5

I've recently moved to Java, but I've had some problems with variable aliasing. I've searched everywhere, but I can't seem to find the proper way to copy the contents of one object to another object without just referencing to the same object. Does anyone have any suggestions?

Edit: What if it is an int that I am having aliasing problems with? Should I be avoiding situations like this in the first place? If so, how?

Improve this question
2
  • Can you post a line or two of what you mean in code? I'm not 100% sure what you're trying to say with "copy the contents of one object to another object without just referencing to the same object". Commented Apr 25, 2009 at 19:51
  • What language uses the term alias? I would think two references to the same object as aliases. However, a deep clone of an object provides a fully new object which really doesn't reference the first object-- so how is it an alias? Commented Apr 25, 2009 at 19:57

5 Answers 5

Reset to default
11

If your class implements the Clonable interface, then you can use the Object.clone() method to create a hard copy. The Wikipedia entry has some good details.

An alternative is to use copy constructors, which according to this page are safer.

Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Just as useful as mihi's answer, but I liked all the useful links on this one. The links give me ways to find out more information if needed. Anyways, I implemented some sort of copy constructor.
clone is very dangerous. It does a "bitwise" copy, which is almost never what you want. Whilst you can, to some extent, insert correcting code you are likely to quietly mess it up from time to time. It also doesn't play well with inheritance.
4

It depends on the "content". For example, you cannot just copy a FileInputStream and then assume that both will continue loading from the same file.

Basically, there are two ways: If the class supports "Cloneable" interface, you can clone it by calling clone(). If not, it often has a copy constructor, that copies in data from another object.

Usually, you will end up with a shallow copy (i. e. all fields of the class are copied, but they point to the same object).

On the other hand, a lot of objects are designed to be immutable (like the String class), and there is no need to copy such an object as it cannot be changed anyway.

Improve this answer

1 Comment

I suspect you can subclass FileInputStream and implement Cloneable, the clone will work just fine. Perhaps a bit strange, but I think it'll actually give you two stream object onto the same FileDescriptor (not checked the source recently).
2

Another option is to design your class to create immutable objects:
http://docs.oracle.com/javase/tutorial/essential/concurrency/immutable.html

This avoids the need for cloning or a copy-constructor because the object cannot be changed once it is created. So multiple variables can point to the same object, but none of them can change the state of the object.

Improve this answer

Comments

1

java.lang.Cloneable is what you are looking for.

Improve this answer

Comments

1

You cannot have an implicit reference to a reference in Java so you cannot alias a variable.

Perhaps if you explain what you are trying to achieve, we can help do that without "aliases"

Edit: You really need to explain what you mean by aliasing an int value. An int value is anonymous at runtime so aliasing it doesn't make any sense.

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.