What is the Difference Between Shallow Copy and Deep Copy in Java?

When working with objects in Java, especially in real-time applications, understanding object copying is very important. One common interview question is:

What is the difference between Shallow Copy and Deep Copy?

Let’s understand this clearly with examples.

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🔹 What is a Shallow Copy?

A shallow copy creates a new object, but it copies the references of nested objects instead of creating new copies of them.

👉 That means both original and copied objects point to the same memory location for internal objects.

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🔹 Example of Shallow Copy

class Address {
    String city;

    Address(String city) {
        this.city = city;
    }
}

class Employee implements Cloneable {
    int id;
    Address address;

    Employee(int id, Address address) {
        this.id = id;
        this.address = address;
    }

    protected Object clone() throws CloneNotSupportedException {
        return super.clone();  // Shallow Copy
    }
}

What Happens?

• A new Employee object is created.

• But the Address object is shared.

• If you modify the address in one object, it affects the other.

👉 Both objects share the same nested object.

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🔹 What is a Deep Copy?

A deep copy creates a new object and also creates separate copies of all referenced objects.

👉 No shared references. Everything is copied independently.

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🔹 Example of Deep Copy

class Employee implements Cloneable {
    int id;
    Address address;

    Employee(int id, Address address) {
        this.id = id;
        this.address = address;
    }

    protected Object clone() throws CloneNotSupportedException {
        Address newAddress = new Address(this.address.city);
        return new Employee(this.id, newAddress);  // Deep Copy
    }
}

What Happens?

• New Employee object is created.

• New Address object is also created.

• Changes in one object do NOT affect the other.

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🔥 Key Differences

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🔹 Real-Time Example

Imagine a banking system:

Account account1 = new Account(balanceDetails);
Account account2 = account1.clone();

If it’s a shallow copy:

• Updating balance in one account may affect the other.

If it’s a deep copy:

• Each account works independently.

In enterprise applications, deep copy is usually preferred when dealing with complex object graphs.

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🔹 How to Implement Deep Copy in Java?

You can implement deep copy using:

✔ Overriding clone() method
✔ Copy constructor
✔ Serialization & Deserialization
✔ Manual copying of fields

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🔹 Interview Follow-Up Questions

Interviewers may ask:

• Does Object.clone() perform shallow or deep copy?

• How does cloning work internally?

• Difference between copy constructor and clone?

• Can we achieve deep copy using serialization?

• Why is deep copy safer?

Understanding this concept is very important for working with:

• DTO objects

• Hibernate entities

• Microservices communication

• Caching systems

• Large enterprise applications

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🎯 Final Summary

• Shallow Copy → Copies object, but shares references

• Deep Copy → Copies object and all nested objects

• Shallow copy is faster but risky

• Deep copy is safer but consumes more memory

Choosing between shallow and deep copy depends on your application requirements.

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