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"Cloneables" Should Implement clone()

Medium
robustnessconventions

What is it?​

This practice addresses the use of the Cloneable interface in Java without overriding the clone() method. Implementing Cloneable without providing a custom clone() method may lead to unexpected behavior and violations of the Cloneable contract.

Why apply it?​

When a class claims to be Cloneable but does not override Object.clone(), it may not adhere to the expected semantics of a deep or shallow copy, potentially causing bugs and maintenance problems. The expected behavior when implementing Cloneable is that calling clone() will not throw a CloneNotSupportedException.

How to fix it?​

Override the clone() method in any class that implements Cloneable. Ensure that it calls super.clone() and properly handles copying, either shallow or deep, depending on the class's needs.

Examples​

Example 1:​

Negative​

A noncompliant example where the class implements Cloneable but does not override clone().

class Foo implements Cloneable {

  public int value;

  // Missing clone method
}

Example 2:​

Positive​

A compliant example where the class properly implements the clone() method, ensuring the object can be cloned safely and predictably.

class Foo implements Cloneable {

  public int value;

  @Override
  public Foo clone() {
    try {
      return (Foo) super.clone();
    } catch (CloneNotSupportedException e) {
      throw new AssertionError(); // Can't happen because Foo is Cloneable
    }
  }
}

Negative​

The negative example shows an incomplete or incorrect cloning implementation, which overlooks overriding clone() when implementing Cloneable.

import java.util.List;

class Entity implements Cloneable {

  public int id;
  public List<Entity> children;

  // Does not override clone method
}

Example 3:​

Positive​

This positive example demonstrates a class implementing Cloneable and performing additional copying logic in the clone() method to achieve a deep copy.

import java.util.List;
import java.util.ArrayList;

class Entity implements Cloneable {

  public int id;
  public List<Entity> children;

  @Override
  public Entity clone() {
    try {
      Entity clone = (Entity) super.clone();
      clone.children = new ArrayList<>(children.size());
      for (Entity child : children) {
        clone.children.add(child.clone());
      }
      return clone;
    } catch (CloneNotSupportedException e) {
      throw new AssertionError(); // Shouldn't happen
    }
  }
}