In Java: How do I create a class that can be initialized and call its methods in one statement?

Question

There is a coding style that I've seen a lot in Android Programming when using Google Services libraries where they use dots to call methods after initializing an instance of a class.

For example, lets say I have a Person class:

public class Person {
    private String firstName;
    private String lastName;    
    private String occupation;
    private String primarySkill;
    private String secondarySkill;

/* ******************
 *   CONSTRUCTOR
 * ******************/
    public Person(String firstName, String lastName) {
        setFirstName(firstName);
        setLastName(lastName);
    }

/* ******************
 *     MUTATORS
 * ******************/
    // firstName Setter:
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    // lastName Setter:
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    // occupation Adder:
    public void addOccupation(String occupation) {
        this.occupation = occupation;
    }

    // primarySkill Adder:
    public void addPrimarySkill(String primarySkill) {
        this.primarySkill = primarySkill;
    }

    // secondarySkill Adder:
    public void addSecondarySkill(String secondarySkill) {
        this.secondarySkill = secondarySkill;
    }   
}

Now, when I create an instance of this class, I want to be able to do the following:

Person bob = new Person("Bob", "Anderson")
            .addOccupation("Student")
            .addPrimarySkill("Java")
            .addSecondarySkill("SQL");

However this gives me syntax errors. How can I build a class that can allow me to do this?

Your methods don't return anything, make them return an instance of the class. — Maroun, Jun 28 '16 at 14:11
This is not magical, if the dot thing works, it is because each of the method calls, returns the object itself. Your methods don't return the object. — Arnaud, Jun 28 '16 at 14:12
This is also called **fluent API** or more or less the **builder pattern**. — Joop Eggen, Jun 28 '16 at 14:15

CPerkins · Accepted Answer · 2016-06-28T15:34:50.577

8

What you're looking for is called Method Chaining. Modify your setters to return this:

// occupation Adder:
public Person addOccupation(String occupation) {
    this.occupation = occupation;
    return this;
}

edited Jun 28 '16 at 15:34

answered Jun 28 '16 at 14:10

CPerkins

8,968
3
34
47

1

Also their return types need to be set to `Person` – Chris Sprague Jun 28 '16 at 14:12
@ChrisSprague good point, thanks. – CPerkins Jun 28 '16 at 14:14
3

I put a bit of formatting in there, hope that's okay with you. Just to make it clear that the "this" to return is the actual keyword, and not "this, the following chunk of code." – yshavit Jun 28 '16 at 14:16
@yshavit good addition. – CPerkins Jun 28 '16 at 14:43

score 1 · Answer 2 · answered Jun 28 '16 at 14:15

You should have "setter" methods returning the type of the object. To build on your addXyx methods:

public Person setLastName(String lastName) {
    this.lastName = lastName;
    return this;
}

public Person addOccupation(String occupation) {
    this.occupation = occupation;
    return this;
}

public Person addPrimarySkill(String primarySkill) {
    this.primarySkill = primarySkill;
    return this;
}

public Person addSecondarySkill(String secondarySkill) {
    this.secondarySkill = secondarySkill;
    return this;
}

With that, you could chain your setter invocations:

Person bob = new Person("Bob", "Anderson")
        .addOccupation("Student")
        .addPrimarySkill("Java")
        .addSecondarySkill("SQL");

score 1 · Answer 3 · answered Jun 28 '16 at 14:15

1

This is a typical case of using a Builder Pattern. The setters in the builder pattern return this

answered Jun 28 '16 at 14:15

toro

194
5

Not actually typical. From the article you linked, the builder pattern "`uses another object, a builder`", where these methods are just setters which use `Method Chaining`. – CPerkins Jun 28 '16 at 15:33

In Java: How do I create a class that can be initialized and call its methods in one statement?

3 Answers3