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Java Forum / First Aid / March 2004copying a class
Is there an easy way to completely copy a class? say I have; FOO A; FOO B; and I do: A = new FOO(); // do some changin of member vars now I want to get a copy of A into B, and be able to modify A but without changing B. It seems that using 1) B = A means B and A point to the same object 2) B = A.clone() means that A and B are different objects but their member vars (e.g. arrays etc.) still point to the same so that if I modify A.array[0] then B.array[0] is also modified. I could over-ride clone() and copy over each variable manually but I thought there may be an easier way? If not, then are there easy ways to copy Hashtables and arrays? Thanks Allan > Is there an easy way to completely copy a class? say I have; > [quoted text clipped - 19 lines] > > If not, then are there easy ways to copy Hashtables and arrays? clone() is the usual way to copy an object, and the reason you need to override it is that different classes will need to have different ideas about what a "copy" is. You've already noticed this in point 2 above: a "shallow copy" is one perfectly reasonable kind of a copy operation, but others can be usefully defined. You define the kind of copy you want by implementing an appropriate clone(). If you want to copy a HashTable, your first task is to decide what you mean by a "copy." One possibility is that you want a new HashTable with the same content as the old; that's a "shallow copy." Another possibility is that you want a new HashTable containing copies of all the content of the old; that's a "deep copy." But even within "deep copy" there are further flavors, because you need to decide what kind of copy to apply to the contained objects: are they in turn to be copied shallowly or deeply? If one of the contained objects is a String, say, do you really need a brand-new String containing the same characters, or will a reference to the original (immutable) String suffice? What if one of the contained objects is a singleton; are you really going to (try to) make a second copy of it? IMHO, the need to make copies is usually -- not always, but usually -- a sign that the design could be improved. Speaking very loosely, the amount of information in your universe does not increase when you make photocopies. If the goal of your application is (in some vague sense) to develop information, the act of copying does not in and of itself do so.  SignatureEric.Sosman@sun.com > > Is there an easy way to completely copy a class? say I have; > > [quoted text clipped - 48 lines] > application is (in some vague sense) to develop information, > the act of copying does not in and of itself do so. Thanks this helps a lot. One little thing. How do I make sure I have a deep copy of an int and String? Does this work? String a = "Hello World\n"; String b; int c = 100; int d; b = a; // does this copy the string? d = c; // i assume this is a copy rather than d being a reference to c Thanks Allan > Thanks this helps a lot. One little thing. How do I make sure I have a > deep copy of an int and String? Does this work? [quoted text clipped - 5 lines] > > b = a; // does this copy the string? No. `a' and `b' are references, and both now refer to the same String object. > d = c; // i assume this is a copy rather than d being a reference to c Yes. `c' and `d' are primitives, not references, so each occurrence has its own unique identity. There is no underlying Object for an `int' (some purists think there ought to be, that Java should have been defined differently, but that's the way things are). I find it helpful to think of "reference" as a primitive type, along with char and boolean and the rest. In this world view, `a' is not a String at all; it's a String *reference* whose value may, from time to time, be changed so it refers to various different String objects. Saying "`a' is a String" is a convenient shorthand, nothing more. In the True Understanding `a' is a variable that can store a primitive of the "reference to String" type, and the String objects themselves are nameless. Bad analogy: The reference is your street adress, and the Object is your house. You can publish your address in any number of directories and other places without building a new house every time you do so. You can burn one of those listings without burning down your house. The reference and the thing referred to are different entities. SignatureEric.Sosman@sun.com > > Thanks this helps a lot. One little thing. How do I make sure I have a > > deep copy of an int and String? Does this work? [quoted text clipped - 8 lines] > No. `a' and `b' are references, and both now refer > to the same String object. so to make a copy, I need to do this: b = new String(a); Is that correct? Thanks Allan > > > Thanks this helps a lot. One little thing. How do I make sure I have a > > > deep copy of an int and String? Does this work? [quoted text clipped - 14 lines] > > Is that correct? Yes. But to return to the question in my first response, why do you want to make this second and redundant copy of "Hello World\n"? The world will not feel twice as welcomed because you've said the same thing twice ... SignatureEric.Sosman@sun.com > > > > Thanks this helps a lot. One little thing. How do I make sure I have a > > > > deep copy of an int and String? Does this work? [quoted text clipped - 19 lines] > "Hello World\n"? The world will not feel twice as welcomed > because you've said the same thing twice ... I do have my reasons. I`m developing a qualitative reasoning inference engine and I require to generate a copy of all the variables into a state. Perhaps I dont need the variable names to be a copy, but everything else does need to be a deep copy. Thanks for your help Allan > I do have my reasons. I`m developing a qualitative reasoning inference > engine and I require to generate a copy of all the variables into a state. > Perhaps I dont need the variable names to be a copy, but everything else > does need to be a deep copy. > Thanks for your help > Allan Aha! I had to solve a very similar problem last month. I implemented Serialisable on my data classes, and simply persisted the object using Serialisation: ByteArrayOutputStream baos = new ByteArrayOutputStream() ObjectOutputStream oos = new ObjectOutputStream( // write whole objects (recursively) new ZipOutputStream( // and compress them (I needed to) baos // into a byte array ) ); oos.writeObject(myData); // process the object byte[] persistedData = baos.toByteArray(); // and recover the data This seems to robustly handle deep copies of all data types automatically. Should you, for some reason, need a shallower copy (I suppose you should not, if your data classes are properly encapsulated), then use 'transient' on members that are not to be persisted, and provide a void readObject(ObjectInputStream) method to restore the value. 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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