首先来看一段Android开发中经常用到的一段代码：

        AlertDialog alertDialog = new AlertDialog.Builder(this)
                .setTitle("头部")
                .setMessage("会话框内容")
                .create();
        alertDialog.show();

什么是Builder模式？为什么这么简洁的代码就能创建一个对话框？

Builder模式的定义：将一个复杂对象的构建与它的表示分离，使得同样的构建过程可以创建不同的表示。

Builder经典模式

Builder经典模式由四个参与者构成：Product、Builder、ConcreteBuilder、Director

上代码

Product.java 被构造的复杂对象

// 
public class Product {
    String partA;
    String partB;

    public String getPartA() {
        return partA;
    }

    public void setPartA(String partA) {
        this.partA = partA;
    }

    public String getPartB() {
        return partB;
    }

    public void setPartB(String partB) {
        this.partB = partB;
    }
}

Builder.java 抽象接口，定义创建Product对象各个组成部件的操作

public interface Builder {
    void buildPartA();
    void buildPartB();
    Product getProduct();
}

ConcreteBuilder.java Builder接口实现类，可定义多个。实际构建Product的地方，返回Product

public class ConcreteBuilderA implements Builder {

    private Product product;

    public ConcreteBuilderA() {
        product = new Product();
    }

    @Override
    public void buildPartA() {
        product.setPartA("buildPartA");
    }

    @Override
    public void buildPartB() {
    }

    @Override
    public Product getProduct() {
        return product;
    }
}

public class ConcreteBuilderB implements Builder {

    private Product product;

    public ConcreteBuilderB() {
        product = new Product();
    }

    @Override
    public void buildPartA() {
        product.setPartA("buildPartB");
    }

    @Override
    public void buildPartB() {
    }

    @Override
    public Product getProduct() {
        return product;
    }
}

Director.java ： Builder接口的构造者和使用者

public class Director {
    private final Builder builder;

    public Director(Builder builder) {
        this.builder = builder;
    }

    public void buildProduct() {
        builder.buildPartA();
        builder.buildPartB();
    }

    public Product getProduct() {
        return builder.getProduct();
    }
}

调用代码：

  Director director = new Director(new ConcreteBuilderA());
  director.buildProduct();

  Director director = new Director(new ConcreteBuilderB());
  director.buildProduct();

上面的是Builder构建的经典模式，下面介绍Builder的变种，在Android中有广泛应用。

public class User {
    // 注意final
    final private String firstName;
    final private String lastName;
    final private int age;
    final private int phone;

    /**
     * 传入UserBuilder
     * @param userBuilder
     */
    private User(UserBuilder userBuilder) {
        this.firstName = userBuilder.firstName;
        this.lastName = userBuilder.lastName;
        this.age = userBuilder.age;
        this.phone = userBuilder.phone;
    }

    public static class UserBuilder {
        // 注意final修饰的变量在构造函数赋值
        final private String firstName;
        final private String lastName;
        private int age;
        private int phone;

        public UserBuilder(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }

        public UserBuilder age(int age) {
            this.age = age;
            return this;
        }

        public UserBuilder phone(int phone) {
            this.phone = phone;
            return this;
        }

        /**
         * 创建私有User
         * @return 返回User
         */
        public User build() {
            return new User(this);
        }
    }
}

调用方式：

new User.UserBuilder("first", "last").age(10).phone(10086).build();

代码解析：
User类的构造方法为私有类，真正实现对User类组成部分的构建是在UserBuilder中，并在UserBuilder的build方法返回User类对象。