建造者模式(Builder Pattern)
类的实例创建和初始化通常有下面两种方式:
- 通过类提供的构造器创建类的实例并初始化类的属性
- 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.
示例代码
- 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();
- 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");
}
}