Odtnhj

I have recently started to read Effective Java by Joshua Bloch. I found the idea of the Builder pattern [Item 2 in the book] really interesting. I tried to implement it in my project but there were compilation errors. Following is in essence what I was trying to do:

The class with multiple attributes and its builder class:

public class NutritionalFacts 
 private int sodium;
 private int fat;
 private int carbo;

 public class Builder 
 private int sodium;
 private int fat;
 private int carbo;

 public Builder(int s) 
 this.sodium = s;
 

 public Builder fat(int f) 
 this.fat = f;
 return this;
 

 public Builder carbo(int c) 
 this.carbo = c;
 return this;
 

 public NutritionalFacts build() 
 return new NutritionalFacts(this);
 
 

 private NutritionalFacts(Builder b) 
 this.sodium = b.sodium;
 this.fat = b.fat;
 this.carbo = b.carbo;
 

Class where I try to use the above class:

public class Main 
 public static void main(String args) 
 NutritionalFacts n = 
 new NutritionalFacts.Builder(10).carbo(23).fat(1).build();
 

I am getting the following compiler error:

an enclosing instance that contains
effectivejava.BuilderPattern.NutritionalFacts.Builder
is required
NutritionalFacts n = new
NutritionalFacts.Builder(10).carbo(23).fat(1).build();

I do not understand what the message means. Please explain. The above code is similar to the example suggested by Bloch in his book.

Make the builder a static class. Then it will work. If it is non-static, it would require an instance of its owning class - and the point is not to have an instance of it, and even to forbid making instances without the builder.

public class NutritionFacts 
 public static class Builder 
 

Reference: Nested classes

You should make the Builder class as static and also you should make the fields final and have getters to get those values. Don't provide setters to those values. In this way your class will be perfectly immutable.

public class NutritionalFacts 
 private final int sodium;
 private final int fat;
 private final int carbo;

 public int getSodium()
 return sodium;
 

 public int getFat()
 return fat;
 

 public int getCarbo()
 return carbo;
 

 public static class Builder 
 private int sodium;
 private int fat;
 private int carbo;

 public Builder sodium(int s) 
 this.sodium = s;
 return this;
 

 public Builder fat(int f) 
 this.fat = f;
 return this;
 

 public Builder carbo(int c) 
 this.carbo = c;
 return this;
 

 public NutritionalFacts build() 
 return new NutritionalFacts(this);
 
 

 private NutritionalFacts(Builder b) 
 this.sodium = b.sodium;
 this.fat = b.fat;
 this.carbo = b.carbo;
 

And now you can set the properties as follows:

NutritionalFacts n = new NutritionalFacts.Builder().sodium(10).carbo(15).
fat(5).build();

To generate an inner builder in Intellij IDEA, check out this plugin: https://github.com/analytically/innerbuilder

You are trying access a non-static class in a static way. Change Builder to static class Builder and it should work.

The example usage you give fails because there is no instance of Builder present. A static class for all practical purposes is always instantiated. If you don't make it static, you'd need to say:

Widget = new Widget.Builder(10).setparm1(1).setparm2(3).build();

Because you would need to construct a new Builder every time.

You need to declare the Builder inner class as static.

Consult some documentation for both non-static inner classes and static inner classes.

Basically the non-static inner classes instances cannot exist without attached outer class instance.

The Builder class should be static. I don't have time right now to actually test the code beyond that, but if it doesn't work let me know and I'll take another look.

This mean that you cant create enclose type. This mean that first you have to cerate a instance of "parent" class and then from this instance you can create nested class instances.

NutritionalFacts n = new NutritionalFacts()

Builder b = new n.Builder(10).carbo(23).fat(1).build();

Nested Classes