Deep Clone Collection Objects in Java [Snippets]

Clone an Object

For example, if I want to have a deep copy of the Dog object, then I would use constructor as:

class Dog {
    private String name;
    private int age;

    public Dog(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }

    //copy constructor to create the copy of the Dog object
    public Dog(Dog dog) {
        this.name = dog.name;
        this.age = dog.age;
    }
    // getters and setters
}

Some of the notable advantages of using them over Objects.clone() are:

• This doesn't force us to implement any interface or throw an exception, but we can surely do it if it is required.
• This allows us to have complete control over object creation; we can write our initialization logic in it.

Clone a Collection

With new feature Streams introduced in Java 8, it has been the easiest ever to clone the collections, because when a stream is created, the instance will not modify its source. This allows the creation of multiple instances from a single source.     

For example, if the  List of Dogscan be cloned as below:

Dog dog1 = new Dog("Puppy", 4);
Dog dog2 = new Dog("Tom", 5);
Dog dog3 = new Dog("Hen", 3);
Dog dog4 = new Dog("Jen", 7);
List<Dog> dogs = new ArrayList<>();
dogs.add(dog1);
dogs.add(dog2);
dogs.add(dog3);
dogs.add(dog4);

//clone with java 8
List<Dog> clonedList = dogs.stream().map(Dog::new).collect(Collectors.toList());

Now, any modification you make to the original collection doesn't affect the cloned collection and vice-versa. 

//clone with java 8
List<Dog> clonedList = dogs.stream().map(Dog::new).collect(Collectors.toList());

assertThat(dogs, is(clonedList));

// modify original list
dogs.remove(0);
// make sure cloned list remains unaffected
assertEquals(3, dogs.size());
assertEquals(4, clonedList.size());

// modify cloned list
clonedList.add(new Dog("New", 3));
// make sure original list remains unaffected
assertEquals(3, dogs.size());
assertEquals(5, clonedList.size());

// modify element in dogs
dog2.setName("Very New Name");

assertEquals("Very New Name", dogs.get(0).getName());
assertEquals("Tom", clonedList.get(1).getName());

The source code for the example presented above is available on GitHub.