建造者(Builder Pattern)

类的实例创建和初始化通常有下面两种方式:

1. 通过类提供的构造器创建类的实例并初始化类的属性
2. 通过无参构造器创建实例, 然后通过set方法初始化实例的属性

建造者模式又是一种创建类实例和初始化的方式.
当一个复杂的对象需要一步步的构造,或者该对象的字段超过4个构造器参数非常多时可以考虑使用建造者模式完成类的实例化.

1. 第一种可以参考Effective Java中的例子, 使用静态内部类实现

public class Computer {
    private String cpu;
    private String ram;
    private String hardDisk;
    private String mainBoard;
    private Computer(Builder builder) {
        this.cpu = builder.cpu;
        this.ram = builder.ram;
        this.hardDisk = builder.hardDisk;
        this.mainBoard = builder.mainBoard;
    }

    public static class Builder {
        private String cpu;
        private String ram;
        private String hardDisk;
        private String mainBoard;

        public Builder(String cpu, String ram) {
            this.cpu = cpu;
            this.ram = ram;
        }

        public Builder createHardDisk(String hardDisk) {
            this.hardDisk = hardDisk;
            return this;
        }

        public Builder createMainBoard(String mainBoard) {
            this.mainBoard = mainBoard;
            return this;
        }

        public Computer build() {
            return new Computer(this);
        }
    }

    // ... getter setter
}

// 可以控制必须初始化的字段,并且可选字段可以通过链式调用一步一步的创建
Computer computer = new Computer.Builder("cpu", "ram")
.createHardDisk("hardDisk").build();

2. 第二种如下

// 被构建的对象
public class Computer {
    private String mainBoard;
    private String hardDisk;
    private String cpu;
    private ram;
    //... getter setter
}

// 抽象的建造者 Builder
public interface Builder {
    void createMainBoard(String mainBoard);
    void createCpu(String cpu);
    void createHardDisk(String hardDisk);
    void createRam(String ram);
    Computer createComputer();
}

// 具体建造者,抽象的建造者的具体实现类
public class ConcreteBuilder implements Builder {
    private Computer computer = new Computer();
    
    @Override
    public void createMainBoard(String mainBoard) {
        computer.setMainBoard(mainBoard);
    }
    // ...其它字段同上
    @Override
    public Computer createComputer() {
        return computer;
    }
}

public class Director {
    private Builder builder;
    public Director(Builder builder) {
        this.builder = builder;
    }
    public Computer createComputer(String cpu, String mainBoard, String hardDisk, String ram) {
        this.builder.createCpu(cpu);
        this.builder.createMainBoard(mainBoard);
        this.builder.createHardDisk(hardDisk);
        this.builder.createRam(ram);
        return this.builder.createComputer();
    }
}

// 测试
public class Test {
    public static void main(String[] args) {
        Builder builder = new ConcreteBuilder();
        Director director = new Director(builder);
        Computer computer = director.createComputer("cpu", "mainBoard", "hardDisk", "ram");
    }
}