问题
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.
回答1:
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
回答2:
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();
回答3:
To generate an inner builder in Intellij IDEA, check out this plugin: https://github.com/analytically/innerbuilder
回答4:
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.
回答5:
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.
回答6:
Once you've got an idea, in practice, you may find lombok's @Builder much more convenient.
@Builder lets you automatically produce the code required to have your class be instantiable with code such as:
Person.builder()
.name("Adam Savage")
.city("San Francisco")
.job("Mythbusters")
.job("Unchained Reaction")
.build();
Official documentation: https://www.projectlombok.org/features/Builder
回答7:
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.
回答8:
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
回答9:
I personally prefer to use the other approach, when you have 2 different classes. So you don't need any static class. This is basically to avoid write Class.Builder when you has to create a new instance.
public class Person {
private String attr1;
private String attr2;
private String attr3;
// package access
Person(PersonBuilder builder) {
this.attr1 = builder.getAttr1();
// ...
}
// ...
// getters and setters
}
public class PersonBuilder (
private String attr1;
private String attr2;
private String attr3;
// constructor with required attribute
public PersonBuilder(String attr1) {
this.attr1 = attr1;
}
public PersonBuilder setAttr2(String attr2) {
this.attr2 = attr2;
return this;
}
public PersonBuilder setAttr3(String attr3) {
this.attr3 = attr3;
return this;
}
public Person build() {
return new Person(this);
}
// ....
}
So, you can use your builder like this:
Person person = new PersonBuilder("attr1")
.setAttr2("attr2")
.build();
回答10:
As many already stated here you need to make the class static.
Just small addition - if you want, there is a bit different way without static one.
Consider this. Implementing a builder by declaring something like withProperty(value) type setters inside the class and make them return a reference to itself. In this approach, you have a single and an elegant class which is a thread safe and concise.
Consider this:
public class DataObject {
private String first;
private String second;
private String third;
public String getFirst(){
return first;
}
public void setFirst(String first){
this.first = first;
}
...
public DataObject withFirst(String first){
this.first = first;
return this;
}
public DataObject withSecond(String second){
this.second = second;
return this;
}
public DataObject withThird(String third){
this.third = third;
return this;
}
}
DataObject dataObject = new DataObject()
.withFirst("first data")
.withSecond("second data")
.withThird("third data");
Check it out for more Java Builder examples.
回答11:
You need to change Builder class to static class Builder. Then it will work fine.
来源：https://stackoverflow.com/questions/5007355/builder-pattern-in-effective-java