How to implement the builder pattern in Java 8?

Question

Implement the builder pattern prior to Java 8 has lots of tedious, nearly duplicated code; the builder itself is typically boilerplate code. Some duplicate code detectors co

Answer 1

We can use Consumer functional interface of Java 8 to avoid multiple getter/setter methods.

Refer the below-updated code with Consumer interface.

import java.util.function.Consumer;

public class Person {

    private String name;

    private int age;

    public Person(Builder Builder) {
        this.name = Builder.name;
        this.age = Builder.age;
    }

    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder("Person{");
        sb.append("name='").append(name).append('\'');
        sb.append(", age=").append(age);
        sb.append('}');
        return sb.toString();
    }

    public static class Builder {

        public String name;
        public int age;

        public Builder with(Consumer<Builder> function) {
            function.accept(this);
            return this;
        }

        public Person build() {
            return new Person(this);
        }
    }

    public static void main(String[] args) {
        Person user = new Person.Builder().with(userData -> {
            userData.name = "test";
            userData.age = 77;
        }).build();
        System.out.println(user);
    }
}

Refer the below link to know the detailed information with the different examples.

https://medium.com/beingprofessional/think-functional-advanced-builder-pattern-using-lambda-284714b85ed5

https://dkbalachandar.wordpress.com/2017/08/31/java-8-builder-pattern-with-consumer-interface/

Answer 2

Building upon this answer, here's a quasi-immutable version of the builder pattern:

import java.util.ArrayList;
import java.util.List;
import java.util.function.BiConsumer;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

/**
 * Responsible for constructing objects that would otherwise require
 * a long list of constructor parameters.
 *
 * @param <MT> The mutable definition for the type of object to build.
 * @param <IT> The immutable definition for the type of object to build.
 */
public class GenericBuilder<MT, IT> {
  /**
   * Provides the methods to use for setting object properties.
   */
  private final Supplier<MT> mMutable;

  /**
   * Calling {@link #build()} will instantiate the immutable instance using
   * the mutator.
   */
  private final Function<MT, IT> mImmutable;

  /**
   * Adds a modifier to call when building an instance.
   */
  private final List<Consumer<MT>> mModifiers = new ArrayList<>();

  /**
   * Constructs a new builder instance that is capable of populating values for
   * any type of object.
   *
   * @param mutator Provides methods to use for setting object properties.
   */
  protected GenericBuilder(
      final Supplier<MT> mutator, final Function<MT, IT> immutable ) {
    mMutable = mutator;
    mImmutable = immutable;
  }

  /**
   * Starting point for building an instance of a particular class.
   *
   * @param supplier Returns the instance to build.
   * @param <MT>     The type of class to build.
   * @return A new {@link GenericBuilder} capable of populating data for an
   * instance of the class provided by the {@link Supplier}.
   */
  public static <MT, IT> GenericBuilder<MT, IT> of(
      final Supplier<MT> supplier, final Function<MT, IT> immutable ) {
    return new GenericBuilder<>( supplier, immutable );
  }

  /**
   * Registers a new value with the builder.
   *
   * @param consumer Accepts a value to be set upon the built object.
   * @param value    The value to use when building.
   * @param <V>      The type of value used when building.
   * @return This {@link GenericBuilder} instance.
   */
  public <V> GenericBuilder<MT, IT> with(
      final BiConsumer<MT, V> consumer, final V value ) {
    mModifiers.add( instance -> consumer.accept( instance, value ) );
    return this;
  }

  /**
   * Instantiates then populates the immutable object to build.
   *
   * @return The newly built object.
   */
  public IT build() {
    final var value = mMutable.get();
    mModifiers.forEach( modifier -> modifier.accept( value ) );
    mModifiers.clear();
    return mImmutable.apply( value );
  }
}

Example usage:

final var caret = CaretPosition
    .builder()
    .with( CaretPosition.Mutator::setParagraph, 5 )
    .with( CaretPosition.Mutator::setMaxParagraph, 10 )
    .build();

When the mutator's reference is released, the state of the returned object is effectively immutable. The CaretPosition class resembles:

public class CaretPosition {
  public static GenericBuilder<CaretPosition.Mutator, CaretPosition> builder() {
    return GenericBuilder.of( CaretPosition.Mutator::new, CaretPosition::new );
  }

  public static class Mutator {
    private int mParagraph;
    private int mMaxParagraph;

    public void setParagraph( final int paragraph ) {
      mParagraph = paragraph;
    }

    public void setMaxParagraph( final int maxParagraph ) {
      mMaxParagraph = maxParagraph;
    }
  }

  private final Mutator mMutator;
  
  private CaretPosition( final Mutator mutator ) {
    mMutator = mutator;
  }

  // ...

