Design: Convention, Status: Accepted

Favor Static Factory Methods Over Constructors

This page describes the structures and conventions used to initialize a class.

Static Factory Methods vs. Constructors

Static factory methods are preferable than constructors for the following reasons:

• Static factory methods provide meaningful names compared with constructors, which improves the readability of the codes by describing the objects being returned.

// With static factory method, giving a hint that a foobar provider with default settings is being created.
FoobarProvider defaultProvider = FoobarProvider.defaultFoobarProvider(); 

// With constructor
FoobarProvider defaultProvider = new FoobarProvider(); 

• They are useful when work with immutable classes as we can reuse the same object instead of creating new object every time when it is invoked.
• Static factory methods can return any subtype of that class and this gives us flexibility to write a separate factory class to return different subclasses if needed.

There are a few disadvantages of static factory methods:

• It might not be straightforward for the users to use static factory method to create new instances compared with constructors, but this can be improved by providing a set of common method names such as create() and smart IDEs such as IntelliJ and Eclipse can also give hints and help auto-completing.
• Classes without public or protected constructors cannot be subclassed, but this could encourage us to use composition instead of inheritance.

In general, we should favor static factory methods over constructors.

Example

public class DefaultCredentialsProvider implements AwsCredentialsProvider, SdkAutoCloseable {

    private static final DefaultCredentialsProvider DEFAULT_CREDENTIALS_PROVIDER = new DefaultCredentialsProvider(builder());

    private DefaultCredentialsProvider(Builder builder) {
        this.providerChain = createChain(builder);
    }

    public static DefaultCredentialsProvider create() {
        return DEFAULT_CREDENTIALS_PROVIDER;
    }
    
    public static Builder builder() {
        return new Builder();
    }

    public static final class Builder { 
      // ...
    }
    // ...
}


// There are two ways to create new instance
DefaultCredentialsProvider defaultCredentialsProvider1 = DefaultCredentialsProvider.create();
DefaultCredentialsProvider defaultCredentialsProvider2 = DefaultCredentialsProvider.builder().build;

Naming Conventions

The naming conventions for the static factory methods:

• create(), create(params) when creating a new instance eg: DynamoDBClient.create()
• defaultXXX() when returning an instance with default settings. eg: BackoffStrategy.defaultStrategy()