Java原型設計模式

原型模式

用原型執行個體指定建立對象的種類，并且通過拷貝這些原型建立新的對象。

public class Work implements Cloneable{
    private String address;
    private int time;

    public void setAddress(String address) {
        this.address = address;
    }

    public void setTime(int time) {
        this.time = time;
    }

    @Override
    public String toString() {
        return "Work{" +
            "address='" + address + '\'' +
            ", time=" + time +
            '}';
    }

    @Override
    protected Work clone() throws CloneNotSupportedException {
        return (Work) super.clone();
    }
}
           

public class Resume implements Cloneable{
    private String name;
    private int age;
    private String company;
    private Work work;

    public Resume(String name){
        this.name = name;
        this.work = new Work();
    }

    private Resume (Work work) throws CloneNotSupportedException {
        this.work = work.clone();
    }

    public void setAge(int age){
        this.age = age;
    }

    public void setCompany(String company){
        this.company = company;
    }

    public void setWork(String address, int time) {
        work.setAddress(address);
        work.setTime(time);
    }

    public void display(){
        System.out.println(toString());
    }

    @Override
    protected Resume clone() throws CloneNotSupportedException {
        Resume resume = new Resume(this.work);
        resume.name = this.name;
        resume.age = this.age;
        resume.company = this.company;
        return resume;
    }

    @Override
    public String toString() {
        return "Resume{" +
            "name='" + name + '\'' +
            ", age=" + age +
            ", company='" + company + '\'' +
            ", work=" + work +
            '}';
    }
}
           

淺拷貝

Java的object.clone()預設為淺拷貝，即對象的基本類型屬性複制值，引用類型屬性複制引用。

假如沒有重寫Resume對象的clone()方法

Resume resume = new Resume("周小狼");
Resume a = resume.clone();
           

兩個對象的name、company、work引用會指向堆中的相同空間，當對象a對work操作會影響resume。

深拷貝

如上代碼所示，重寫Resume的clone方法，在方法中new一個resume對象，調用private構造器，傳入目前對象的work屬性，構造器再調用work的clone方法拷貝屬性，此處要注意，如果work的屬性中存在引用類型，還需要繼續遞歸下去，才能實作兩個resume對象的完全獨立。（String類型是不可變，當修改String類型的值實際上是開辟新空間寫入新值，将引用指向新空間，不會影響到其他對象）

測試

public class Test {

    public static void main(String[] args) throws CloneNotSupportedException {
        Resume resume = new Resume("周小狼");
        resume.setAge(15);
        resume.setCompany("網魚");
        resume.setWork("同仁堂",2);

        Resume a = resume.clone();
        a.setCompany("林小媛");
        a.setWork("長運堂",1);
        resume.display();
        a.display();
    }
}
           

測試結果

Resume{name='周小狼', age=15, company='網魚', work=Work{address='同仁堂', time=2}}
Resume{name='周小狼', age=15, company='林小媛', work=Work{address='長運堂', time=1}}
           

參考資料

《大話設計模式》

Java深拷貝和淺拷貝