Deep Dive into Java: What\'s the Difference Between Shallow Copy, Deep Copy, and Clone?

Understanding Deep Copy, Shallow Copy, and Clone in Java

The concept of copying an object often involves misunderstandings when discussing "deep copy," "shallow copy," and "clone." In the Java context, it's crucial to distinguish between "copying a value" and "copying an object."

Shallow Copy vs. Deep Copy

In a shallow copy of an object, only the first level of the object is copied. For example:

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

In this case, eg2 would have its foo set to 1 and bar pointing to the same array as eg1. A deep copy, on the other hand, would create a copy of the entire object graph, including any nested child objects:

<code class="java">Example eg2 = new Example(eg1.foo, Arrays.copy(eg1.bar));  // Deep copy</code>

However, the exact depth of a copy in Java remains ambiguous, as it doesn't provide a clear definition of what constitutes a "level."

Clone Method

The clone method exists in all Java classes and aims to create a copy of the target object. Nonetheless, its semantics are ambiguous:

• The Java API doesn't specify whether clone produces a shallow or deep copy.
• It doesn't even guarantee that clone will create a new object.

The javadoc for clone states that it may return the original object, or the copy may not be equal to the original. Additionally, the behavior of clone varies across different Java classes, making it difficult to derive a consistent model for all object types.

Conclusion

While the terms "shallow copy," "deep copy," and "clone" are often used in Java, their precise meaning can vary significantly. Shallow copies duplicate the top level of an object, while deep copies delve further, but the concept of "depth" remains subjective. The clone method also lacks a clear definition, potentially leading to unforeseen issues when used across different classes. Developers should carefully consider the desired copying semantics when dealing with Java objects.

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