Abstract Factory
通过工厂类创建某一对象，而不是直接使用new关键字。
public class Computer(){
}
public class NoteBookComputer extends Computer(){
}
public abstract class ComputerFactory(){
    public abstract Computer createComputer();
}
public class NoteBookComputerFactory extends ComputerFactory{
     public Computer createComputer() {
         return new NoteBookComputer();
    }
}
client code:
ComputerFactory factory=new NoteBookComputerFactory();
Computer noteBook=factory.createComputer();