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I have an array and i want to create an independent copy of it. Changes in one of those should not impact the other at all.

Currently everything i tried just created two references on one object. The default clone() method did not work aswell.

JButton[][] old = game.getFields().clone(); // getFields() returns a two dimensional array
schtever
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M.Dietz
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2 Answers2

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Method to use for Copy of an array is mainly dependent upon the exact scenario. As long as we’re using a primitive type array, we can use any of the methods offered by the System and Arrays classes.

Using System Class

int[] array = {23, 43, 55};
int[] copiedArray = new int[3];
System.arraycopy(array, 0, copiedArray, 0, 3);

Using Arrays Class

int[] array = {23, 43, 55, 12};
int newLength = array.length;
int[] copiedArray = Arrays.copyOf(array, newLength);

For non-primitive types,we need to do a deep copy of an array.

For deep copy it already answered: How do you make a deep copy of an object in Java?

NullPointer
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Suppose you have the object object. The default clone method will create a new instance of object, i.e. object != object.clone(); but the references to other mutable objects therein will not be new instances as well. The new instance of object will not be independent in the sense that its child objects can be modified without affecting the child objects of the original. For primitive types, however, it works just fine because they are passed as values rather than references when constructing the clone. This is called a "shallow copy."

For a "deep copy," you need to actually create new instances of every mutable child object you wish to be able to manipulate independently of the original object. You could write some code to do this, or look at deep copy APIs.

To write your own solution, you'd be looking at creating a method that achieves the same thing as a "copy constructor" - a constructor which takes the same object type as a parameter and uses its values (not references) to construct its own instance. This can be difficult if you don't own the object though, due to method and field security. The clone() approach is no good since you don't own the object, and it'd function the same as writing a custom copy method anyway.

Your best bet would be to serialise and deserialise it, and there are already some powerful APIs for this. Take a look at Google's GSON API. Here's a quick link found via Google that would be worth a read: http://www.baeldung.com/java-deep-copy

Oracle on .clone(): https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html#clone()

Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object. The general intent is that, for any object x, the expression:

 x.clone() != x

will be true, and that the expression:

 x.clone().getClass() == x.getClass()

will be true, but these are not absolute requirements. While it is typically the case that:

 x.clone().equals(x)

will be true, this is not an absolute requirement.

By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == x.getClass().

By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.

The method clone for class Object performs a specific cloning operation. First, if the class of this object does not implement the interface Cloneable, then a CloneNotSupportedException is thrown. Note that all arrays are considered to implement the interface Cloneable and that the return type of the clone method of an array type T[] is T[] where T is any reference or primitive type. Otherwise, this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.

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