Programming Design Notes

在某些時候，為免改寫原本 Object 的數據，而需要複製一個一模一樣的 Object 出來，例如記下當時的 Object 狀態，然後備份等等。如果使用 Getter 和 Setter 一個一個數據去複製，一定會大大增加程式碼行數，亦只能對已知的 Object 去複製。當這個 Object 新增一項數據時，便要再重寫複製方法，所以這方法是很沒有效率的。

Xstream 原本是 Java 操作 XML 的一個 Library，除此之外亦可以使用 Xstream 去複製 Object (將 Java Object 轉換成 XML 再轉換為 Java Object)。

Xstream 官方網址: http://xstream.codehaus.org/

下載後將 .jar 加到 class path。

假設現在有 Parent 和 Info 2 個 Object，這 2 個 Object 都是作為傳遞數據的 Object。

Parent:

package xstream.sample;

public class Parent {

    private String firstName;
    private String lastName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "Parent [firstName=" + firstName + ", lastName=" + lastName
                + "]";
    }

}

Info:

package xstream.sample;

import java.util.LinkedList;
import java.util.List;

public class Info {

    private String firstName;
    private String lastName;
    private int age;
    private List<String> hobbies;
    private Parent parent;
    
    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public int getAge() {
        return age;
    }

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

    public List<String> getHobbies() {
        return hobbies;
    }

    public void setHobbies(List<String> hobbies) {
        this.hobbies = hobbies;
    }

    public void addHobby(String hobby) {
        if (hobbies == null)
            hobbies = new LinkedList<String>();
        hobbies.add(hobby);
    }
    
    public Parent getParent() {
        return parent;
    }

    public void setParent(Parent parent) {
        this.parent = parent;
    }

    @Override
    public String toString() {
        return "Info [firstName=" + firstName + ", lastName=" + lastName
                + ", age=" + age + ", hobbies=" + hobbies + ", parent="
                + parent + "]";
    }
    
}

付責複製的 Function:

package xstream.sample;

import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.io.xml.DomDriver;

public class Main {

    private static final XStream XSTREAM = new XStream(new DomDriver());
    
    @SuppressWarnings("unchecked")
    public static <T> T cloneObject(T src){
        return (T) XSTREAM.fromXML(XSTREAM.toXML(src));
    }

}

測試的程式:

package xstream.sample;

import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.io.xml.DomDriver;

public class Main {

    private static final XStream XSTREAM = new XStream(new DomDriver());
    
    public static void main(String[] args) {
        //Create info object
        Info info = new Info();
        Parent parent = new Parent();
        
        //Set data
        info.setFirstName("Lawrence");
        info.setFirstName("Cheung");
        info.setAge(24);
        info.addHobby("Sports");
        info.addHobby("Reading");
        
        parent.setFirstName("Tom");
        parent.setLastName("Cheung");
        info.setParent(parent);
        
        //Print my info
        System.out.println("Original object: " + info);
        
        System.out.println("Now, we are going to clone info object");
        System.out.println();
        Info clonedInfo = cloneObject(info);
        
        //Print cloned info
        System.out.println("Cloned object: " + clonedInfo);
        System.out.println();
        
        //Modify original info
        System.out.println("Change original info age to 25 and parent first name set to John.");
        info.setAge(25);
        info.getParent().setFirstName("John");
        
        System.out.println();
        System.out.println("Original object: " + info);
        System.out.println("Cloned object: " + clonedInfo);
    }
    
    @SuppressWarnings("unchecked")
    public static <T> T cloneObject(T src){
        return (T) XSTREAM.fromXML(XSTREAM.toXML(src));
    }

}

結果顯示如下:

Original object: Info [firstName=Cheung, lastName=null, age=24, hobbies=[Sports, Reading], parent=Parent [firstName=Tom, lastName=Cheung]]
Now, we are going to clone info object

Cloned object: Info [firstName=Cheung, lastName=null, age=24, hobbies=[Sports, Reading], parent=Parent [firstName=Tom, lastName=Cheung]]

Change original info age to 25 and parent first name set to John.

Original object: Info [firstName=Cheung, lastName=null, age=25, hobbies=[Sports, Reading], parent=Parent [firstName=John, lastName=Cheung]]
Cloned object: Info [firstName=Cheung, lastName=null, age=24, hobbies=[Sports, Reading], parent=Parent [firstName=Tom, lastName=Cheung]]

可以看出連 Parent 在內也一起複製了，就算更改了原本的 Info Object 也不會改動到複製出來的 Object，即是 2 個變數的 Pointer 不是指向同一個位置。

相關書籍: