some other method that performs the real instance creation. Object.clone creates a new object and copies all fields of this object into it in a bit by bit fashion. class Rabbit implements Cloneable { ... public Object clone() { try { return super.clone(); } catch ( CloneNotSupportedException e ) { return null; } } ... } // end Rabbit SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > Object.clone creates a new object and copies all fields of this object > into it in a bit by bit fashion. [quoted text clipped - 15 lines] > ... > } // end Rabbit Should have read the javadoc... thanks for the reminder! On Wed, 02 Nov 2005 13:23:59 +0100, Eike Preuss <usenet@eikepreuss.REMOVE.de> wrote, quoted or indirectly quoted someone who said : >How does readObject() work? It does not actually execute the constructor. At the JVM level it does a new without calling the constructor. They might have pulled that off by composing or patching a class file manually. I don't know how you could get that effect in Java itself. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > It does not actually execute the constructor. At the JVM level it does > a new without calling the constructor. They might have pulled that > off by composing or patching a class file manually. I don't know how > you could get that effect in Java itself. By having the constructor not assign any values to fields*, and add an init() method that does the initialization. * This of course includes assignments in declarations: int foo = 5; is the same as int foo; public void <init>() { foo = 5; } On 02 Nov 2005 16:51:33 +0100, Tor Iver Wilhelmsen <jadedgamer@hotmail.com> wrote, quoted or indirectly quoted someone who said : >By having the constructor not assign any values to fields*, and add an >init() method that does the initialization. [quoted text clipped - 10 lines] > foo = 5; >} readObject has to work with arbitrary objects. It can't go composing constructors or methods for them. It can't even count on a no-arg constructor existing. I spent a while looking at the code in java.util.ObjectInputStream. It is pretty hairy. readOrdinaryObject(boolean unshared) uses desc.newInstance() to create an empty shell of an object later filled in with the read. That puzzled me on two counts: 1. what if there is no no-arg constructor? 2. I distinctly read somewhere that readObject does NOT invoke the constructor. The explanation is in the javaDoc for ObjectStreamClass.newInstance: Creates a new instance of the represented class. If the class is externalizable, invokes its public no-arg constructor; otherwise, if the class is serializable, invokes the no-arg constructor of the first non-serializable superclass. Throws UnsupportedOperationException if his class descriptor is not associated with a class, if the associated class is non-serializable or if the appropriate no-arg constructor is inaccessible/unavailable. On read, the code in the outer layer constructors of the serialized objects does not get called, just the inner non-serializable layers, e.g. Object. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > clone() is a user implemented create and copy operation, Object#clone() > does absolutely nothing. Take a second look at the JavaDoc for Object.clone(), you'll find it does a lot more than you think. -- chris Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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