From here, the CaretPosition can freely reference its internal Mutator instance, which handily provides the opportunity to avoid violating encapsulation by otherwise exposing get accessor methods on the immutable class without necessity.

This is only quasi-immutable because the values can be changed if a handle to the mutable instance is retained. Here's how immutability can be violated:

final var mutable = CaretPosition.builder()
    .with( CaretPosition.Mutator::setParagraph, 5 )
    .with( CaretPosition.Mutator::setMaxParagraph, 10 );
final var caret = mutable.build();
mutable.setParagraph( 17 );
System.out.println( "caret para: " + caret.toString() );

Should caret.toString() include the paragraph value, the resulting string will contain the value 17 instead of 5, thereby violating immutability. Another downside to this approach is that if validation is performed at build() time, the second call to setParagraph will not be passed through the validator.

Ways to avoid this include:

• Immutable copy constructor. Copy the mutable member variables into the immutable instance, which entails duplicating all member variables.
• Mutator copy constructor. Copy the Mutator into a new object reference, which avoids duplicating all member variables while building a truly immutable instance of the desired type.
• Clone. Clone the mutator when constructing the immutable instance, which requires either implementing Serializable everywhere or using a deep-copy library.
• Library. Scrap this solution for Project Lombok, AutoValue, or Immutables.

The Mutator copy constructor option would resemble:

private Mutator() {
}

private Mutator( final Mutator mutator) {
  mParagraph = mutator.mParagraph;
  mMaxParagraph = mutator.mMaxParagraph;
}

Then the change to CaretPosition is trivial---instantiate the Mutator using its copy constructor:

private CaretPosition( final Mutator mutator ) {
  mMutator = new Mutator( mutator );
}

Answer 3

I have recently tried to revisit the builder pattern in Java 8, and I am currently using the following approach:

public class Person {

    static public Person create(Consumer<PersonBuilder> buildingFunction) {
        return new Person().build(buildingFunction);
    }

    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    private Person() {

    }

    private Person build(Consumer<PersonBuilder> buildingFunction) {
        buildingFunction.accept(new PersonBuilder() {

            @Override
            public PersonBuilder withName(String name) {
                Person.this.name = name;
                return this;
            }

            @Override
            public PersonBuilder withAge(int age) {
                Person.this.age = age;
                return this;
            }
        });

        if (name == null || name.isEmpty()) {
            throw new IllegalStateException("the name must not be null or empty");
        }

        if (age <= 0) {
            throw new IllegalStateException("the age must be > 0");
        }

        // check other invariants

        return this;
    }
}

public interface PersonBuilder {

    PersonBuilder withName(String name);

    PersonBuilder withAge(int age);
}

Usage:

var person = Person.create(
    personBuilder -> personBuilder.withName("John Smith").withAge(43)
);

Advantages:

• A clean builder interface
• Little to no boilerplate code
• The builder is well encapsulated
• It's easy to segregate the optional attributes from the mandatory attributes of the target class (the optional attributes are specified in the builder)
• No setter needed in the target class (in DDD, you generally don't want setters)
• Use of a static factory method to create an instance of the target class (instead of using the new keyword, so it's possible to have several static factory methods, each with a meaningful name)

Possible drawbacks:

• The calling code can save a reference to the passed-in builder and later screw up the mounted instance, but who will do that?
• If the calling code saves a reference to the passed-in builder, a memory leak can occur

Possible alternative:

We can setup a constructor with a building function, as follows:

public class Person {

    static public Person create(Consumer<PersonBuilder> buildingFunction) {
        return new Person(buildingFunction);
    }

    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    private Person(Consumer<PersonBuilder> buildingFunction) {
        buildingFunction.accept(new PersonBuilder() {

            @Override
            public PersonBuilder withName(String name) {
                Person.this.name = name;
                return this;
            }

            @Override
            public PersonBuilder withAge(int age) {
                Person.this.age = age;
                return this;
            }
        });

        if (name == null || name.isEmpty()) {
            throw new IllegalStateException("the name must not be null or empty");
        }

        if (age <= 0) {
            throw new IllegalStateException("the age must be > 0");
        }

        // check other invariants
    }
}

Answer 4

The GenericBuilder

The idea for building mutable objects (immutable objects are addressed later on) is to use method references to setters of the instance that should be built. This leads us to a generic builder that is capable of building every POJO with a default constructor - one builder to rule them all ;-)

The implementation is this:

public class GenericBuilder<T> {

    private final Supplier<T> instantiator;

    private List<Consumer<T>> instanceModifiers = new ArrayList<>();

    public GenericBuilder(Supplier<T> instantiator) {
        this.instantiator = instantiator;
    }

    public static <T> GenericBuilder<T> of(Supplier<T> instantiator) {
        return new GenericBuilder<T>(instantiator);
    }

    public <U> GenericBuilder<T> with(BiConsumer<T, U> consumer, U value) {
        Consumer<T> c = instance -> consumer.accept(instance, value);
        instanceModifiers.add(c);
        return this;
    }

    public T build() {
        T value = instantiator.get();
        instanceModifiers.forEach(modifier -> modifier.accept(value));
        instanceModifiers.clear();
        return value;
    }
}

The builder is constructed with a supplier that creates new instances and then those instances are modified by the modifications specified with the with method.

