Deep Copy vs. Shallow Copy vs. Clone: Which Java Copy Mechanism Should You Use?

Deep Copy, Shallow Copy, and Clone in Java

When manipulating objects in Java, understanding the nuances of copy semantics is crucial. Deep copy, shallow copy, and clone are commonly used terms to describe different approaches to object duplication.

Shallow Copy

A shallow copy duplicates the outermost level of an object. The new object has separate fields, but any references held within those fields are shared with the original object.

Example:

<code class="java">Example eg1 = new Example(1, new int[]{1, 2});
Example eg2 = new Example(eg1.foo, eg1.bar);</code>

In this shallow copy, eg2 has a new foo and reference to the same bar array as eg1.

Deep Copy

A deep copy traverses multiple levels of an object, creating new copies of all nested objects. The new object is completely independent of the original.

Example:

<code class="java">Example eg1 = new Example(1, new int[]{1, 2});
Example eg2 = new Example(eg1.foo, Arrays.copy(eg1.bar));</code>

In this deep copy, eg2 has a new foo and a new array bar, which is a copy of the one in eg1.

Clone

The clone method, which exists for all objects and arrays, is intended to produce a copy. However, its behavior is not standardized:

• It may return a shallow or deep copy.
• It may not always create a new object.
• It may not guarantee object equality.

Conclusion

Deep copying is recommended when complete independence between objects is necessary. Shallow copying is suitable when object references are sufficient. The behavior of the clone method is inconsistent and should be used with caution.

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