建造者模式

建造者模式

建造者模式（Builder Pattern）使用多个简单的对象一步一步构建成一个复杂的对象。

这种类型的设计模式属于创建型模式，它提供了一种创建对象的最佳方式。

解决问题：

当一个类的构造函数超过4个，而且某些参数是可选的，我们通常有两个方法来构建他的对象。

public class Computer {
    private String cpu;//必须
    private String ram;//必须
    private int usbCount;//可选
    private String keyboard;//可选
    private String display;//可选
}

第一：折叠构造函数模式

public class Computer {
     ...
    public Computer(String cpu, String ram) {
        this(cpu, ram, 0);
    }
    public Computer(String cpu, String ram, int usbCount) {
        this(cpu, ram, usbCount, "罗技键盘");
    }
    public Computer(String cpu, String ram, int usbCount,
                    String keyboard) {
        this(cpu, ram, usbCount, keyboard, "三星显示器");
    }
    public Computer(String cpu, String ram, int usbCount,
                    String keyboard, String display) {
        this.cpu = cpu;
        this.ram = ram;
        this.usbCount = usbCount;
        this.keyboard = keyboard;
        this.display = display;
    }
}

第二种：Javabean 模式

public class Computer {
        ...

    public String getCpu() {
        return cpu;
    }
    public void setCpu(String cpu) {
        this.cpu = cpu;
    }
    public String getRam() {
        return ram;
    }
    public void setRam(String ram) {
        this.ram = ram;
    }
    public int getUsbCount() {
        return usbCount;
    }
...
}

弊端：

第一种主要是使用及阅读不方便。

第二种方式在构建过程中对象的状态容易发生变化，造成错误。

解决问题

适用建造者模式

• 目标类以及静态建造类

package org.kouhao.design.patterns.建造者模式;

/**
 * 目标类：电脑
 *
 * @author admin
 */
public class Computer {
    private String cpu;//必须
    private String ram;//必须
    private int usbCount;//可选
    private String keyboard;//可选
    private String display;//可选

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

    public Computer(ComputerBuilder computerBuilder) {
        this.cpu = computerBuilder.cpu;
        this.ram = computerBuilder.ram;
        this.usbCount = computerBuilder.usbCount;
        this.keyboard = computerBuilder.keyboard;
        this.display = computerBuilder.display;
    }

    public void setUsbCount(int usbCount) {
        this.usbCount = usbCount;
    }

    public void setKeyboard(String keyboard) {
        this.keyboard = keyboard;
    }

    public void setDisplay(String display) {
        this.display = display;
    }

    @Override
    public String toString() {
        return "Computer{" +
                "cpu='" + cpu + '\'' +
                ", ram='" + ram + '\'' +
                ", usbCount=" + usbCount +
                ", keyboard='" + keyboard + '\'' +
                ", display='" + display + '\'' +
                '}';
    }

    public static class ComputerBuilder {
        private String cpu;//必须
        private String ram;//必须
        private int usbCount;//可选
        private String keyboard;//可选
        private String display;//可选

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

        public ComputerBuilder usbCount(int usbCount) {
            this.usbCount = usbCount;
            return this;
        }

        public ComputerBuilder keyboard(String keyboard) {
            this.keyboard = keyboard;
            return this;
        }

        public ComputerBuilder display(String display) {
            this.display = display;
            return this;
        }

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

• 客户端

package org.kouhao.design.patterns.建造者模式;

/**
 * @author admin
 */
public class Client {
    public static void main(String[] args) {
        Computer.ComputerBuilder computerBuilder = new Computer.ComputerBuilder("Inter", "").
                usbCount(1).display("display").keyboard("keyboard");
        computerBuilder.build();
    }
}