The GenericBuilder would be used for Person like this:

Person value = GenericBuilder.of(Person::new)
            .with(Person::setName, "Otto").with(Person::setAge, 5).build();

Properties and further Usages

But there is more about that builder to discover.

For example, the above implementation clears the modifiers. This could be moved into its own method. Therefore, the builder would keep its state between modifications and it would be easy create multiple equal instances. Or, depending on the nature of an instanceModifier, a list of varying objects. For example, an instanceModifier could read its value from an increasing counter.

Continuing with this thought, we could implement a fork method that would return a new clone of the GenericBuilder instance that it is called on. This is easily possible because the state of the builder is just the instantiator and the list of instanceModifiers. From there on, both builders could be altered with some other instanceModifiers. They would share the same basis and have some additional state set on built instances.

The last point I consider especially helpful when needing heavy entities for unit or even integration tests in enterprise applications. There would be no god-object for entities, but for builders instead.

The GenericBuilder can also replace the need for different test value factories. In my current project, there are many factories used for creating test instances. The code is tightly coupled to different test scenarios and it is difficult to extract portions of a test factory for reuse in another test factory in a slightly different scenario. With the GenericBuilder, reusing this becomes much easier as there is only a specific list of instanceModifiers.

To verify that created instances are valid, the GenericBuilder could be initialized with a set of predicates, which are verified in the build method after all instanceModifiers are run.

public T build() {
    T value = instantiator.get();
    instanceModifiers.forEach(modifier -> modifier.accept(value));
    verifyPredicates(value);
    instanceModifiers.clear();
    return value;
}

private void verifyPredicates(T value) {
    List<Predicate<T>> violated = predicates.stream()
            .filter(e -> !e.test(value)).collect(Collectors.toList());
    if (!violated.isEmpty()) {
        throw new IllegalStateException(value.toString()
                + " violates predicates " + violated);
    }
}

Immutable object creation

To use the above scheme for the creation of immutable objects, extract the state of the immutable object into a mutable object and use the instantiator and builder to operate on the mutable state object. Then, add a function that will create a new immutable instance for the mutable state. However, this requires that the immutable object either has its state encapsulated like this or it be changed in that fashion (basically applying parameter object pattern to its constructor).

This is in some way different than a builder was used in pre-java-8 times. There, the builder itself was the mutable object that created a new instance at the end. Now, we have a separation of the state a builder keeps in a mutable object and the builder functionality itself.

In essence
Stop writing boilerplate builder patterns and get productive using the GenericBuilder.

Answer 5

public class PersonBuilder {
    public String salutation;
    public String firstName;
    public String middleName;
    public String lastName;
    public String suffix;
    public Address address;
    public boolean isFemale;
    public boolean isEmployed;
    public boolean isHomewOwner;

    public PersonBuilder with(
        Consumer<PersonBuilder> builderFunction) {
        builderFunction.accept(this);
        return this;
    }


    public Person createPerson() {
        return new Person(salutation, firstName, middleName,
                lastName, suffix, address, isFemale,
                isEmployed, isHomewOwner);
    }
}

Usage

Person person = new PersonBuilder()
    .with($ -> {
        $.salutation = "Mr.";
        $.firstName = "John";
        $.lastName = "Doe";
        $.isFemale = false;
    })
    .with($ -> $.isHomewOwner = true)
    .with($ -> {
        $.address =
            new PersonBuilder.AddressBuilder()
                .with($_address -> {
                    $_address.city = "Pune";
                    $_address.state = "MH";
                    $_address.pin = "411001";
                }).createAddress();
    })
    .createPerson();

Refer: https://medium.com/beingprofessional/think-functional-advanced-builder-pattern-using-lambda-284714b85ed5

Disclaimer: I am the author of the post

Answer 6

You can check the lombok project

For your case

@Builder
public class Person {
    private String name;
    private int age;
}

It would generate the code on the fly

public class Person {
    private String name;
    private int age;
    public String getName(){...}
    public void setName(String name){...}
    public int getAge(){...}
    public void setAge(int age){...}
    public Person.Builder builder() {...}

    public static class Builder {
         public Builder withName(String name){...}
         public Builder withAge(int age){...}
         public Person build(){...}
    }        
}

Lombok do it on the compilation phase and is transparent for developers.