Design Patterns Tutorials and Guides

Java Builder Pattern

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Java Builder Pattern

The builder design pattern is one of the creational design patterns. You might aware of creational design patterns that all these patterns will be used to create and configure objects. Here we will focus on the builder diesign pattern in java.

Builder Design Pattern

Builder Design Pattern will be very helpful when we create immutable objects. While creating the immutable object We just need to pass required fields in object construction as we can not pass later after object construction.

Let's consider below given example used in one of our applications.

Laptop.java
public class Laptop {
	 
	 private final String processor;
	 private final Integer ram;
	 private final String storage;
	 private final Integer capacity;
}

In the Laptop class, it may have some mandatory fields and some optional fields. Values for these all fields should be passed while creating the object itself as it is an immutable class.

We need to create different types of constructors. Let's create all types of constructors to understand the problem.

All Constructors of Laptop
private Laptop(String processor)
private Laptop(String processor, Integer ram)
private Laptop(String processor, Integer ram, String storage)
private Laptop(String processor, Integer ram, String storage, Integer capacity)

All these constructors will be a bit messy and code file size will be increased. So to overcome this problem we have a builder design pattern. Here we will make use of the method chain.

Method Chain

To complete the sequence of steps in the creation/making of some class object we can use a method chain. Method chain will be in one statement and it consists of many numbers methods. Notice each method's return type should be this. After completion of some action and then will return this object.

For suppose to create the laptop with builder pattern we use method chain as given below.

Method Chain
new Laptop.LaptopBuilder().processor("Intel Core I5").ram(4).storage("SSD").capacity(500).build();

So a sequence of steps executes throughout this method chain and performed some actions to create a Laptop object here.

Builder Design Pattern Java Example

Now we will focus on the same Laptop builder example. Here we are going to create a builder class for Laptop assembling.

Rules for Builder design pattern

Create a Laptop class with a static builder class in it. And in builder class has a different method to chain all the required actions.

Laptop.java
public class Laptop {

    private final String processor;
    private final Integer ram;
    private final String storage;
    private final Integer capacity;

    public Laptop(LaptopBuilder laptopBuilder) {
        this.processor = laptopBuilder.processor;
        this.ram = laptopBuilder.ram;
        this.storage = laptopBuilder.storage;
        this.capacity = laptopBuilder.capacity;
    }

    //getters

    public final static class LaptopBuilder {

        private String processor;
        private Integer ram;
        private String storage;
        private Integer capacity;


        public LaptopBuilder processor(String processor) {
            this.processor = processor;
            return this;
        }

        public LaptopBuilder ram(Integer ram) {
            this.ram = ram;
            return this;
        }

        public LaptopBuilder storage(String storage) {
            this.storage = storage;
            return this;
        }

        public LaptopBuilder capacity(Integer capacity) {
            this.capacity = capacity;
            return this;
        }

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

    
}

Notice Laptop contains a Constructor with Builder object as input. And LaptopBuilder contains methods of chain to be executed while creating Laptop object.

The client for the Builder demonstration would be as given below.

LaptopBuilderDemo.java
public class LaptopBuilderDemo {
    public static void main(String[] args) {
        Laptop laptop = new Laptop.LaptopBuilder().processor("Intel Core I5").ram(4).storage("SSD").capacity(500).build();
        System.out.println(laptop);
    }
}

Code repo for this example is Java-Builder-Design-Pattern

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