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Table of Contents
Understand the Default Behavior: clone() Does a Shallow Copy
Make a Deep Copy When Needed
Consider Alternatives When Cloning Gets Tricky
Watch Out for Common Gotchas
Home Java javaTutorial How to properly clone an object in Java?

How to properly clone an object in Java?

Jul 09, 2025 am 12:41 AM

Cloning in Java requires understanding shallow vs deep copying. 1. The default clone() method performs a shallow copy, duplicating the object but not its referenced objects, leading to shared references. 2. For deep copying, manually clone nested objects by overriding clone() in all related classes. 3. Alternatives like copy constructors, serialization, or libraries offer simpler solutions for complex cases. 4. Common issues include accessibility, handling immutable fields, and correctly copying collections. Modern projects often prefer copy constructors over clone().

How to properly clone an object in Java?

Cloning an object in Java might seem straightforward, but if you're not careful, you can end up with shared references or incomplete copies. The key is understanding the difference between shallow and deep copying and knowing when to use each.

How to properly clone an object in Java?

Understand the Default Behavior: clone() Does a Shallow Copy

Java provides the clone() method through the Object class. If you want to use it, your class must implement the Cloneable interface — otherwise, it throws a CloneNotSupportedException.

But here's the catch: by default, clone() only does a shallow copy. That means if your object contains references to other objects, those references are copied over, not the actual objects they point to.

How to properly clone an object in Java?

For example:

class Person implements Cloneable {
    String name;
    Address address; // another object

    public Person clone() {
        return (Person) super.clone();
    }
}

In this case, both the original and the cloned Person will point to the same Address object. Changes made via one will affect the other.

How to properly clone an object in Java?

So if you need a full copy, you have to handle nested objects yourself.


Make a Deep Copy When Needed

To do a true deep copy, you need to manually clone any referenced objects inside your clone() method.

Here’s how:

  • Make sure all nested classes also support cloning.
  • Override clone() for each nested class.
  • In the parent class, explicitly clone the nested objects.

Example:

class Person implements Cloneable {
    String name;
    Address address;

    public Person clone() {
        Person p = (Person) super.clone();
        p.address = this.address.clone(); // deep copy of Address
        return p;
    }
}

class Address implements Cloneable {
    String city;

    public Address clone() {
        return (Address) super.clone();
    }
}

This way, both the Person and its internal Address are properly duplicated.

Note: This approach works well for small object graphs. If your object has many nested or complex dependencies, consider alternatives like serialization or libraries.


Consider Alternatives When Cloning Gets Tricky

If implementing clone() becomes too tedious or error-prone, there are simpler ways:

  • Manual copy constructor: Create a constructor that takes an existing instance and copies its values. It gives you full control and avoids some pitfalls of clone().

    class Person {
        public Person(Person source) {
            this.name = source.name;
            this.address = new Address(source.address);
        }
    }
  • Serialization trick: If your class implements Serializable, you can serialize and deserialize the object to make a deep copy. Works well but may be slower.

  • Use libraries: Tools like Dozer, ModelMapper, or even Jackson can help with deep copying complex objects.


  • Watch Out for Common Gotchas

    • Forgetting to override clone() as public — otherwise, you’ll get accessibility issues.
    • Not handling immutable fields correctly — sometimes they don’t need cloning.
    • Using clone() on collections — remember to copy lists, maps, etc., manually unless they’re immutable.

    Also, keep in mind that the Object.clone() method is kind of outdated. Many modern Java projects avoid it entirely in favor of copy constructors or builders.


    All right, that’s the core of cloning in Java. It’s not rocket science, but you do need to know what’s going on under the hood.

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