But why?

Imagine the headache constructing complex objects from constructor that has more than 5 arguments.

Then you need to remember what is what & you explicitly need to send null for any property that you do not want to set.

Take for example below snippet:

// The complex entity
public class IceCream {

    private int scoops;
	private Type type;
	private Flavour flavour;
	private Brand brand;
	private float price;

    // a big exhaustive constructor
    public IceCream(int scoops, Type type, 
                        Flavour flavour, Brand brand, float price) {
        // the usual code here
    }
}

// Using the above complex entity
public class IceCreamMachine {
    public IceCream orderAnIceCream() {
        return new IceCream(1, CUP, MANGO, BASKINS, 125);
    }
}

Builder Pattern

Builder Pattern addresses this problem statement by following a step-by-step approach. Essentially it creates the entire object from small steps.

This comes handy when we have a complex object & a constructor does not help with being informative.

How its done?

The implementation is simple. We just create a separate class that provides steps (methods) to set individual properties with ease. Finally it will return the desired object.

• Create your entity as normal but for the constructor make it private & let it accept a single argument as builder

  public class IceCream {
  
    private int scoops;
    private Type type;
    private Flavour flavour;
    private Brand brand;
    private float price;
  
    public IceCream(Builder builder) {
        this.scoops = builder.scoops;
        this.type = builder.type;
        this.flavour = builder.flavour;
        this.brand = builder.brand;
        this.price = builder.price;
    }
  }
  

• Create a static inner class as builder
• The builder is required to have same properties as your entity
• The builder then also provide methods for setting each property & return itself
• Below is a snippet for builder

  public class IceCream {
    // leaving out the fields just for now
    public static class Builder {
        // same properties as IceCream entity
        private int scoops;
        private Type type;
        private Flavour flavour;
        private Brand brand;
        private float price;
  
        // keeping a default constructor if we need 
        // to set all properties one by one
        private Builder() {
            super();
        }
  
        // parameterized constructor to create 
        // builder with bare-minimum properties
        private Builder(Flavour flavour, Brand brand) {
            this.brand = brand;
            this.flavour = flavour;
        }
  
        public Builder addScoops(int scoops) {
            this.scoops = this.scoops + scoops;
            return this;
        }
  
        public Builder selectType(Type type) {
            this.type = type;
            return this;
        }
  
        public Builder selectFlavour(Flavour flavour) {
            this.flavour = flavour;
            return this;
        }
  
        public Builder selectBrand(Brand brand) {
            this.brand = brand;
            return this;
        }
  
        public Builder calculatePrice() {
            this.price = (this.brand.getRate() 
                            + this.type.getRate() 
                            + this.flavour.getRate()) * this.scoops;
            return this;
        }
  
        // method invokes the parent class constructor
        public IceCream build() {
            return new IceCream(this);
        }
  
        // static method for easy object creation
        public static Builder createInstance(Flavour flavour, Brand brand) {
            return new Builder(flavour, brand);
        }
  
        // static method for easy object creation
        public static Builder createInstance() {
            return new Builder();
        }
    }
  }
  

• Below snippet shows how to construct objects using this builder

  // using builder with parameters
  IceCream vanillaSmall = Builder.createInstance(Flavour.VANILLA, 
                                        Brand.BASKINS)
                                    .addScoops(1)
                                    .selectType(Type.CUP)
                                    .calculatePrice()
                                    .build();
  // using builder with all steps explicitly defined
  IceCream mangoLarge = Builder.createInstance()
                            .selectFlavour(Flavour.MANGO)
                            .selectBrand(Brand.KWALITY)
                            .addScoops(3)
                            .selectType(Type.CUP)
                            .calculatePrice()
                            .build();
  

• The way we now initialize objects is very clean & readable.

Caveats?

Yes like everything else, we need to maintain the builder with same properties which is cumbersome.