Creational_Prototype Pattern

✅

• create a new instance
• by copying an existing instance

• the existing instance has a clone() method

• the existing instance == new copied instance ➡️ false
• the existing instance.equals(new copied instance) ➡️ true

✅ Diagram

👍🏻

• when creating an instance is resource expensive
• 👀 connecting to database, connecting to network

👎🏻 If we did not have prototype pattern…

• cannot clone an existing instance

GithubIssue githubIssue1 = new GithubIssue(repository);

//before prototype
//have to create new instance with constructor
GithubIssue githubIssue2 = new GithubIssue(repository);
GithubIssue githubIssue3 = githubIssue.clone(); // This will not work❌

✅ clone() from Object class

• Java by default provides the clone() method from Object class

✔️ Cloneable interface

• the class we want to clone must implement Cloneable()

public class GithubIssue implements Cloneable{
    private int id;
    private String title;
    private GithubRepository repository;

    //then override clone()
    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    //and override equals()
    //bc cloned instance and existing instance should be equal
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof GithubIssue that)) return false;
        return id == that.id && Objects.equals(title, that.title) && Objects.equals(repository, that.repository);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, title, repository);
    }
}

✔️ Main class

• ❓ existing instance and cloned instance will have same memory? ❌
• ❓ existing instance and cloned instance will have same value? equals()? ⭕️

• ❓ existing instance and cloned instance will have same .getClass()? ⭕️

• ⚠️ Default clone() from Java is a swallow copy
• so, the Repository of existing instance and cloned instance are the same ⭕️

GithubIssue githubIssue1 = new GithubIssue(repository);

GithubIssue githubIssue3 = (GithubIssue) githubIssue1.clone();

// githubIssue1 == githubIssue3  => false
// githubIssue1.equals(githubIssue3) => true
// githubIssue1.getClass() == githubIssue3.getClass() => true
// githubIssue1.getRepository() == githubIssue3.getRepository() => true, shallow copy

Swallow copy 🆚 Deep copy

• In GithubIssue, it has Repository as its field
• in swallow copy, the cloned instance will have the same Repository as the existing instance
• if existing instance’s Repository changes,

• cloned instance’s Repository will also change

• In deep copy, cloned instance will have its own Repository
• Even if existing instance’s Repository changes,
• cloned instance’s Repository will NOT change

✅ Create my own clone() for deep copy

• If I want cloned instance to have a complete separate Repository
• should not use clone() from Java ❌
• Can create my own clone()

✔️ Create my own clone()

• This way, we can have deep copy

public class GithubIssue implements Cloneable{
    private int id;
    private String title;
    private GithubRepository repository;

    //create new repository
    //and return the newly created instance
    @Override
    protected Object clone() throws CloneNotSupportedException {
        GithubRepository repository = new GithubRepository(); //create new repository
        repository.setUser(this.repository.getUser());
        repository.setName(this.repository.getName());

        GithubIssue issue = new GithubIssue(repository); //new instance
        issue.setId(this.id);
        issue.setTitle(this.title);

        return issue;
    }