Advantage
- It simulates named optional parameters which is easily used to client API.
- Detect the invariant failure(validation error of field) as soon as the invalid parameters are passed, instead of waiting for build to be invoked.
- The builder can fill in some fields automatically such as a serial number.
- Avoid Class.newInstancebreaks compile-time exception checking
Disadvantage
-
A builder must be created first. It's time cost and verbose. It would be better to be used when there are more than 4 parameters.
Demo
package com.effectivejava.creatingobject;
// Builder Pattern
public class NutritionFacts {
private final int servingSize;
private final int servings;
private final int calories;
private final int fat;
private final int sodium;
private final int carbohydrate;
public static class Builder {
// Required parameters
private final int servingSize;
private final int servings;
// Optional parameters - initialized to default values
private int calories = 0;
private int fat = 0;
private int carbohydrate = 0;
private int sodium = 0;
public Builder(int servingSize, int servings) {
this.servingSize = servingSize;
this.servings = servings;
}
public Builder calories(int val) {
calories = val;
return this;
}
public Builder fat(int val) {
fat = val;
return this;
}
public Builder carbohydrate(int val) {
carbohydrate = val;
return this;
}
public Builder sodium(int val) {
sodium = val;
return this;
}
public NutritionFacts build() {
return new NutritionFacts(this);
}
}
private NutritionFacts(Builder builder) {
servingSize = builder.servingSize;
servings = builder.servings;
calories = builder.calories;
fat = builder.fat;
sodium = builder.sodium;
carbohydrate = builder.carbohydrate;
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
// TODO Auto-generated method stub
return String
.format("NutritionFacts[servingSize:%d, servings:%d, calories: %d, fat: %d, sodium: %d, carbohydrate: %d]",
this.servingSize, this.servings, this.calories,
this.fat, this.sodium, this.carbohydrate);
}
}
NutritionFacts nutritionFacts = new NutritionFacts.Builder(10, 100)
.calories(3).carbohydrate(2).fat(1).sodium(4).build();
System.out.println(nutritionFacts.toString());
/*******************************************/
// A builder for objects of type T
public interface Builder<T> {
public T build();
}
Tree buildTree(Builder<? extends Node> nodeBuilder) { ... }
Summary
The Builder pattern is a good choice when designing classes whose constructors or static factories would have more than a handful(>4) of parameters.